An automated method for polynya detection using a geomorphon algorithm - Mia Hurst; Lars Boehme (Ocean Science, 2026)
Polynyas, persistent areas of open water within sea ice, are critical features of polar marine systems, facilitating ocean-atmosphere heat exchange, deep water formation, nutrient cycling, and biological productivity. However, current remote sensing approaches typically delineate polynyas using either sea ice concentration thresholds or thin sea ice thickness retrievals. While these methods have proven highly valuable, they can struggle to capture complex polynya morphology, especially fine-scale coastal features, and can be time-consuming to use and inconsistent across spatial scales. This study presents a novel application of a geomorphon pattern recognition algorithm, originally developed for terrestrial landform classification, to automate polynya detection using sea ice concentration data from the Advanced Microwave Scanning Radiometer 2 (AMSR2) ARTIST Sea Ice (ASI) product. Focusing on two key Southern Ocean regions, the Weddell and Amundsen Seas, we assess the algorithm's performance through a comprehensive sensitivity analysis involving multiple geomorphon parameter combinations and comparisons with traditional sea ice concentration threshold-based methods. By identifying morphological analogues, such as depressions and valleys in sea ice concentration data, the geomorphon method accurately captures polynya morphology, achieving F1-scores of 0.88–0.94 relative to threshold-based detections, while producing area estimates consistent with published observations. Additional sensitivity analyses examining both geomorphon parameter selection and uncertainty in the underlying sea ice concentration observations indicate that derived polynya areas are robust to plausible observational errors. The method's scalability and self-adaptive lookup distance allows detection of both large-scale open-ocean polynyas, and fine-scale coastal polynyas. The automated nature of the method enables efficient processing of large sea ice concentration data products while avoiding reliance on predefined concentration thresholds. Although further testing across additional regions, datasets, and polynya types is required, these results demonstrate that morphology-based classification provides a promising framework for automated and reproducible polynya detection and the generation of consistent long-term polynya records.

Data-driven enhancement of ocean surface forcing for accurate floating debris transport modelling in the East/Japan Sea - Hong Thi My Tran; Young-Gyu Park; Jun Myoung Choi (Ocean Science, 2026)
Floating debris transport in marginal seas is controlled by a combination of geostrophic currents, wind-driven currents, wave-induced drift, and direct wind forcing, but the relative importance of these processes remains difficult to quantify without in-situ trajectory observations. This study evaluates near-surface forcing combinations for particle tracking in the East/Japan Sea using 33 GPS-tracked surface drifters deployed off the Korean coast in November 2021. The drifters revealed a clear cross-basin transport pathway toward the Japanese coastline and an early bifurcation near a hyperbolic-type Lagrangian structure. We used a particle tracking model to test combinations of geostrophic current, Ekman current, Stokes drift, and windage against the observed drifter trajectories using MAE and NCLS metrics. For undrogued DX drifters, the best performance was obtained by combining geostrophic current, Stokes drift, and windage. For drogued DO drifters, which sampled a deeper effective depth, the inclusion of all four forcing components provided the best agreement with observations. The optimized forcing combinations outperformed benchmark simulations driven by the GOFS analysis and GLORYS reanalysis in this specific event, primarily because they better reproduced the Lagrangian structure associated with the early trajectory bifurcation. Seasonal simulations showed that winter winds enhance eastward transport and beaching along Japan, whereas weaker summer winds allow mesoscale eddies to broaden particle dispersion. The present framework represents the fluid-dynamical component of near-surface particle transport and does not explicitly include plastic-specific fate processes such as fragmentation, biofouling-induced buoyancy changes, aggregation, settling, or resuspension.

Chlorophyll a variation trends in marginal seas: assessing the impact of global warming and anthropogenic activities using time-series satellite data (1998–2020) - Nan Yao; Xiaoyu Zhang; Lei Bi; Shuchang Ma; Andrew B. Cundy; Haiyan Jin; Renyi Liu (Ocean Science, 2026)
Global warming has been identified as the primary cause of the decline of surface chlorophyll a (Chl a) concentrations in the oceans. Conversely, increasing Chl a concentrations have been observed in a number of marginal seas over recent decades due to the increasing anthropogenic input of key nutrients. With the intensification of global warming, however, its impact on Chl a in coastal waters along with the superimposed effects of human regulation of nutrients emissions has been rarely studied. To address this research gap, in this study, we provided a comparative analysis of enclosed versus open marginal seas, revealing divergent physiological and ecological responses to rising sea-surface temperature (SST) across different nutrient regimes. We utilized time series of ocean color satellite data from 1998 to 2020 to examine the spatiotemporal distribution of Chl a in a range of marginal seas, and we considered its relationship with environmental factors, in particular with SST, photosynthetically active radiation (PAR), and surface wind speed (SWS). The results suggested that the sea areas examined, with their varying mixing and water exchange characteristics and degrees of human influence, have had different responses in terms of their Chl a trends to increasing SST. Specifically, in eutrophic enclosed seas with weak hydrodynamic exchange capacity, such as the Bohai Sea, increasing SST did not suppress Chl a concentration; instead, we observed a continuous increase in Chl a in the center of the sea. In comparison, the open marginal seas examined showed strong negative relationships between SST and Chl a with increasing distance offshore regardless of the degree of pressure from human activities. This result indicated that the expected global warming effects driving reductions in Chl a have been extending to nearshore and marginal sea areas. This trend may be exacerbated by stricter environmental management policies imposed in recent years, which have reduced anthropogenic nutrient input. Distinct from the noted effect of global warming, PAR and SWS have shaped Chl a in ways that are strongly modulated by geography and climate. PAR is the dominant positive driver only in the Amazon Estuary, where equatorial cloudiness and high turbidity create a light-limited regime, so any increase in PAR directly stimulates phytoplankton. In mid-latitude open waters, PAR is secondary to SST. Its seasonal rise is coupled to SST and therefore it is negatively correlated with Chl a after thermal stratification reduces nutrient supply. SWS has emerged as a key driver in the three open regimes (the East China Sea > the Eastern Coastal Waters of the United States > the Amazon Estuary) by injecting nutrient-rich cold deep water and episodically raising Chl a. Inside the two enclosed seas (the Bohai Sea and the Gulf of Mexico), correlations with both PAR and SWS have been weak (|r|

Evaluate the impact of a 4 h tandem phase on the continuity of nadir altimetry measurements between S3 and S3NG-T - Noémie Lalau; Michaël Ablain; Thomas Vaujour; François Boy; Gerald Dibarboure; Alejandro Egido (Ocean Science, 2026)
The upcoming Sentinel-3 Next Generation Topography (S3NG-T) mission, designed to succeed the current Sentinel-3 (S3) mission, will operate on the same ground tracks as the current S3 constellation to maximise continuity of measurements, but with a fixed 4 h temporal lag due to satellite design constraints. This configuration prevents the implementation of a classical near-simultaneous tandem phase, traditionally used for inter-mission cross-calibration, and raises concerns regarding the impact of short-term oceanic variability on continuity assessment. In this study, we evaluate the feasibility and expected performance of a 4 h delayed tandem phase for cross-calibrating S3 and S3NG-T. Using tandem datasets from Sentinel-3A/B and Jason-3/Sentinel-6 missions, combined with SWOT KaRIn observations, we develop a methodology to quantify the oceanic variability introduced by a 4 h delay and to evaluate its effect on the accuracy of inter-mission offset estimates. Results indicate that the classical tandem configuration achieves regional inter-mission Sea Level Anomaly (SLA) offset uncertainties of approximately 2 mm over a three-month period. In contrast, a 4 h delayed tandem phase increases this uncertainty to about 7 mm in the same period, but still performs significantly better than non-tandem scenarios. Extending the 4 h tandem phase to one year enables the detection of systematic instrumental errors of ±3.5 mm amplitude, sufficient to ensure continuity between S3 and S3NG-T. These findings demonstrate that, despite additional oceanic variability, a 4 h tandem configuration remains a viable and effective strategy for cross-calibration, especially when supported by improved environmental corrections and by extending the observation duration to a full year.

High current speed events in a harbor channel driven by resonant sub-hourly sea level dynamics: an example from Varna, Black Sea - Jüri Elken; Laura Piho; Maarja Kruusmaa (Ocean Science, 2026)
Current and sea-level observations in the 2.4 km-long, 300 m-wide channel between Varna Lake and the Black Sea, conducted by three novel Hydromast stations with a 1 min resolution, revealed that moderate sub-hourly dynamics were occasionally interrupted by high-amplitude oscillations with a period of a few tens of minutes. The resonant excitation of the events was studied using barotropic 1D analytical and numerical methods. Basic features of high-intensity sub-hourly variations can be interpreted as linear barotropic long waves in a sea-channel-lake system with resonant and damped forcing. The observed 37 min oscillations, identified from the power spectra, spectrogram, and wavelet analysis, can be explained as the first mode of the channel-lake system. Another period of 19 min resembles the zero, quarter-wave mode of the channel and the second mode of the channel-lake system. During the measurement period, two high-amplitude events were highlighted. One of the events is interpreted as having been caused by a meteotsunami; moderate meteorological conditions prevailed; strong channel oscillations began abruptly and lasted for a few cycles. The second event occurred more than 5 h after landward winds up to 20 m s−1. The observed maximum changes in current and sea level – up to 0.8 m s−1 and 0.8 m, respectively, over 10 min – are harmful to ship navigation, harbor operations, and coastal management. Building on these observations, approaches for detecting and forecasting strong sub-hourly oscillations are discussed.

Intraseasonal variability and eddy-induced structural modulation of the North Pacific intermediate water revealed by multi-mooring observations - Qiang Ren; Yansong Liu; Yansheng Zhang; Shumin Tu; Wei Huang; Feng Nan; Ran Wang; Xinyuan Diao; Jianfeng Wang; Xinchuang Liu; Zifei Chen; Fei Yu (Ocean Science, 2026)
The North Pacific Intermediate Water (NPIW) plays an important role in regulating thermohaline structure and biogeochemical distributions in the North Pacific. However, limited continuous observations have hindered our understanding of its short-term variability and structural response to mesoscale processes. Based on 1 year mooring observations from three sites (M1–M3) in the western North Pacific, this study investigates the intraseasonal structural variability of the NPIW and its modulation by mesoscale eddies. The NPIW thickness exhibits pronounced intraseasonal variability, with a dominant period of approximately 60–80 d that is coherent among the three mooring sites. Unlike previous studies that mainly focused on temperature and salinity anomalies, this study introduces NPIW thickness as a structural diagnostic parameter to characterize the vertical compression and expansion of the intermediate layer associated with mesoscale variability. The results reveal a clear inverse relationship between NPIW thickness and salinity, indicating that anticyclonic conditions are generally associated with thinner and more saline intermediate layers, whereas cyclonic conditions correspond to thicker and relatively fresher layers. Spatial composite analyses further show that thickness and salinity responses exhibit clear regional differences, with stronger variability near the western boundary, likely related to complex water-mass redistribution and possible mixing processes. These findings provide quantitative observational evidence for intraseasonal variability in NPIW thickness and highlight its usefulness as a diagnostic indicator for mesoscale–intermediate water interactions in the North Pacific.

Thermohaline gradients and frontal regimes in the northwestern Tropical Atlantic - Dante C. Napolitano; Jonathan Gula; Solange Coadou-Chaventon; Sabrina Speich; Cesar B. Rocha; James C. McWilliams; Dongxiao Zhang; Xavier Carton (Ocean Science, 2026)
At the edge of the Amazon River plume, stirring by the North Brazil Current (NBC) and its eddies creates sharp surface thermohaline gradients on horizontal scales of 𝒪 (1–100) km. This study provides a comprehensive picture of these gradients and fronts associated with the region's distinctive dynamics. Saildrone observations show that the plume amplifies density gradient variability at all scales from 1–100 km, with frontal sharpness up to 75× stronger inside the plume than outside, with differences reaching 100× at scales below 3 km. Density gradients are partially reinforced or compensated by temperature-salinity variations, with net frontogenesis observed in both regions. To expand in-situ observations, we use a 1 km resolution CROCO (Coastal and Regional Ocean COmmunity model) simulation to assess the spatial distribution of surface fronts and their spatio-temporal variability. We characterize three distinct frontal regimes: (i) broken-up fronts parallel to shore occupy the plume core over the continental shelf, (ii) thin elongated fronts associated with NBC-plume interactions connect nearshore and offshore regions, and (iii) pools of anisotropic fronts driven by the seasonal mixed-layer cycle are present offshore. Salinity dominates density gradients throughout the year north of 15° N, whereas near-shore fronts exhibit seasonal shifts in temperature-salinity dominance linked to the Amazon discharge seasonality and NBC strength. Within the plume, freshwater filaments stirred by NBC rings systematically generate density-compensated fronts on their inner edge and reinforced fronts on their outer rim, a pattern with implications for energy cascades and tracer export.

Water mass modification of the warm Atlantic Inflow towards the Arctic across the Iceland-Faroe Ridge - Guðrið Eriksdóttir; Bogi Hansen; Karin Margretha H. Larsen; Steffen M. Olsen; Andrea M. U. Gierisch; Sólveig Rósa Ólafsdóttir (Ocean Science, 2026)
The warm Atlantic inflow across the Iceland-Faroe Ridge (IFR) is the strongest of the three branches carrying warm water to the Nordic Seas and further into the Arctic Ocean. This branch (IF-inflow) carries almost 50 % of the total Atlantic water inflow. After crossing the ridge, a large part of this water is converted to dense water that returns to the Atlantic as cold overflow, which contributes to the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). A better understanding of the temperature and salinity variations of the IF-inflow is therefore important for assessing both regional and global climate. Based on satellite altimetry and drifter observations, we document the pathway of the Atlantic water through the Iceland Basin to the IFR and show how this pathway is affected by variable intrusion of the Subpolar Gyre into the basin with a narrower, faster and southward-shifted pathway during periods of strong Subpolar Gyre intrusion. The variable intrusion is furthermore shown to be the main cause of the long-term salinity variations upstream as well as downstream of the IFR while the temperature has increased due to the general warming of the oceans in addition to the variations caused by variable intrusion. As the Atlantic water crosses the IFR, it is cooled by at least 1 °C and freshened by at least 0.1 g kg−1. We show that this transformation mainly is caused by mixing with Arctic water masses, rather than by air-sea-interactions, and that most of the modification takes place upstream of the ridge crest. The Atlantic water changes character from an almost barotropic to a much more baroclinic flow over the ridge enabling water masses, otherwise constrained to follow isobaths, to cross the ridge. We find that the cooling and freshening of the Atlantic water across the IFR are relatively constant throughout the whole period from 1993 to 2023, except for the last few years. The freshening across the ridge implies that the salinity difference between IF-inflow water and the deep waters northeast of the ridge has been reduced by roughly 20 %–30 % after crossing the ridge, which weakens the potential for dense-water formation downstream. Updated transport estimates show a slight strengthening for this AMOC branch. From 1993 to 2023, the volume transport of the IF-inflow increased by 12±7 %, while the heat transport relative to 0 °C increased by 16±8 %.

Impact of mesoscale eddy parameterization on Arctic Atlantic Water circulation and heat transport in the eddy-permitting grey zone - Per Pemberton; Iréne Wåhlström; Sam T. Fredriksson (Ocean Science, 2026)
The Arctic Ocean is undergoing rapid change, yet many CMIP-type climate models struggle to accurately represent its circulation and water masses. A key feature of the system is the topographically controlled boundary currents that transport warm, saline Atlantic Water northward at intermediate depths into the Atlantic Water layer. An important process affecting these boundary currents is the lateral flux of heat and salt driven by mesoscale eddies. Because the deformation radius is relatively small in the Arctic Ocean, numerical simulations require kilometer-scale resolution to fully capture eddy dynamics. Most of the current climate models, however, operate in a non-eddying regime, relying on mesoscale eddy parameterizations – typically combining isopycnal diffusion (Redi) with eddy-induced advection (Gent and McWilliams, GM). As horizontal resolution increases, future models will shift from an eddy-parameterized to eddy-permitting regime, entering a grey zone where eddies are only partially resolved and the role of GM parameterization becomes less straightforward. This study investigates the use of GM parameterization in eddy-permitting models, focusing on its effect on the northward transport of Atlantic Water in the Nordic Seas and Arctic Ocean. We conduct realistic simulations where we vary GM diffusivity strength and test two different GM scalings. These experiments are compared with a high-resolution reference simulation and observational data. Further, we show how the GM parameterization modulates the topographically steered boundary current through a reduction in baroclinicity of the large-scale circulation, how transport across Greenland–Scotland Ridge and Fram Strait is controlled by its strength; and how resolved and parameterized eddy heat fluxes contribute to the redistribution of heat. Our results suggest that mesoscale eddy buoyancy fluxes remain insufficiently resolved at eddy-permitting Arctic resolutions, supporting the continued use of GM-type parameterizations in the grey-zone regime. However, there are some obvious limitations with the tested GM formulations in our experiments that warrants further improvement of the GM scheme in an Arctic context.

Sea surface salinity downscaling using deep generative diffusion models - Enzo Forestier; Luther Ollier; Roy El Hourany; Jacqueline Boutin; Carlos Mejia; Sylvie Thiria (Ocean Science, 2026)
High-resolution satellite observations are essential for studying fine-scale ocean processes. Yet, present satellite sea surface salinity (SSS) products remain too coarse to resolve many fine-scale structures. We investigate denoising diffusion models as a generative framework for SSS downscaling in a controlled proof-of-concept experiment based on GLORYS reanalysis fields. A multichannel diffusion prior is trained on 1/12° SSS, sea surface temperature (SST), and sea surface height (SSH) fields, and is then conditioned at inference time on a synthetically degraded coarse SSS observation (1/3°) together with high-resolution (1/12°) auxiliary SST and/or SSH. Conditioning is performed through pseudo-inverse guidance, which steers the generated samples toward states that are compatible with the coarse observation while remaining within the learned GLORYS-consistent multivariate distribution. We also test a gradient-enhancement procedure designed to increase contrast during inference. Experiments in the Gulf Stream region compare models conditioned on SST only, SSH only, and both variables. Validation over the year 2020 uses root-mean-square error (RMSE), structural similarity (SSIM), gradient distributions, and temporal Fourier spectra. In the present GLORYS configuration, conditioning on SST substantially improves accuracy relative to SSH alone; combining SST and SSH yields further gains, comparable to a strong convolutional baseline under RMSE/SSIM, while additionally providing an ensemble of plausible reconstructions. The gradient-enhanced sampler increases structural contrast but can risk amplifying part of the variability, illustrating a trade-off between pixel-wise accuracy and structural realism. Overall, the results support guided diffusion as a promising framework for SSS downscaling and uncertainty-aware reconstruction, while showing that transfer to real satellite SSS products will require product-aware observation operators, uncertainty weighting, and independent in-situ validation.

Eddy kinetic energy and energy conversion rates along the Atlantic Water boundary current north of Svalbard - Kjersti Kalhagen; Ilker Fer; Till M. Baumann; Jon Albretsen; Lukas Frank (Ocean Science, 2026)
On the continental slope north of Svalbard, the boundary current carrying Atlantic Water (AW) loses heat as it flows eastward. This cooling cannot be fully attributed to atmospheric heat loss or turbulent mixing. Lateral exchange, potentially linked to mesoscale activity, has previously been proposed as a contributing factor, based on limited observations of eddies. Here, we analyse a year-long dataset of hydrography and velocity observations from two mooring arrays, supplemented by output from an eddy-resolving ocean model, to quantify the seasonal variability of eddy kinetic energy (EKE) and baroclinic and barotropic energy conversion rates over time-scales from days to months. Both EKE and conversion rates peak in autumn and winter, coinciding with the strongest boundary current and the warmest AW. Local EKE variability, however, is only weakly associated with conversion rates, suggesting advection from upstream generation sites or unresolved variability from limited measurements. Conversion is mainly baroclinic, through boundary current instability, providing conditions favourable for offshore propagation of warm-core eddies. Modelled conversion rates have a complex spatial structure with substantial values on the offshore, deeper side of the boundary current with comparable contributions from baroclinic and barotropic processes. Resulting mesoscale activity enhances lateral stirring and heat loss from the boundary current, particularly in winter and spring, contributing to the along-stream cooling of AW.

North Atlantic response to a quasi-realistic Greenland meltwater forcing in eddy-rich EC-Earth3P-VHR hosing simulations - Eneko Martin-Martinez; Eduardo Moreno-Chamarro; Fraser William Goldsworth; Jin-Song von Storch; Cristina Arumi; Daria Kuznetsova; Saskia Loosveldt-Tomas; Pierre-Antoine Bretonnière; Pablo Ortega (Ocean Science, 2026)
The vast majority of studies examining the impact of freshwater from ice sheet melting on the Atlantic Meridional Overturning Circulation (AMOC) use climate models that cannot resolve mesoscale ocean processes and do not include an accurate spatio-temporal distribution of the freshwater forcing. These two factors critically affect the nature of the AMOC response. Our study partially fills that gap with a set of three hosing experiments using perpetual 1950 radiative forcing with the global configuration of the eddy-rich EC-Earth3P-VHR climate model. The model is forced for 21 years with a spatial and monthly distribution of Greenland meltwater fluxes derived from a product based on observations and model simulations. An annual average close to 0.04 Sv is released on top of the simulated model river runoff, which is vertically distributed in the coastal points connected to each hydrological basin. Within the first year, we observe a response of reduced salinity in the Greenland and Labrador currents. Since the beginning of the experiments, these boundary currents also experience an acceleration and cooling. The cooling arises because freshwater-induced stratification suppresses vertical mixing, reducing the entrainment of warmer subsurface waters into the surface layer of the boundary currents. The meltwater fluxes also lead to a rapid weakening of the AMOC at subpolar latitudes due to circulation adjustments forced by the changes in density gradients, with the salinity-driven density reduction outweighing the temperature-driven density increase. Around year 7, deep mixing in the Labrador Sea begins to weaken as freshwater anomalies accumulate through lateral exchanges with the boundary currents. This weakening in deep mixing affects the Deep Western Boundary Current (DWBC), which warms up at the OSNAP section, and sequentially further weakens the AMOC, resulting in an even stronger reduction that reaches also the subtropical latitudes. After the 21 simulated years, the AMOC has weakened by almost 3 Sv at 60.2° N in density space, which represents a ∼20 % reduction of the climatological value in the control, 14.9 Sv. In the context of the North Atlantic, a basin-wide decrease of 1.3 Sv is measured between 10–65° N, which represents about 10 % reduction of the control reference, 13 Sv. This AMOC reduction is strong enough for some global climate impacts to emerge, such as a “bipolar seesaw” temperature response.

Climate modes synergistically influence marine heatwaves in the North Sea - Yuxin Lin; Zhiqiang Liu; Feng Zhou; Qicheng Meng; Wenyan Zhang (Ocean Science, 2026)
Global shelf seas have experienced unprecedented marine heatwaves (MHWs) in recent decades. Although MHWs have been extensively studied at the global scale, their regional variability and underlying mechanisms remain poorly understood, particularly in shelf seas influenced by multiple climate modes. Here, we examine MHW variability in the Northeastern Atlantic shelf using a correlation-based k-means clustering approach. Two distinct subregions with contrasting seasonal patterns are identified. In winter, the southern North Sea experiences increased MHW frequency, intensity, and duration. This enhancement is linked to a positive East Atlantic Pattern, which intensifies westerly winds and enhances warm Atlantic inflow through both atmospheric and oceanic pathways. In contrast, the northern North Sea shows enhanced MHW frequency and duration in summer, while MHW intensity weakens. This summer response is modulated by Atlantic Multidecadal Variability, with its positive phase strengthening Pacific–Atlantic connections via Rossby wave propagation, altering cloud cover and surface radiative forcing. A shallow mixed layer, enhanced stratification, and circulation-induced upwelling favor frequent and persistent but less intense summer MHWs. This north–south contrast demonstrates that different combinations of atmospheric and oceanic processes shape MHW variability across the shelf, providing a diagnostic and mechanistic framework for understanding regional MHW variability and its potential predictability.

Documenting the 2015–2017 freshening of the eastern Eurasian Basin of the Arctic Ocean and evaluating its drivers and consequences - Dolly More; Igor V. Polyakov; Andrey V. Pnyushkov (Ocean Science, 2026)
The Arctic Ocean is undergoing rapid change, with freshwater playing a central role in shaping stratification, vertical heat exchange, and sea-ice loss. Using long-term observations from the Nansen and Amundsen Basins Observational System (NABOS), we document a strong freshening event in the eastern Eurasian Basin between late 2015 and early 2017. During this period, salinity in the upper 175 m decreased by ∼ 0.5 psu, equivalent to an additional ∼ 0.6 m of freshwater, relative to the preceding (2013–2015) and following (2017–2018) years. The anomaly originated on the Kara Sea shelves in 2014–2015, when exceptional Yenisey and Ob discharge provided a combined freshwater surplus of ∼ 0.78 m, sufficient to explain the observed freshening. Trajectory analysis traced the freshwater anomaly to the Kara Sea, with transport times of 8–9 months to the shelf and 22–23 months to offshore. The resulting enhanced stratification suppressed upper-ocean currents by ∼ 22 % and vertical shear by ∼ 50 %. These changes enabled thicker sea ice to persist through the summers of 2016–2017, in contrast to near-ice-free conditions in adjacent years. While wind anomalies aided the retention of freshwater along the slope, anomalous river discharge was the dominant driver of the event. Overall, the 2015–2017 event demonstrates how episodic river discharge events can restructure upper-ocean stratification, reduce oceanic heat fluxes, and lead to delayed melt and increased summer sea ice, highlighting the sensitivity of upper-ocean processes and sea ice to episodic freshwater forcing in the Arctic.

Bottom topography effects on the internal wave climate in the Ionian Sea - Florian Kokoszka; Stefania Sparnocchia; Davide Cavaliere; Vincenzo Artale; Mireno Borghini; Beatrice Giambenedetti; Federico Falcini (Ocean Science, 2026)
The abyssal Ionian Sea is a deep region of interest for the entire ocean circulation of the Mediterranean Sea, since it plays an important role in the ventilation processes of the whole basin. Here we investigate spatial patterns of internal wave climate over the bottom of the Ionian sub-basin. To identify regional features of the internal wave field in terms of vertical shear and strain, we analyze LADCP and CTD profiles, measured across the basin in 2007, covering various seafloor morphologies (shelf, shelf break, and abyssal plain). By introducing broadband statistical quantities derived from the shear–strain variance partition, our results show that increasing seafloor roughness reduces the absolute values of shear-to-strain ratio, a pattern also influenced by correlations between slope and roughness. Roughness appears to constrain waves toward higher frequencies, with high shear-to-strain ratios associated with lower frequencies and flatter propagation angles, and low ratios linked to higher frequencies and steeper beams. Spectral analyses indicate that rougher regions enhance strain variance at small vertical scales while reducing shear variance at larger scales, leading to flatter shear spectra in the low-wavenumber band. Together, these findings suggest that roughness redistributes energy from large-scale (low-mode, low-vertical-wavenumber internal waves with vertical scales O(102–103 m)) toward small-scale (high-vertical-wavenumber internal waves with vertical scales O(10–101 m)), fundamentally altering the balance of internal wave energy across scales. These results provide useful knowledge for ad hoc finescale parameterization based on seafloor topography.

The answer is blowing in the wind: seasonal hydrography and mixing of the inner sea of Tierra del Fuego, Southern Patagonia - Manuel I. Castillo; Constanza Zuñiga; Carmen Barrios-Guzmán; Natalia Cisternas; José Garcés-Vargas; Mauricio F. Landaeta; Andrea Piñones; Marcela Rojas; Alicia I. Guerrero; Maritza Sepúlveda (Ocean Science, 2026)
This study characterizes seasonal hydrography and mixing processes in Almirantazgo Fjord, a sensitive ecosystem in southern Chilean Patagonia. Although estuarine and tidal forcing conventionally explain fjord dynamics, wind stress effects remain less understood in this high-latitude region. The study analyses a comprehensive six-month dataset including a moored time-series of temperature, salinity, and dissolved oxygen, cross-fjord CTD transects, and hydrographic profiles derived from seal-deployed sensors. Observations indicate distinct seasonality, shifting from a stratified water column in summer – defined by low-salinity surface water from glacial melt – to a mixed winter state with significantly reduced vertical stability. The analysis identifies persistent, topographically channelled up-fjord winds as a primary physical driver. By applying the Wedderburn number (Wb) and mechanical energy balance calculations, we determined that strong wind stress perturbs the pycnocline (Wb > 1). During stratified summer periods, strong wind events (above the 90th percentile) generated wind power of the same order of magnitude as that of the estuarine circulation. Under such conditions, wind forcing amplifies vertical mixing, modulates the pressure gradient, and supports oxygenation in the upper and subsurface layers. First-order estimates indicate that the upper brackish layer is flushed in approximately one week, reflecting a dynamic surface exchange governed by the interplay of freshwater buoyancy and wind stress. These results indicate that wind constitutes a primary mechanism regulating the hydrographic structure and biogeochemical function of the Tierra del Fuego inner sea.

Linking large-scale climate modes to local wave climate and storm surge: insights from a weather typing approach - Zehua Zhong; Hachem Kassem; Ivan D. Haigh; Dafni E. Sifnioti; Ye Liu; Paula Camus (Ocean Science, 2026)
Understanding temporal variations in nearshore sea states is crucial, as they affect shoreline evolution and coastal hazard potential. Local sea state conditions are influenced by large-scale climate modes, yet the underlying mechanisms remain not fully understood. Previous studies have mainly established the climate–sea state links through correlation analyses or other statistical methods. This study investigates whether weather typing, a statistical downscaling method, can provide a physically interpretable link between climate modes and local wave and storm surge variability. The analysis was conducted at Hartlepool, UK, where 36 weather types were previously developed to assess the exposure to coastal hazards for a local nuclear power station. Six climate indices were examined, and we found that the North Atlantic Oscillation (NAO) and the Scandinavian pattern (SCAND) have significant correlations with local wave and storm surge variables. The analysis reveals that, in response to the phases of NAO or SCAND, storm surge distributions exhibit changes in the mean and standard deviation, peak wave period distributions shift between bimodal and near-unimodal shapes, and wind waves and swell show different dominant directions. Using weather types, these response patterns can be traced back to synoptic circulation conditions characterized by different prevailing winds, spatial patterns of storm activity, and local atmospheric pressure. NAO and SCAND modify the occurrence probabilities of these synoptic conditions, thereby providing a probabilistic link between large-scale climate modes and local sea states. This research demonstrates the potential of weather types to offer new perspectives on the impact of climate modes on local sea states.

Improving ocean bottom pressure fields using space gravity data in state estimation - Rui M. Ponte; E. Nishchitha S. Silva; Ou Wang; Ichiro Fukumori; Mengnan Zhao (Ocean Science, 2026)
Ocean bottom pressure (pb) is critical for monitoring and understanding ocean variability, yet global observations from GRACE and GRACE Follow-On suffer from limited spatiotemporal coverage. State estimation methods allow for the dynamical interpolation of sparse data by optimally combining observations with models. Here we examine the effects of assimilating GRACE data (local pb anomalies and global mean), along with other datasets, on state estimates produced by the project for Estimating the Circulation and Climate of the Ocean (ECCO). The ECCO optimization leads to large adjustments in pb fields at monthly and longer timescales. A substantial part of those adjustments is directly induced by GRACE constraints, with largest impacts occurring at high latitudes. Additionally, the mean ocean mass constraint is essential for mitigating large imbalances in freshwater fluxes derived from atmospheric reanalyses (used as prior forcing) and for producing a realistic barystatic sea level curve. Interpretation of remaining ECCO and GRACE differences highlights issues with non-oceanographic data signals. Our findings indicate that GRACE data contain information complementary to that available in other datasets, quantifying their value for determining pb and associated circulation fields.

TS-Cast: deep learning for subsurface ocean reconstruction from satellite observations in the northwestern Pacific - Jeong-Yeob Chae; Kathleen A. Donohue; Jae-Hun Park (Ocean Science, 2026)
Since the 1990s, satellite observations have been providing reliable estimates of ocean surface state, including absolute dynamic topography (ADT), sea surface temperature (SST), and sea surface salinity (SSS) at sufficient space and time resolution to characterize ocean dynamics. Together with the extensive hydrographic dataset from Argo and ship-based hydrographic profiles, these measurements provide a comprehensive view of oceanic conditions. While ADT reflects full ocean dynamics, its steric component represents the integrated information for subsurface water properties. However, relating surface variables to subsurface profiles remains challenging because surface signatures are often non-linearly related to interior structures, and satellite data contain inherent non-steric signals. To address these limitations, we introduce TS-Cast, a novel uncertainty-aware deep neural network. Unlike direct regression models, TS-Cast is designed to adjust monthly climatological profiles as a physical prior and learns to dynamically adjust them. By using a 31 s sequence of satellite inputs (SST, SSS, and ADT) and quantifying prediction uncertainty, the model effectively captures the temporal variation of mesoscale dynamics. It was trained on approximately 155 000 Argo and ship-based thermohaline profiles in the northwestern Pacific. TS-Cast’s capability is demonstrated by comparisons with independent time-series data from moorings that measured temperature and salinity or vertical acoustic travel time. The network significantly improves upon the climatological baseline, achieving an overall Root Mean Square Error (RMSE) of

Filamentogenesis and Filamentolysis of a Low-Density Filament: Dynamic Processes in the Near-Surface Ocean Under Tidal Forcing - Michelle Albinus; Thomas H. Badewien; Lisa Gassen; Oliver Wurl; Jens Meyerjürgens (Ocean Science, 2026)
This study investigates the dynamics and alteration of a low-density filament driven by freshwater-induced buoyancy embedded within a tidal mixing front, focusing on its spatial and temporal evolution in the near-surface layer (0.1–10 m) of the water column. A high-resolution, multi-sensor dataset, consisting of surface drifters, a drifting sensor chain, and an autonomous surface vehicle equipped with an Acoustic Doppler Current Profiler, temperature, and conductivity sensors, was used to observe patterns of divergence, vorticity, and vertical velocities. The measurements resolved three phases of the filament occurring on length scales of O(0.1–2 km) and time scales of minutes to one hour: (I) establishment of the filament in the overlying first meter and filamentolysis

Wave-induced sediment resuspension potential in the Finnish Archipelago, Baltic Sea: integrating field measurements with large-scale numerical model simulations - Jan-Victor Björkqvist; Mari Savela; Heidi Pettersson; Victor Alari; Alf Norkko (Ocean Science, 2026)
Sediment resuspension, driven by wind-wave-induced shear stress, is a key process influencing coastal water quality, biogeochemical cycles, and the transport of pollutants and organisms. The critical shear stress, τcr, is a central parameter in sediment transport models, since initiation of motion can occur when wave-induced shear stress exceeds the critical value. In this study, we implemented a high-resolution (20 m) spectral wave model to simulate near-bottom orbital velocities across the complex archipelago of southwestern Finland. We then used laboratory measurements from in situ sediment samples to determine a model for the critical shear stress that accounts for physical properties using the median grain size and the dry bulk density, and the time-varying biological variation using chlorophyll a. Our proposed model, τcrd50,ρB,Chla(t), explained 66 % of the variation of the measured critical shear stress for our data collected from three different sediment types (Mud, Sand and Mixed sediments). The modelled mean critical shear stress differed between sediment classes, with values of 0.49 N m−2 for Mud, 1.56 N m−2 for Sand, and 1.02 N m−2 for Mixed sediments. The variability in the critical shear stress around the mean values driven by a non-constant biological contribution was approximately 30 % for Mud and Sand, and approximately 50 % for Mixed sediments. Finally, we used a class-level map of the sea floor and the in situ grain size data to translate the wave model orbital velocities to near-bottom shear stresses. Based on the numerical model data, the critical shear stresses from the newly proposed model, τcr(d50,ρB,Chla(t)), were rarely exceeded based on only wave-induced motions in most of the model grid, but could, nonetheless, be exceeded to up around 10 % of the times in smaller areas. This study highlights the importance of incorporating both physical and biological factors – and their temporal dynamics – into sediment transport models to achieve reliable predictions of critical shear stresses and resuspension potential.

A T-DINEOF model for multiple oceanic variables reconstruction - Bo Ping; Ruiting Yang; Yunshan Meng; Fenzhen Su; Cunjin Xue (Ocean Science, 2026)
Satellite-derived oceanic data are frequently affected by cloud cover, resulting in spatiotemporal gaps. The Multi-DINEOF method is widely used to reconstruct multiple oceanic variables. However, Multi-DINEOF essentially remains a matrix-based DINEOF approach and does not fully leverage the correlations among multiple variables. To address this limitation, this study proposes the T-DINEOF model, aiming to improve the accuracy of reconstructing multiple oceanic variables simultaneously. When applied to sea surface temperature (SST), sea surface chlorophyll a (SCHL), and sea surface wind (SSW) collectively, T-DINEOF reduces root mean square error (RMSE) by 12.9 %, mean absolute error (MAE) by 13.8 %, and mean absolute percentage error (MAPE) by 11.9 % compared to Multi-DINEOF. For each individual oceanic variable, T-DINEOF outperforms both Multi-DINEOF and the original DINEOF methods, reducing RMSE by 9.0 % and 14.7 %, MAE by 10.5 % and 14.6 %, and MAPE by 13.7 % and 13.4 % for SST; reducing RMSE by 9.3 % and 11.8 %, MAE by 9.9 % and 13.4 %, and MAPE by 8.3 % and 11.8 % for SCHL; and reducing RMSE by 16.6 % and 3.7 %, MAE by 16.8 % and 3.5 %, and MAPE by 16.4 % and 3.1 % for SSW. Additionally, T-DINEOF proves effective in regions with a high proportion of missing data and in cases of low data correlation.

A Digital Twin Ocean: can we improve coastal ocean forecasts using targeted marine autonomy? - Dale Partridge; Deep Banerjee; David Ford; Ke Wang; Jozef Skákala; Juliane Wihsgott; Prathyush P. Menon; Susan Kay; Daniel Clewley; Andrea Rochner; Emma Sullivan; Matthew Palmer (Ocean Science, 2026)
This study outlines the development and testing of a Digital Twin Ocean (DTO) framework, aimed at improving coastal ocean forecasts through the use of autonomous underwater gliders. A fleet of gliders were deployed in the western English Channel during August-September 2024 to collect measurements of temperature, salinity, chlorophyll and oxygen, aiming to track the movement of the harmful algal bloom Karenia mikimotoi. Measurements were assimilated into a very high resolution (1.5 km) numerical forecast model, with an implementation of biogeochemistry data assimilation for this purpose. The model forecast was then used by a probabilistic uncertainty model to plan a series of waypoints to navigate the glider fleet towards features of interest. By utilising a continuous feedback loop of measurement, prediction, guidance, and refinement a system with real time coupling between the real ocean environment and its digital counterpart has been established. Building upon a prior pilot study of Ford et al. (2022), this work improves every element of the system to address several limitations of the prior configuration. Whilst a bloom was present in the wider area, measurements and modeling suggest it didn't enter the glider operation zone. Despite this and other operational challenges the mission clearly demonstrates the benefits of such a system. The ability to simultaneously track multiple features of interest, namely chlorophyll maxima and oxygen minima, would not have been possible with a single glider resulting in significant benefits to the system. Furthermore, the improvement to biogeochemical forecasting has been demonstrated through a series of post mission experiments, highlighting the advantages of high temporal resolution observations and increased spatial resolution of the model.

A multidecadal sea level rise and its hiatus in the tropical Atlantic margin off northwest Africa - Hamed D. Ibrahim; Yunfang Sun (Ocean Science, 2026)
Satellite and reanalysis data sets are analyzed to explain sea level changes in the tropical North Atlantic margin off northwest Africa. The study domain sea level was rising as far back as 1986 and a pause in sea level rise (hiatus) began around 2010 and stopped in 2019. Characteristics of sea level anomaly and its drivers during a period of rise (1996–2004) and the hiatus period (2010–2018) are analyzed and compared. Results show that the most effective cause of domain-wide sea level rise during the period of rise is seawater expansion owing to changes in density structure (steric expansion), with almost equal contribution from temperature-driven (thermosteric) expansion and salinity-driven (halosteric) expansion. The cause of the domain-wide pause in sea level rise is a large thermosteric contraction that counteracted halosteric expansion and mass accumulation. Multidecadal sea level increase, defined here as the difference between the mean sea level during the period of rise and the hiatus period, is owing to steric expansion, vertical land motion, and mass accumulation, which contributed 56 %, 24 %, and 16 %, respectively. There are, however, regional differences in the patterns of multidecadal steric and mass adjustment. In the northern subdomain where ocean processes predominate mass-driven sea level variability, the steric adjustment is dominated by halosteric expansion, whereas in the southern subdomain where atmosphere-ocean processes predominate mass-driven sea level variability, the steric adjustment is dominated by thermosteric expansion. The accumulation of low-salinity water in the northern subdomain and precipitation in the southern subdomain appears to be associated with a mutual adjustment of vertical and horizontal velocity distribution inside the domain and west of it in the area of the Guinea Dome, a permanent upwelling region where isotherms are displaced upwards. The low-salinity water influx to the northern subdomain is linked to changes in the southward-flowing Canary Current. A probable hypothesis inferred from correlation and potential vorticity analysis is that the Canary Current source region was freshened by currents that supply water to the region via two pathways: an open ocean path that is consistent with the Azores Current, and a Western Europe coastal ocean path that is consistent with the Portugal Current and Portugal Coastal Current system. The results obtained highlight a multidecadal linkage between sea level anomalies in the eastern tropical North Atlantic margin and salinity anomalies elsewhere in the North Atlantic.

The past evolution of marine heatwaves and their drivers in the southern North Sea - Tobias Schulzki; Franziska U. Schwarzkopf; Arne Biastoch (Ocean Science, 2026)
Marine heatwaves (MHWs) are defined as prolonged periods of anomalously high ocean temperatures. These events can have severe impacts on marine ecosystems and, if they occur at the surface, can feed back on the atmosphere, changing inland air temperatures and precipitation. We use a comprehensive set of model, reanalysis, and observational datasets to investigate recent changes in North Sea MHWs. All datasets show a significant warming trend, accompanied by a marked increase in the frequency of MHWs. In contrast, the maximum intensity of MHWs has decreased in many regions of the North Sea, including the German Bight. If the linear trend in temperature is removed, only a few MHWs have been detected after 2019, suggesting natural variability has damped the effect of the long-term warming. While distinct weather patterns are associated with the onset of MHWs, their occurrence alone is not sufficient to trigger them. As the heat content is an integrated quantity, the ocean temperature at the beginning of the season (ocean preconditioning) is a key factor, in addition to prevailing weather patterns during the season. As a consequence, only in winter we find a significant dependency of MHWs on established climate indices. In our study region, MHWs result from a combination of short-term weather-related variability and longer-term seasonal to decadal variability. Furthermore, we find that the evolution of the surface temperature in the German Bight is largely determined by local atmospheric conditions rather than remote variability in the Atlantic. Although the inflow of warm water through the English Channel is important, it is the atmosphere that controls its volume transport and temperature. Whether the atmospheric conditions themselves are linked to remote variability in the Atlantic Ocean remains to be studied.

Modulation of internal tides properties off the Vitória–Trindade ridge during contrasted seasons from altimetry and a regional ocean model - Perrine Bauchot; Ariane Koch-Larrouy; Michel Tchilibou; Loren Carrère; Fabrice Hernandez; Guillaume Morvan; Jérôme Chanut (Ocean Science, 2026)
The incoherent fraction of internal tides, generated through interactions with mesoscale eddies and other transient oceanic features, remains poorly understood and challenging to predict. This limits our ability to accurately represent energy transfers and mixing induced by these waves. The Vitória–Trindade Ridge off the Brazilian shelf is a relevant natural laboratory to investigate these processes, as a hotspot for internal tides generation embedded in a region of intense mesoscale activity. To assess how seasonal stratification and mesoscale variability modulate internal tides, we compared a 27-year satellite altimetry record with a high-resolution (1/36°) regional simulation using NEMO v4.0.2. This joint analysis allows us to characterize the generation, propagation, and dissipation of internal tides under two contrasted regimes: austral winter (defined here from May to October) marked by a deep pycnocline, and austral summer (defined here from November to April) with a shallower and sharper seasonal pycnocline. Both model and observations depict six intense, in-phase beams of the baroclinic flux propagating southward from the ridge. The first two have a wavelength of 100 km approximately corresponding to the mode-1 of propagation, while more distant beams are spaced by about 50 km only, which likely corresponds to the mode-2 of propagation. Quantification from the model shows that generation rates are 5 %–15 % higher in summer than in winter. Dissipation occurs predominantly near the ridge (45 %) but also extends offshore (40 %), reaching beyond 2–3 mode-1 wavelengths. In the open ocean, dissipation is up to 40 % stronger in winter, leading to a weaker baroclinic flux propagating on shorter distances compared to summer. Altimetry confirms seasonal variations in both wavelength and amplitude, especially for mode-2 internal tides. Finally, a representative case of interaction between internal tides and a mesoscale eddy is documented under summer conditions, showing deviation and diffraction of the baroclinic flux. This study demonstrates that mesoscale variability and seasonal stratification jointly modulate the coherence and energy pathways of internal tides. These findings are essential for improving predictions of the incoherent tide and for interpreting high-resolution altimetric observations.

Dynamically downscaled future projections of the Northwest Atlantic Ocean across low to high emissions scenarios - Dongmin Kim; Andrew C. Ross; Sang-Ik Shin; Fabian A. Gomez; Jasmin G. John; Denis L. Volkov; Sang-Ki Lee; Michael A. Alexander; Charles A. Stock (Ocean Science, 2026)
We used a high-resolution (1/12°) Modular Ocean Model version 6 implementation for the Northwest Atlantic Ocean (MOM6-NWA12) to dynamically downscale Geophysical Fluid Dynamics Laboratory Earth System Model version 4.1 (GFDL-ESM4.1) projections for the 21st century. Simulations were conducted under four different Coupled Model Intercomparison Project Phase 6 emission scenarios. MOM6-NWA12 accurately simulates the spatial patterns of sea surface temperature, salinity, and dynamic sea surface height (SSH) during the historical period. In particular, the Gulf Stream's strength, position, recirculation, and separation from the US East Coast are significantly improved in MOM6-NWA12 compared to the coarse-resolution GFDL-ESM4.1. Projected end-of-century warming varied strongly between scenarios, from ∼4 °C under prior “worst case” emissions scenarios (SSP-585), 2∼3 °C under intermediate scenarios (SSP-245, SSP-370) more consistent with current trajectories, to ∼1 °C under aggressive mitigation (SSP-126). Consistent with a significant weakening of the Atlantic Meridional Overturning Circulation projected by GFDL-ESM4.1, MOM6-NWA12 shows a substantial volume transport reduction in the Western Boundary Current (WBC) system (i.e., Yucatan Current, Florida Current, Antilles Current, and the Deep Western Boundary Current) toward the late 21st century (between 23 % and 38 %, varying by scenario). This projected weakening of the WBC system and the associated reduction in the coastal upwelling of cold, fresh subsurface waters lead to a significant increase in ocean temperature, salinity, and dynamic SSH along the US southeast and northeast Coasts, particularly in the South Atlantic Bight.

Interaction of AMOC and intrinsic multidecadal Southern Ocean variability - Emma J. V. Smolders; René M. van Westen; Henk A. Dijkstra (Ocean Science, 2026)
A strongly-eddying version (0.1° horizontal resolution) of the Parallel Ocean Program (POP) shows pronounced intrinsic multidecadal variability in the Southern Ocean, the so-called Southern Ocean Mode (SOM). This Southern Ocean multidecadal variability is induced by eddy-mean flow interaction and deep convection. The SOM variability propagates through the global ocean and influences the strength of the Atlantic Meridional Overturning Circulation (AMOC) by about 3 Sv. The opposite role on how the AMOC influences the SOM is unknown, as this requires long simulations and preferably with different AMOC background states. Here, using the results of a simulated AMOC collapse in the strongly-eddying ocean-only POP version, we find that the amplitude of the SOM is substantially reduced following an AMOC collapse. Associated changes in horizontal and vertical density variations lead to a weakening of the Antarctic Circumpolar Current transport and a shutdown of deep convection in the Weddell Sea. In contrast, these changes promote deep convection events and the emergence of multidecadal variability in the Pacific sector of the Southern Ocean. A mechanical energy budget analysis shows both a reduction in the wind input and a disruption of the phase difference between wind work and the potential to kinetic energy conversion. The results highlight the strong connection between the AMOC and intrinsic multidecadal variability in the Southern Ocean.

Investigating the predictability of marine heatwaves at subseasonal to seasonal timescales in New Caledonia, South Pacific - Inès Mangolte; Sophie Cravatte; Alexandre Ganachaud; Christophe Menkes (Ocean Science, 2026)
Marine Heatwaves (MHWs) have emerged as one of the most important threat for marine ecosystems, with impacts such as coral bleaching, massive fish mortality and displacement of mobile fauna. In the context of climate change, it is urgent to develop strategies such as subseasonal to seasonal forecasting to help human societies adapt and react to the increasing frequency, duration and intensity of these events. Here we evaluate the predictability of MHWs at the scale of a South Pacific island country, New Caledonia, using ensemble forecasts from a dynamical coupled ocean-atmosphere model. We show that implementing a probabilistic approach where we extract information from the dispersion in the ensemble results in a higher skill than a deterministic approach where we simply compute the ensemble average. We find that longer, more intense, and wider MHWs, are more predictable than weaker, less intense, and shorter MHWs. We also find that the longest and widest MHWs occur in the cold season (June–October) during strong La Niña episodes, and that they can successfully be predicted up to 7 months in advance. In contrast, MHWs occurring during the warm season have poor or no predictability of more than a few weeks in advance. We discuss how this information can be efficiently transferred to marine stakeholders in terms of the usefulness and useability of the forecast. We recommend that future research should focus on identifying the drivers of different types of MHWs in order to understand their sources of predictability.

Stratified-turbulence observations dominated by slantwise downward convective warm-water periods in the deep Mediterranean - Hans van Haren (Ocean Science, 2026)
A three-dimensional mooring array holding nearly 3000 high-resolution temperature sensors in 2500 m deep Mediterranean-Sea waters is used for a yearlong study on different sources of turbulent waterflows, which are vital for life. Although temperature differences are found never larger than 0.01 °C, daily, weekly, and seasonal variations are observed. With a delay of about a week, the deep-sea stratified turbulence tracks atmospheric disturbances, which are found 35 % more energetic in winter than in summer. About half the time, relatively warm stratified waters are moved to near the seafloor from 100's of meters higher levels. Combined internal-wave and sub-mesoscale eddy-induced motions lead to slantwise downward convective warm-water periods that are half an order of magnitude more turbulent than those induced via general geothermal heating from below, and about one order of magnitude more turbulent than those from open-ocean processes. The analysis estimates that eddy-induced stratified turbulence is likely more important for deep-sea life than rare, not observed, deep dense-water formation at the abyssal-plain mooring site.

Estuarine mixing - Hans Burchard; Knut Klingbeil; Xiangyu Li; Lloyd Reese; W. Rockwell Geyer (Ocean Science, 2026)
This review paper presents, explains and discusses major aspects of estuarine mixing which is defined as the destruction of salinity variance. Due to the large amounts of brackish water in estuaries produced by mixing of fresh river discharge and salty ocean water, mixing is one major characteristic of what is an estuary. In this review, mixing is quantified locally as well as on estuary-wide scales. Diagnostics of integrated mixing are given for estuarine volumes bounded by transects as well as isohalines (surfaces of constant salinity) moving with the flow. It is shown how entrainment across a moving isohaline surface depends on gradients of turbulent salt flux and mixing per salinity class. Various relations are derived that link estuarine salt mixing to other estuarine properties such as the freshwater discharge and the bulk estuarine circulation. For estuaries bounded towards the ocean by a fixed transect, the Knudsen mixing law is explained, where estuarine mixing is the product of the Knudsen salinities of inflowing and outflowing water masses and the river discharge. When the estuarine volume is bounded by a moving isohaline surface of salinity S, mixing inside the estuary is simply the product of S2 and the river discharge. Major processes that drive estuarine mixing are presented on various time scales (tidal, fortnightly, weather and discharge time scales) and spatial scales (channel-shoal interaction, mixing fronts). As underlying methods for the quantification of mixing, observational concepts, as well as numerical modelling methods such as consistent turbulence closure modelling and numerical mixing analyses are presented. As an outlook, some future perspectives are sketched. Many of the concepts presented in this review are illustrated using simulation results from a numerical model setup of the Elbe River estuary.

Along-channel variability of Total Exchange Flow in a narrow, well-mixed estuary: influence of the M4 tide - Manuel Díez-Minguito; Hans Burchard (Ocean Science, 2026)
This study provides preliminary estimates of Total Exchange Flow (TEF) along the Guadalquivir River Estuary (Spain) at different cross-sections during low river flows. The analysis combines 3 years of observations from a real-time monitoring network with an analytical exchange flow scenario featuring a sectionally-homogeneous M2+M4 oscillating tidal flow and salinity. Exchange profiles and volume and salinity transports sorted by salinity classes are computed. The results indicate that bulk along-channel TEF estimates decrease upstream. The largest net incoming water volume transport, on the order of 300 m3 s−1, is attained at the lower part of the estuary, near where the largest salinity gradient is observed. This value is about 12-fold the normal river flow from the head dam at Alcalá del Río. Knudsen bulk quantities are consistent with the weakly-stratified character of the Guadalquivir estuary, whose mixing completeness is larger than 67 % at all cross-sections. The covariance between M2 salinity and current seems to play a more important role in exchange flow in the Guadalquivir estuary than the effects due to tidal asymmetry. Overall, the inclusion of the M4 improves TEF estimates in ∼ 10 % in the Guadalquivir estuary. A sensitivity analysis shows that in other estuaries and semi-enclosed basins the effects of the M4 could be even larger. The inclusion of the M4 constituent changes the exchange flow profile by salinity class, modifying the range of salinities of both outflows and inflows.

Radionuclides as tracers of Arctic outflows: pathways, water mass mixing, and freshwater input to Davis Strait and the Labrador Sea - Lisa G. T. Leist; Maxi Castrillejo; Kumiko Azetsu-Scott; Craig Lee; Jed Lenetsky; Marc Ringuette; Christof Vockenhuber; Habacuc Pérez-Tribouillier; Catherine Jeandel; Jean-Éric Tremblay; Núria Casacuberta (Ocean Science, 2026)
In Davis Strait, one of two primary Arctic gateways between the Arctic and Atlantic Oceans, the exchange between northward-flowing Atlantic waters and southward-flowing Arctic waters possibly influences deep-water formation in the subpolar North Atlantic. Yet, these circulation pathways and mixing processes, that may be changing, remain uncertain, underscoring the need for new tracers to resolve their structure and variability. This study employs observations from 2022 and 2024 of two artificial radionuclides, 129I and 236U, as new tracers of water masses in Baffin Bay, Davis Strait and the Labrador Sea. Samples were collected during three expeditions: the AZOMP occupation of the AR7W Line in May 2022, Davis Strait Observation Program in October 2022, and the Amundsen Expedition as part of the Refuge Arctic and Transforming Climate Action programme in September–October 2024. By defining the characteristic 129I and 236U concentrations of the main inflowing waters, we examined the distribution, origin and formation of key water masses in Baffin Bay. Our results reveal a substantial contribution (75 %) of West Greenland Shelf Water to Arctic Water on the surface of central Baffin Bay. 236U-rich Arctic-Atlantic-derived water entering Baffin Bay via Lancaster Sound contributes 40 %–60 % to the formation of the Transition Water observed in Baffin Bay. In contrast, cold Arctic Water appears to originate mainly from Nares Strait, and is derived primarily (up to 70 %) from Arctic-Atlantic Water. Notably, the contribution of the Baffin Bay Transition Water to the formation of LSW was significant, exceeding 20 %. However, the binary mixing model showed limitations in quantifying the origin of North East Atlantic Deep Water due to low tracer concentrations and the likely influence of multiple water mass sources. This study offers novel insights into the origin and transformation of water in Baffin Bay and the Labrador Sea and enhances our understanding of the complex interactions between the Arctic Ocean and the subpolar North Atlantic.

Greenland tip jet and deep convection in the Irminger Sea: disentangling the roles of heat loss and wind stress - Aleksandr M. Fedorov; M. Femke De Jong; Claudia E. Wieners; Elodie Duyck; Henk A. Dijkstra (Ocean Science, 2026)
The strength of the Atlantic Meridional Overturning Circulation (AMOC) depends on deep-water formation in the Subpolar Gyre, particularly in the Irminger Sea, where convection is strongly modulated by short-lived but intense Greenland tip jet wind events. These mesoscale westerlies induce substantial surface heat loss and impose wind stress on the ocean, jointly influencing convective intensity. Using the high-resolution Parallel Ocean Program (POP) within the Community Earth System Model (CESM), we disentangle the thermal and mechanical effects of tip jet on mixed layer deepening through three ensemble experiments: full-forcing (heat loss + wind stress anomalies), heat-only, and wind-only, each compared to a climatological control run. All forced cases show a significant December–April deepening of the mixed layer relative to the control. The heat-only and full-forcing experiments produce similar mixed layer depth (MLD) increases (+1200 m; reaching ∼1800 m), confirming that surface heat loss is the primary driver of deep convection. The wind-only case shows a smaller but still significant increase (+400 m; MLD ∼1000 m), associated with enhanced early-winter mixing and wind-driven salinity increases in the upper ocean. This wind stress forcing erodes the fresh surface layer, reduces buoyancy, and promotes shear-driven mixing in December so that climatological winter heat loss can deepen the mixed layer more efficiently. Because wind stress is not projected to weaken under future warming, its mechanical influence may help delay or modulate the decline of convection in the Irminger Sea as surface heat loss decreases.

Gliding through marine heatwaves: subsurface biogeochemical characteristics on the Australian continental shelf - Daneeja Mawren; Julia Araujo; Romain Le Gendre; Jessica A. Benthuysen; Franck Eitel Kemgang Ghomsi; Jayanthi S. Saranya; Amandine Schaeffer (Ocean Science, 2026)
Marine heatwaves (MHWs) disrupt ecosystems across multiple trophic levels by altering oxygen and biological productivity through the water column and yet, most studies focus on the surface, overlooking subsurface processes that shape ecosystem responses. To address this gap, we analysed 16 years of routine and event-based glider observations on the continental shelf around Australia to present the first comprehensive assessment of the subsurface biogeochemical response during surface MHWs across four contrasting coastal regions. Across all regions and seasons, the distribution of chlorophyll concentrations shifted towards a decline in the mixed layer and an increase below the mixed layer during MHWs, modulated by the event categories. Dissolved oxygen shows a more complex distribution, which also varies during moderate and strong MHW events, arguably with more variation in the mixed layer than below. When regional and seasonal specificities are taken into account, the subsurface characteristics of MHWs vary in accordance with the environmental setting, including the continental shelf structure, tropical or sub-tropical regime, and boundary current influence, especially through the changes in stratification. Summer surface MHWs were characterised by a shallower mixed layer depth than normal conditions and enhanced stratification, confining warming to the upper ocean, while other seasons allow deeper penetration under weakly stratified conditions. The depth of maximum stratification therefore emerged as a useful proxy for the vertical extent of MHWs. The interaction between physical processes, such as seasonal circulation and stratification, and biological feedback, including the presence of deep chlorophyll maxima and potential oxygen production, highlights the complex biogeochemical responses to MHWs, and underscores the importance of region-specific dynamics and the need for more consistent observation strategy, including biogeochemical processes.

Sea ice melt drives vertical p CO 2 variability modulating air–sea gas exchange - Henry C. Henson; Dorte H. Søgaard; Bjarne Jensen; Kunuk Lennert; Tim Papakyriakou; Mikael K. Sejr; Jakob Sievers; Søren Rysgaard; Lise Lotte Sørensen (Ocean Science, 2026)
Strong spatial and temporal gradients in salinity, temperature, and carbonate chemistry in Arctic coastal surface waters complicate the estimation of air-sea carbon dioxide (CO2) exchange, particularly during sea ice breakup. The present study evaluates the applicability of the widely used bulk flux model under such conditions. This approach assumes homogeneous surface conditions and no vertical pCO2 gradients in the bulk seawater. However, our observations in a stratified Arctic fjord reveal pronounced vertical variability in pCO2 within the upper water column, including non-linear gradients near the air-sea interface. This results in, widely varying flux estimates depending upon the depth of the pCO2 measurement used to establish air-sea disequilibrium. Importantly, similarly structured nonlinear pCO2 profiles were observed across distinct fjord systems and years, suggesting that this vertical heterogeneity may be a characteristic feature of Arctic stratification during sea ice breakup. We recommend incorporating both micrometeorological techniques and high-resolution vertical profiling in Arctic fjords to improve flux estimates of CO2 in this rapidly changing region.

On the fate of the Irminger Current water and its impact on the convection region in the Irminger Sea – a Lagrangian model study - Nora Fried; Renske Gelderloos; Oliver J. Tooth; Caroline A. Katsman; M. Femke de Jong (Ocean Science, 2026)
The Irminger Sea is one of the few places in the North Atlantic where dense water masses are formed through deep convection. In addition to atmospheric forcing, wintertime convection in the Irminger Sea interior can be impacted by the extent of restratification in the preceding years. In the Irminger Sea, the cold central basin is contrasted to the Irminger Current (IC) which carries warm and saline waters of subtropical origin. In this study, we investigate the potential impact of the IC on restratification of the Irminger Sea's deep convection area (DCA), using one-year output of a high-resolution regional model combined with Lagrangian particle tracking. We release particles over the upper 1500 m of the IC in the eastern Irminger Sea and track them forward-in-time for six months. Of those particles, 39 % follow the boundary current circulation and 60 % enter the interior Irminger Sea. Around 1 % leaves the Irminger Sea through Denmark Strait and across the ridge to the Iceland Basin. From all particles released, about 26 % reach the DCA, steered by mesoscale variability. In this model simulation, the majority of those particles (95 %) enter the DCA in the upper 1500 m of the water column. On their way to the DCA, the IC waters cool and freshen but on average remain lighter than waters in the DCA and therefore have the potential to restratify the DCA. This westward spread of light IC waters constrains the extent of the DCA to the western Irminger Sea by enhancing the stratification in the eastern part of the basin.

Intraseasonal modulation of sea surface temperatures in the Tropical North Atlantic by African Easterly Waves - Marc Kakante Mendy; Florent Gasparin; Manon Gévaudan; Moussa Diakhaté; Issa Sakho; Julien Jouanno (Ocean Science, 2026)
The sea surface temperature (SST) variability in the Tropical North Atlantic (TNA) plays a crucial role in the regional climate by modulating the Intertropical Convergence Zone (ITCZ) and influencing precipitation, convective systems, and tropical cyclones. While atmospheric synoptic-scale intraseasonal variability in this region is dominated by African Easterly Waves (AEWs), their impact on SST remains poorly understood. This study investigates the modulation of SST by AEWs using a regional configuration of a coupled ocean-atmosphere model and moored surface buoy air-sea observations. The results reveal a significant AEWs signature in SST anomalies, with typical temperature fluctuations of approximately ±0.3 °C (reaching up to ±0.5 °C for the strongest events). A heat budget analysis shows that AEWs mainly influence SST through modulation of the latent heat flux, shortwave radiation, and vertical mixing. The contribution of ocean mixing and that of the air-sea fluxes appear to be of similar order. The dominant 3–5 d AEWs exhibit a stronger impact than their 6–9 d counterparts. These findings highlight the role of AEWs in driving SST variability and mixed-layer dynamics, underscore the importance of accurately representing them in coupled climate models, and call for further investigation into their influence on the mean and seasonal upper-ocean state.

Stratification and mixed layer depth around Iceland: Characterization and inter-annual variability - Angel Ruiz-Angulo; Esther Portela; Charly de Marez; Andreas Macrander; Sólveig Rósa Ólafsdóttir; Thomas Meunier; Steingrímur Jónsson; M. Dolores Pérez-Hernández (Ocean Science, 2026)
The ocean around Iceland is a key region where major water masses and currents interact, influencing the global ocean circulation. Here, we analyze 29 years (1990–2019) of quarterly hydrographic section data collected around Iceland. The hydrographic properties around Iceland show important spatial variability. Based on temperature, salinity, and stratification structure, we classified the Icelandic waters in three distinct regions: the south, the north, and northeast regions. The warm and salty Atlantic Waters that dominate the south show the deepest winter mixed layers (∼ 500 m) while the north and northeast show shallower depths (∼ 100 m). Based on the decomposition of total stratification into temperature and salinity contributions, we find that the subsurface stratification is mainly controlled by temperature in the south and by salinity in the northwest, while in the north, the North Icelandic Irminger Current and East Icelandic Current alternate seasonally, shifting the region between temperature-dominated and salinity-dominated stratification. The interannual variability of the mixed layer and of its thermohaline properties is also large around Iceland. Mixed layer waters were generally colder in the 1990's, then warmed until approximately 2015, and became colder again from 2015 to 2018. In the northeast, a multidecadal mixed layer warming trend emerges from the interannual variability as the Atlantic Water progresses northeastward, which is responsible for transforming locally the upper stratification from salinity-dominated into temperature-dominated. This is associated with the “Atlantification” of the Arctic. Within the mixed layer south of Iceland, density has continuously decreased since the mid 1990's. Elsewhere, we observe density-compensated changes in mixed layer temperature and salinity, without clear long trends. This study provides an unprecedented and detailed description of the seasonal to multi-decadal variability of the mixed layer depth and stratification around Iceland, showing links between this regional variability and changing North Atlantic under global warming.

Response of the Nordic Seas to the 2–6 February 2020 marine cold air outbreak in the GLORYS12 ocean reanalysis - Svenya Chripko; Thomas Spengler; Stefanie Semper; Kjetil Våge (Ocean Science, 2026)
Marine Cold Air Outbreaks (MCAOs) play a crucial role in wintertime water mass transformation in the Nordic Seas. However, due to the spatio-temporal variability of atmospheric forcing and lateral ocean transport, the processes by which MCAOs influence the ocean remain unclear. Using the global eddy-resolving ocean and sea ice reanalysis GLORYS12, we investigate the mechanisms driving the ocean response over different regions of the Nordic Seas to the particularly intense 2–6 February 2020 MCAO event. To assess the impact of the MCAO on the ocean, we quantify the contributions of the mean surface turbulent heat flux relative to the mean change in ocean heat content during the event. The western part of the Nordic Seas (Greenland Sea and northern interior Iceland Sea) was primarily affected by the air–sea heat exchanges, with an overall mixed-layer cooling of approximately 0.02 °C d−1 during the event in the interior Greenland Sea and a deepening of more than 30 m d−1 in some areas. In the eastern part (Norwegian Sea), on the other hand, the air–sea heat exchanges were masked by stronger lateral oceanic heat transport, with a cooling or warming of an order of magnitude higher. In the interior part of the northern Iceland Sea, the mixed-layer depth increased by approximately 5 m d−1, while it decreased near the boundary current in the western Iceland Sea by approximately 8 m d−1. GLORYS12 generally provides consistent results with observational studies from the Iceland and Greenland Seas during MCAOs, highlighting the large spatial variability in the ocean mixed-layer response to a single MCAO event in the Nordic Seas.

Tide of the Time: Global tidal characteristics observed from in-situ measurements - Michael G. Hart-Davis; Roman Sulzbach; Stefan A. Talke; Ivan D. Haigh; Marta Marcos; Philip Woodworth; Richard Ray; Ole B. Andersen; Florent Lyard; Ergane Fouchet; Denise Dettmering; Maik Thomas; et al. (Ocean Science, 2026)
Tide gauges have been critical sources for sea level research, enabling the development of tidal theory and an understanding of local variations that occur across the global oceans. Tides play important roles in a variety of oceanographic and geodetic applications, and characterizing their spatial variability is valuable for applications ranging from fishing to flood risk management. This manuscript presents the coastal characteristics of ocean tides based on 3591 high-frequency tide gauge observations from the recently updated Global Extreme Sea Level Analysis (GESLA) database. These characteristics range from tidal datums such as Mean High Water (MHW) and the Great Diurnal Range to metrics like the Age of the Tide, Form Factors, updated amplitude trend estimates, and new insights into the regional duration of high tides. Our analysis finds that 125 out of 237 long-time series show statistically significant trends in one or more constituents, from −1.47 to +1.80 mm yr−1, while the duration of the high-water stand during spring tides is shown to vary from 1 to 14 h, for an inundation depth of 20 cm. It is anticipated that the results presented will be useful not only to tidal experts but also to a wide range of cross-disciplinary researchers and local communities, aiding their understanding of a vital component of the global Earth system.

Ocean salinity across space-time scales: from water cycle indicator to dynamical driver - Lisan Yu (Ocean Science, 2026)
Ocean salinity plays a complementary role in the climate system: it integrates changes in the global water cycle while also helping drive ocean circulation through its control on seawater density. Salinity has long been viewed as the ocean's “rain gauge”, a largely passive recorder of surface evaporation, precipitation, and runoff. Yet salinity also shapes the currents and mixing that redistribute heat and freshwater, raising a central question: when does salinity mainly record climate forcing, and when does it actively influence climate dynamics? This review synthesizes two decades of satellite and in situ observations within a regime-dependent framework in which salinity's function is set by the competition among freshwater forcing, advection, and mixing timescales. At basin scales (> 1000 km) over decades, salinity tracks water-cycle change through pattern amplification, with fresh regions freshening and salty regions becoming saltier. At regional to mesoscale (∼ 10–500 km) and seasonal-to-interannual timescales, salinity traces circulation pathways; subsurface anomalies often reflect subduction and ventilation histories from years earlier. At submesoscales (O(10 km)) and synoptic timescales (hours to days), salinity becomes dynamically active, sharpening density fronts, modulating stratification, and altering mixing in ways that feed back on its own transport and air–sea exchange. Understanding ocean climate response requires resolving regime boundaries where these balances shift. The critical observational gap is global sea-surface salinity at O(10 km), where salinity transitions from passive tracer to active driver yet current satellite products cannot resolve this scale. Observations at regime boundaries would show how water-cycle intensification and ocean circulation changes interact, improving projections of climate change, ocean heat storage and distribution, and ecosystem dynamics at regional and global scales.

Monitoring surface gravity wave variability with State of Polarization sensing on a subsea telecommunication cable - Julián D. Pelaez Quiñones; Steinar Bjørnstad; Peter J. Thomas; Per Lunde (Ocean Science, 2026)
State of polarization (SoP) sensing is a fibre-optic interrogation technique that recovers cumulative polarization variations of probe lasers transmitted between opposite ends of optical fibres. These changes can be triggered by various external stimuli interfering with the guided light in the fibre, including surface gravity wave-induced motions, as evidenced by prior studies. In this study, we advance this knowledge by quantitatively assessing the capacity of SoP sensing to continuously capture surface gravity wave height and frequency variability in the ocean along a subsea telecommunication cable via comparison with reference in situ and validated model data. Our SoP dataset consists of nearly 11 months of data collected at the landing site of a ∼240 km back-looped fibre link across the Norwegian trench over the North sea, between the city of Egersund in the Southwestern coast of Norway and an offshore bottom-fixed platform. Spectral and correlation analyses demonstrate the capacity of SoP to detect both, wind waves and swells acting simultaneously over different regions of the cable in the 0.03–0.6 Hz range. During the observation period, wind waves affected mostly the offshore end of the cable, while swell signatures dominated in the coastal region, close to the landing site. Correlation coefficients of up to nearly 70 % are obtained between SoP data and the wave analysis model for the full observation period. We discuss various physical aspects related to the SoP measurements and outline some of their limitations for sea state monitoring, chiefly in terms of interpretation capacity and sensitivity. Our results also evidence that SoP sensing systems can be a practical way to obtain dynamic, first-order estimates of the sea state variability where cables are sufficiently shallow. A non-negligible portion of the extensive infrastructure of submarine telecommunication cable networks has thus the potential to be turned into arrays of remotely-operated sensors of opportunity that are complementary to well-established oceanographic instruments.

Obtaining accurate, high-frequency and long-term seawater pH data by using coupled lab-on-chip and optode sensing technologies - Anthony J. Lucio; Dirk Koopmans; Martin Arundell; Socratis Loucaides; Allison Schaap (Ocean Science, 2026)
The marine science community requires accurate, cost-effective, and reliable pH sensors capable of long-term, stable operations in-situ from coastal to deep-sea environments. Spectrophotometric pH sensors, based on lab-on-chip (LOC) technology, offer measurement frequencies of every 10 min and provide good performance (pH difference 0.02 RMSE) relative to validation samples with long-term use. However, for applications where higher-frequency measurements are important, this maximum sample rate may be limiting, in addition to the power requirements needed to operate the sensor. In contrast, commercially available pH optodes (PyroScience GmbH) are relatively inexpensive, consume little power and are contained within a comparatively small form-factor package, but with intense use the pH sensitive membrane can photo-oxidise, causing signal drift. The combination of LOC and optode technologies, however, can be used to provide long-term, high-frequency and high-stability in-situ pH data, but protocols to correct for sensor drift need to be developed and evaluated. To examine sensor drift and develop protocols to account for it, we suspended two LOC pH sensors with two pH optodes at 0.5 m depth from a floating pontoon within a harbour in Southampton, UK for six months (June–December 2023). This is a highly dynamic tidal environment with substantial biofouling. The optode (AquapHOx-L-pH, PyroScience GmbH) and an independent pH sensor (Deep SeapHOx V2, Sea-Bird Scientific) measured at a high frequency (e.g., ≤ 5 min) alongside a LOC pH sensor measuring at a lower frequency (e.g., ≤ 2 h). Triplicate lab validated co-samples were collected each week, in addition to dedicated sensors monitoring the temperature, salinity, dissolved oxygen and tidal height. We find good agreement, i.e., mean ΔpH = -0.022±0.023 between the SeapHOx and LOC sensors (3182 data points in common), in addition to individual performances of 0.02 RMSE relative to validation samples. As expected, we found significant signal drift (e.g., generally ≤ 0.012 pH d−1) and pH offsets (e.g., 0.1–0.2) with the optodes after intensive use in a high biofouling environment. However, by coupling LOC pH data to high frequency optode data, we corrected the optode signal drift/offset and achieved a similar field performance (∼ 0.02 RMSE relative to validation samples) as the SeapHOx sensor even when using ultra-low LOC pH sensor measurement frequencies (e.g., several days to weeks per LOC measurement). Overall, this work provides the oceanographic community with guidelines on how to achieve accurate, rapid, and long-term pH measurements, while also balancing power requirements, by combining two complementary pH sensing technologies.

Analytical approaches for wave energy dissipation induced by wave-generated turbulence and random wave-breaking - Yongzeng Yang; Fuwei Wang; Meng Sun; Xingjie Jiang; Xunqiang Yin; Yongfang Shi; Yong Teng (Ocean Science, 2026)
This paper is dedicated to investigate the dissipation effects of wave-generated turbulence reacting on ocean waves and to estimate the energy loss due to wave-breaking theoretically. An analytical dissipation source function induced by wave-generated turbulence in the water was proposed through the equilibrium solutions of a second-order turbulence closure model between the wave shear instability generations and the turbulent kinetic energy (TKE) dissipations under the well-founded structural equilibrium closure assumptions. And an improved postbreaking spectrum expression, based on the breaking wave statistical method, was presented to depict more explicitly the intermittent wave-breaking events. Comparisons of the TKE dissipation rates indicate that the model results agree well with the laboratory observations which were directly generated by the waves themselves, but for mechanical and wind waves, dispersions suggest the presence of other dynamic processes. The modeled attenuation coefficients of the breaking spectrum correspond to the decreasing tendency measured from the lake experiment, which yields valuable insights of the physics of dominant breaking, but the statistical approach is less well-suited for simulating the rapid transient regime of wave breaking as well as for the higher frequency in the equilibrium range. Evaluations for more complex situations will be addressed in the future series of papers.

Quantifying farmed kelp atmospheric CO 2 uptake and release through localized air-sea flux measurements in the Northern Gulf of Alaska - Josianne Haag; Cale A. Miller; Jonah Jossart; Amanda L. Kelley (Ocean Science, 2026)
The rapid growth of mariculture in the United States, particularly in Alaska, has ignited interest in the co-benefit of using farmed kelp as a mitigation strategy against anthropogenic carbon dioxide (CO2) released to the atmosphere. Here, we quantified the air-sea CO2 flux in two kelp farms in the Northern Gulf of Alaska with differing oceanographic conditions and farming practices to determine the carbon sequestration potential over the growing season. Sensors were deployed on two subsurface moorings placed in proximity of one another at each farm site: one “inside” and one “outside” as a reference upstream of the farm. Both sensor arrays conducted hourly measurements of pH or partial pressure of CO2 (pCO2), temperature, salinity, and oxygen during the time from seed line outplanting in winter (November to January) to spring harvest (April or May) in 2024. In Windy Bay, nominal differences in carbonate chemistry parameters were detected between the inside and outside moorings from November until March, at which time seawaterpCO2 within the farm decreased with respect to the reference site. In Kalsin Bay, the inside mooring displayed winter seawater pCO2 values 100 µatm greater than those recorded at the outside mooring, likely due to the colonization of a biofilm on the kelp lines while young sporophytes were not yet mature; however, seawater pCO2 values at both moorings converged in spring. Integrated over the entire deployment, one farm strengthened the bay's carbon influx and one farm reversed the natural sink by releasing carbon over the deployment period: 800.1 ± 145.8 mol m−2 in Kalsin Bay and -9.23±3.6 mol m−2 in Windy Bay. This study highlights the nuance of farmed kelp carbon capture by demonstrating that a farm site can influence overall air-sea CO2 flux, whereby farming structures create artificial habitat and that kelp farms are not always a net sink for atmospheric carbon.

Internal tides–cyclonic eddy interaction and intermodal energy pathways: evidence from 3 km NEMO-AMAZON36 simulations - Fabius Kouogang; Ariane Koch-Larrouy; Xavier Carton; Fernand Assene; Guillaume Morvan; Moacyr Araujo (Ocean Science, 2026)
The interaction between internal tides (ITs) and mesoscale features plays a key role in ocean energy dissipation. Understanding how IT energy is transformed in energetic western boundary regions remains a central challenge, particularly in regions of vigorous mesoscale activity. To this aim, we apply vertical mode decompositions to the high-resolution (3 km) simulations during September–December 2015. This study shows that the IT vertical mode and the precise point of IT-eddy encounter determine whether the IT energy propagates freely, deviates, or is trapped, and how topography and coherent eddies synergistically scatter energy between baroclinic modes off the Amazon shelf. Three representative interaction cases, each captured in a separated 25 h snapshot, were examined: undisturbed propagation until crossing the Ceará Rise seamount, interaction with a cyclonic eddy (CE) core, and interaction with a CE eastern edge. The principal findings establish two points. First, an IT response (propagation, deviation or scattering) is dually controlled by its vertical mode, and the mesoscale encounter location along with the associated background conditions (currents and stratification). In the absence of a strong eddy, the Mode-1 IT propagates as a coherent beam with a long propagation range (O(1100 km)). In the presence of a strong CE, however, the IT beams are disrupted, preventing sustained long-range transmission. Within the eddy core, the Mode-1 IT is coherently refracted northward (∼ 35° relative to its northeastward incident direction) while maintaining high energy fluxes exceeding 200 W m−1. At the CE edge, Mode-1 is diffracted into two distinct branches, with one propagating northward (∼ 39°) and the other eastward (∼ 35°). In contrast, the IT higher modes are highly susceptible to blocking and trapping: Mode-2 energy, despite initial amplitudes comparable to Mode-1, is strongly blocked at the CE-seamount interface, while Mode-3 remains weak (below 200 W m−1) and less propagative. Second, intermodal energy transfer is governed by a hierarchical synergy between the seamount and CE's background flow. In the absence of an eddy, the seamount drives a forward energy cascade (O(10-8Wmkg-1)) from the Mode-1 IT to higher modes. In contrast, in the presence of a CE, the CE's strong horizontal shear triggers a competing inverse energy cascade (O(10-8Wmkg-1)) from the background flow to the IT modes. This interaction is critical for the extreme damping of Mode-2 and explains the observed redistribution of energy fluxes. These results provide mechanistic insight into the fate of IT energy in complex oceanic environments and advance understanding of multi-scale ocean dynamics.

Cuban coral traces annual hydrologically driven variability in δ 234 U values since the end of the Little Ice Age - Sahra Greve; Norbert Frank; Paolo Montagna; Carlos Manuel Alonso-Hernández; Miguel Gomez-Batista; Eric Douville; Sophie Warken (Ocean Science, 2026)
The natural uranium isotope ratio of 234U / 238U in seawater behaves conservatively at basin scale, yet it can be regionally affected by continental freshwater discharge at decadal to centennial timescales. Here, we analyse annual variations in the 234U / 238U isotope ratio, expressed as ‰-deviation from radioactive equilibrium as δ234U, of a coral from Cuba. Over the past 237 years, the mean δ234U value of the coral was 145.7±0.1 ‰ (1σM), which is identical to that of modern open ocean seawater, whereas the average variation over the past century has been ±3.7 ‰. This moderate variability is, however, significantly greater than the external precision and reproducibility of measurements of ±0.55 ‰ (2σM). Moreover, the δ234U values coincide inversely with regional precipitation, suggesting excess 234U contribution from regional freshwater runoff. The most important finding is a strong increase in annual δ234U variability to ±8.1 ‰ during the end of the Little Ice Age (LIA, 1778–1847). We suggest that the increased δ234U dynamics reflect substantial excess 234U contributions from the Mississippi, far greater variability in the local freshwater fluxes to the Gulf of Mexico, and/or reduced advective currents during the LIA. This study demonstrates that yet unexplored variability in coral δ234U records within the presently known range of seawater δ234U may be attributed to local and advected freshwater sources, which opens a new pathway for reconstructing these processes over time. Moreover, it places strong constraints on the initial δ234U variability of fossil corals in light of ultra high-precision 230Th/U dating.

High-latitude eddy statistics from SWOT compared with in situ observations - Charly de Marez; Arne Bendinger; Ahmad Fehmi Dilmahamod (Ocean Science, 2026)
Mesoscale eddies play a key role in the transport of heat, salt, and momentum in the ocean, yet their statistical characterization at high latitudes has remained elusive due to the coarse resolution of conventional satellite altimetry. Here we present a statistical description of the mesoscale eddies in the Labrador Sea at an unprecedented resolution, using observations from the Surface Water and Ocean Topography (SWOT) mission, significantly extending previous estimates derived from lower-resolution altimetry products. We apply an eddy-detection algorithm directly to the native 2 km SWOT swaths, without gridding or assimilation, and validate the detections against in situ measurements from shipboard current profiler data from one cruise in 2024, as well as against a statistically derived shipboard current-profiler-based eddy census. The comparison demonstrates good agreement in eddy size and intensity, confirming SWOT's ability to resolve high-latitude mesoscale structures previously undetectable or distorted in gridded altimetry. The SWOT-derived eddy census based on two full calendar years reveals a predominance of energetic anticyclones (Irminger Rings) in the basin interior and smaller cyclones along the continental slopes, with clear seasonal variability linked to boundary current instability. These findings provide the first observational benchmark for mesoscale activity in the Labrador Sea and illustrate SWOT’s potential to extend eddy statistics to high-latitude and ice-influenced regions, opening the way for a global assessment of mesoscale variability.

Determination of the height of deep-sea mooring lines above seafloor using turbulence measurements - Hans van Haren (Ocean Science, 2026)
Height variations O(1) m of closely spaced moored oceanographic instrumentation are difficult to measure in the deep sea, requiring high-accuracy pressure sensors preferably on all instruments in a mooring-array. In this paper, an alternative method for relative height determination is presented using 2 m spaced high-resolution temperature sensors moored on multiple 9.5 m-spaced lines in the deep Western Mediterranean Sea. While it was anticipated that height variations between lines could be detected under near-homogeneous conditions via adiabatic lapse rate O(10−4 °C m−1) by the 3 × 10−5 °C-noise-level sensors, such was prevented by the impossibility of properly correcting for short-term bias due to electronic drift. Instead, a satisfactory height determination was achieved during a period of relatively strong stratification and large turbulence activity. By band-pass filtering data of the highest-resolved turbulent motions across the strongest temperature gradient, significant height variations were detectable to within ±0.2 m.

Climatology and annual cycle of global ocean dissolved oxygen represented by multiple observational gridded products - Juan Du; Lijing Cheng; Takamitsu Ito; Hernan E. Garcia; Zhankun Wang; Jonathan D. Sharp; Christopher J. Roach; Shoshiro Minobe; Yuntao Zhou; Bin Lu; Gian Giacomo Navarra; Seth M. Bushinsky (Ocean Science, 2026)
Ocean dissolved oxygen (O2) is an essential climate variable crucial for sustaining the marine life; thus, changes of O2 at various spatiotemporal scales should be quantified and understood. Here, we study the climatology and annual cycle of O2 at regional to global scales using eight available gridded observational products. These datasets are generated by different groups using different primary data selection, quality control, bias correction, and interpolation methods, including statistical and machine-learning-based mapping methods. A common set of metrics was collaboratively developed by the community of the Gridded Observational Dataset Intercomparison Project-Dissolved Oxygen (GODIP-DO) to facilitate the inter-comparison. We find that global mean O2 profiles are consistent among all products (±3 µmol kg−1), with the well-established decrease from high surface values to a minimum ∼ 1000 m, and subsequent increase to higher O2 at depth, although local differences could reach ±25 µmol kg−1 (0–1000 m). The hemispheric O2 annual cycle correlates strongly with ocean temperature changes, suggesting the key driver of temperature for the O2 annual cycle. However, there is substantial variation in the global mean 0–100 m O2 annual cycle, the magnitude ranges from −1 to 0.8 µmol kg−1, with a standard deviation of the datasets of ∼ 0.3 µmol kg−1. Average oxygen minimum zones (OMZ) volume among the products is 80.92 × 106 km3 (±1.95 %) for a 60 µmol kg−1 threshold and 152.00 × 106 km3 (±1.72 %) for a 90 µmol kg−1 threshold. Our results help to depict and understand the spread among the available O2 gridded datasets.

Modelling primary production: multitude of theories, or multitude of languages? - Jozef Skákala; Shubha Sathyendranath; Yuri Artioli; Deep S. Banerjee; Heather Bouman; Robert J. W. Brewin; Momme Butenschön; Stefano Ciavatta; Stephanie Dutkiewicz; Yanna Fidai; David Ford; Grinson George; et al. (Ocean Science, 2026)
Marine primary production, converting approximately 50 Gt of inorganic carbon into organic carbon per year, is an important component of the global carbon cycle, and a major determinant of past, present and future climate. Large-scale, long-term estimates of marine primary production rely primarily on two types of models: satellite-based models that make extensive use of remote-sensing data, and ecosystem models providing numerical simulation of ecological processes embedded in general ocean circulation models. Intercomparison exercises of model outputs (both within and across the two model types) have consistently revealed high discrepancies between estimated global ocean primary production, including divergent magnitudes and even opposite trends. Model-observation comparisons are also complex, because paucity of data, differences in measurement techniques, and evolving methodologies could all lead to difficulties with the interpretation of results. These uncertainties limit the applications of primary production models (both satellite-based and ecosystem), especially in the climate context, where an important question is whether climate change will drive significant future changes in regional or global primary production. Both satellite-based and ecosystem models rely on a range of fixed model parameters, whose values need to be carefully estimated and tested. In this paper, we suggest that such model parameters represent an underappreciated but important source of inter-model differences. With the proliferation of both satellite and in situ observations of relevant variables at global scales, and the availability of powerful statistical tools such as data assimilation and machine learning, we argue that time is right to systematically examine model parameters, gaining both better insights into parameter values and how those values might vary in space and time. We argue that such spatio-temporal parameter variability can be theoretically justified for ecosystem models with complexity similar to those commonly used within Earth System Models (ESMs) in climate studies. The spatially and temporally varying parameter values could serve to unify models that are structurally different. An important aspect of this unification could be the ability to infer the spatio-temporal variability of parameters in the less complex models from the emergent behaviour of the more complex ones. This could include ecosystem model simulations of nutrients, temperature, phytoplankton classes, or vertical distributions informing satellite-based models. We conclude that better understanding of model parameter roles and integration (or inter-calibration) of different types of models could reduce discrepancies among the primary production models and improve the reliability of marine primary production projections.

The Scotland–Canada overturning array (SCOTIA): twenty years of meridional overturning in the subpolar North Atlantic - Alan D. Fox; Neil J. Fraser; Kristin Burmeister; Sam C. Jones; Stuart A. Cunningham; Lewis A. Drysdale; Ahmad Fehmi Dilmahamod; Johannes Karstensen (Ocean Science, 2026)
The Atlantic meridional overturning circulation (AMOC) is expected to decline dramatically over the 21st century, with severe impacts for Northern Hemisphere climate. After 20 years of sustained monitoring in the subtropics, a detectable AMOC weakening trend is now beginning to emerge. However, continuous observations at subpolar latitudes are currently too short-lived to determine any weakening signal above the large-amplitude interannual variability. Here, we introduce a new subpolar observing configuration, SCOTIA (Scotland–Canada overturning array), combining parts of the existing OSNAP mooring array with scattered CTD and Argo data, to extend the record of subpolar AMOC backward in time to cover the subtropical monitoring period, 2004–2024. SCOTIA facilitates a rigorous comparison of the decadal-scale variability in transports and overturning at subpolar and subtropical latitudes. Our results show subpolar AMOC varies on pentadal to decadal timescales with an amplitude comparable to that observed in the subtropics. Anomalously high overturning during 2016–2020 was driven by increased southward transports in the density classes associated with Labrador Sea Water. We find no statistically significant trend in subpolar AMOC during the period 2004–2024.

Opinion: status, plans and needs of Southern Ocean modelling - Torge Martin; Carolina O. Dufour; Andrew J. S. Meijers; Alyce M. Hancock (Ocean Science, 2026)
In preparation for the SOOS/OCEAN:ICE Workshop on ice-ocean observation harmonization and future priorities agenda, a survey targeting the modelling community was conducted to assess research priorities for the Southern Ocean and Antarctica. While this initiative was tailored mostly towards physical ocean and ice modelling, its outcome specifically supports the design of field activities from the open Southern Ocean to the Antarctic shelf for the forthcoming Antarctica InSync campaign and is aligned with broader strategic planning efforts ahead of the next International Polar Year. The survey results are a useful basis to further communication between modeling and observing science communities. We believe this is crucial for optimizing campaign planning, achieving enhanced data usage and improving numerical experiments.

Sea-surface temperature variability and climate drivers in Cuba's Jardines de la Reina National Park (2003–2022) - Maibelin Castillo-Alvarez; Oscar Pizarro; Alain Muñoz-Caravaca; Iván Pérez-Santos; David Carrasco; David Francisco Bustos-Usta; Laura Castellanos-Torres (Ocean Science, 2026)
Coral reef systems on the southeastern Cuban shelf are exposed to rapid warming and increasingly frequent marine heatwaves (MHWs). However, the physical drivers of local sea-surface temperature (SST) variability remain poorly quantified. The present study examines seasonal-to-decadal SST variability in and around the Jardines de la Reina National Park (JRNP) and investigates the extent to which atmospheric–ocean processes and large-scale climate modes influence that variability. The study analyses daily 1 km Multi-Scale Ultra High Resolution (MUR) SST from 2003–2022, in conjunction with ERA5 surface heat fluxes and GLORYS12 mixed-layer fields. A mixed-layer heat budget, compiled from daily values and averaged to monthly values, is used to attribute the seasonal cycle; long-term trends, MHWs and modes of variability (Empirical Orthogonal Funtions – EOFs) can be used to explain interannual to decadal changes and their links to El Niño Southern Oscillation (ENSO), the Western Hemisphere Warm Pool (WHWP), the Tropical North Atlantic (TNA) and the North Atlantic Oscillation (NAO). Net air–sea heat exchange sets the seasonal evolution of SST, whereby horizontal advection provides a smaller modulation near the shelf break; a characteristic ∼2-month lead of heat flux over temperature is consistent with mixed-layer heat storage. This thermodynamic control explains a marked autumn–winter shelf–offshore contrast between the shallow gulfs (Gulfs of Ana María – GAM and Guacanayabo – GG) and the adjacent Caribbean Sea. Superimposed over this area is a warming trend of ∼0.28 °C per decade (strongest in winter/transition months, peaking around April at ∼0.48 °C and November at ∼0.35 °C per decade) and a step-like shift in 2011–2013 towards a persistently warmer state. MHWs intensified during the second decade; the mean event-wise maximum intensity was higher inside GAM, while upper categories occurred more frequently offshore. EOF1 (87.5 %) is a basin-wide mode linked on an interannual basis to ENSO/WHWP and latent-heat flux and at low frequency to the NAO, while EOF2 (6.2 %) captures a shelf–offshore dipole related to TNA. Our results provide a physical basis for which to issue early warnings from forecasts of net heat flux and mixed-layer depth, thus encouraging the use of regional high-resolution modeling and targeted observations. Key limitations to the present study include a 20-year MHW baseline and an under-resolution of currents in highly shallow and complex bathymetry.

Transient tracer observations in the Gulf of St. Lawrence reveal shift from younger to older inflow waters - Lennart Gerke; Toste Tanhua; William A. Nesbitt; Samuel W. Stevens; Douglas W. R. Wallace (Ocean Science, 2026)
The deep waters of the Gulf of St. Lawrence (GSL) have experienced a significant reduction in dissolved oxygen content during the past decades. One widely documented driver of this deoxygenation is a change in the composition of the deep inflowing water that ventilates the Gulf, which consists of a mixture of warmer, less-oxygenated North Atlantic Central Waters (NACW) and cooler, more-oxygenated Labrador Current Waters (LCW). While previous studies have inferred changes in this inflow from hydrographic and biogeochemical observations, direct constraints on deep-water ventilation timescales have been lacking. Here, we present the first spatially-comprehensive measurements of transient tracers (SF6 and CFC-12) in the GSL, allowing direct estimates of tracer-derived mean ages and ventilation timescales throughout the deep layer. Mean ages increase systematically with density and exhibit a pronounced along-channel gradient, with younger waters (30–40 years) in the Lower St. Lawrence Estuary and older waters (50–60 years) toward Cabot Strait. This west-east increase in ventilation age is more gradual than corresponding changes in temperature and salinity, indicating that tracer-derived ages respond differently to variations in source water composition than hydrographic properties. The observed gradual age structure is consistent with a greater influence of older, earlier ventilated NACW in the eastern Gulf, while younger ages inland reflect a continued contribution of LCW. These results demonstrate that, as of 2022, the shift toward longer deep-water ventilation timescales, and associated changes in inflow properties toward higher NACW composition, is ongoing rather than having reached a steady end state, contrary to earlier predictions.

A method for quantifying correlation in the shape of oceanographic profile data - Mark Taylor; Stephanie Henson (Ocean Science, 2026)
Vertical profiles are a common type of oceanographic observation, involving measurements of a variable across a range of depths, and are widely used to identify physical and biogeochemical features of the water column. Recent studies have shown that oceanographic profiles can be represented as functional data objects, where each profile is treated as a single datum and expressed as a function of pressure. This study applies a recently developed technique, which defines a scalar correlation coefficient for functional data, to the analysis of oceanographic profiles. The method represents each profile using basis functions, whose associated weightings are termed basis coefficients, and quantifies dependence through the variability of these coefficients. An important advantage of this method is that the resulting correlation coefficient reflects similarities in overall profile shape, not just correlations between values at specific depths. Two applications of this method are explored: calculating the correlation coefficient between two different oceanographic variables, and estimating the temporal autocorrelation function of a single variable. Each application is demonstrated using two case study datasets: (1) the Coastal Endurance Washington Offshore Profiler Mooring and (2) biogeochemical-Argo floats. The first case study demonstrates how the method can be used to identify physical drivers of variability in biogeochemical profile structure. The second case study reveals regional differences in relationships between profiled variables and their temporal autocorrelation characteristics. This technique has broad potential for application to data from moorings, autonomous platforms, and ocean models, with possible use in observing system optimisation, data assimilation, and the analysis of vertically structured ocean processes.

Dynamic and steric sea-level changes due to a collapsing AMOC in the Community Earth System Model - René M. van Westen; Caroline A. Katsman; Dewi Le Bars (Ocean Science, 2026)
A collapse of the Atlantic Meridional Overturning Circulation (AMOC) leads to a redistribution of dynamic sea level (DSL) across the global ocean surface. Here, we investigate DSL and steric sea-level responses under different AMOC strengths using the Community Earth System Model and two stand-alone ocean configurations (strongly eddying and parameterising eddy effects) with the Parallel Ocean Program. For our analysis, we employ various quasi-equilibrium freshwater hosing experiments in which AMOC collapses were reported. As the AMOC begins to collapse, the DSL substantially rises over the Atlantic Ocean and Arctic Ocean. Regions outside the Atlantic basin display a relatively small DSL drop. The largest DSL trends are found over the North Atlantic Ocean and reach +6 mm yr−1 over a 100-year period, with DSL trends near densely-populated coastal regions of up to +4 mm yr−1. This is a considerable contribution to local sea-level rise compared to the observed global mean sea-level rise of +3.3 mm yr−1. DSL trends obtained from the quasi-equilibrium experiments include a contribution from the freshwater hosing itself (≈ +0.2 mm yr−1), which is typically a factor 10–20 smaller than DSL response during the AMOC collapse. Moreover, a collapsed AMOC increases the net oceanic heat uptake leading to more than 50 cm of global mean thermosteric sea-level rise on millennial timescales (> 2000 years). These results highlight the potential value of accounting for an AMOC collapse scenario when developing or applying sea-level rise projections for the North Atlantic Ocean.

Marine heatwaves across the central South Pacific: characteristics, mechanisms, and modulation by El Niño Southern Oscillation - Bastien Pagli; Takeshi Izumo; Alexandre Barboni; Carla Chevillard; Cyril Dutheil; Raphaël Legrand; Christophe Menkes; Claire Rocuet; Sophie Cravatte (Ocean Science, 2026)
Marine heatwaves (MHWs) are intensifying with climate change, endangering ecosystems such as coral reefs. Yet their regional characteristics and drivers remain poorly understood in many parts of the Pacific. Here we provide a comprehensive assessment of MHWs in the central South Pacific and across the five archipelagos of French Polynesia (FP; representing more than 5 million km2 of maritime area, a region as vast as Europe), using sea surface temperature observations and an ocean reanalysis to investigate underlying mechanisms. MHW characteristics vary widely across the region: its northern and southern parts (the Marquesas and Austral archipelagos, respectively) experience the highest number of MHW days and the strongest cumulative intensities, especially during the warm season (November–April). In contrast, its central part (the Society, Tuamotu, and Gambier Islands) exhibits more moderate MHW characteristics. Heat budget analyses highlight the seasonally and regionally diverse mechanisms shaping MHWs. In central FP during the warm season (austral summer), most MHWs are driven by air–sea heat fluxes, while in the northern part, those driven by oceanic horizontal advection dominate. During the cold season (austral winter), more MHWs driven by horizontal advection are observed in the whole region since the thicker seasonal mixed layer reduces the proportion of MHWs driven by air–sea fluxes. El Niño–Southern Oscillation (ENSO) strongly modulates MHW occurrences: El Niño favors MHW occurrences in northeastern FP, while La Niña increases MHW occurrence in the southwest with different spatial extent depending on ENSO flavors (Central or Eastern Pacific ENSO events). This modulation arises from reduced wind-evaporation cooling with reduced wind speed, shoaled mixed layers, and enhanced horizontal heat advection, occurring primarily to the northeast of French Polynesia during El Niño and to the southwest during La Niña. These results greatly improve our understanding of MHW characteristics, dynamics and variability in this ecologically-fragile region.

The impact of suspended sediments on exchange flow in a macrotidal, hyperturbid estuary - Cristian M. Rojas; Lauren Ross; Betty John Kaimathuruthy; Isabel Jalón-Rojas; Aldo Sottolichio; Nicolas Huybrechts (Ocean Science, 2026)
This study investigates the sensitivity of the total exchange flow (TEF) with regard to suspended sediment concentrations (SSC) in water density calculations. We consider two scenarios of an idealized model based on a macrotidal, highly turbid estuary: one where density depends on both salinity and SSC and another where density depends only on salinity. These models are used to understand how TEF volume inflow (Qin) varies along the estuary over fortnightly and monthly time scales. The bulk values of TEF inflow between the two model scenarios show small differences (less than 4 %) over 5 months. However, differences in TEF bulk volume inflow reach 7 % downstream of the estuarine turbidity maximum (ETM), 22 % at the ETM, and nearly 70 % upstream of the ETM when quantified only over spring tides. The gradient Richardson number indicates that sediment-induced stratification suppresses shear production ∼ 30 km further upstream when suspended sediments are an active tracer. The most profound impact on TEF occurs during spring tides at, and upstream of, the ETM where sediment contributions to density exceed those from salinity. The sediment contribution leads to increased stratification in the upper reaches of the estuary, effectively reducing the longitudinal density gradient and therefore the exchange flow as much as 2000 m3 s−1, indicating a dominant role of suspended sediments in attenuating exchange in hyperturbid estuaries. The results from this work highlight the importance of SSC on density calculations in estuarine studies of the exchange flow.

Physical and biological processes driving seasonal variability of Nitrate budget and biological productivity in the Gabon-Congo upwelling system - Landry Junior Mbang Essome; Gaël Alory; Casimir Yelognissé Da-Allada; Isabelle Dadou; Roy Dorgeless Ngakala; Guillaume Morvan (Ocean Science, 2026)
The Gabon-Congo upwelling system, located in the southeastern Gulf of Guinea, is a highly productive marine ecosystem influenced by both local and remote physical forcing. This study investigates the seasonal variability of the nitrate budget and biological productivity in this region using a high-resolution (1/36°) coupled physical-biogeochemical simulation with the NEMO-PISCES model. The analysis highlights the relative contributions of physical and biological processes in modulating nitrate concentrations in both the mixed layer and the euphotic zone. Results reveal a semi-annual cycle of nitrate, with two upwelling periods (May–August and December) and two downwelling periods (January–April and October–November). These cycles are primarily driven by the passage of coastal trapped waves (CTWs) forced by equatorial Kelvin waves, inducing vertical thermocline displacements and regulating nitrate availability in the euphotic zone. The nitrate budget analysis shows that the vertical diffusion linked to internal tide and local wind, and vertical advection linked to the CTWs, are the dominant process supplying nitrate to the mixed layer during the main upwelling season. However, near the Congo River mouth (5.5–6° S), the horizontal advection plays a key role, supplying significant amounts of nitrate through the river plume. In the lower euphotic layer, the vertical mixing contributes to the nitrate loss during the upwelling but becomes a source of nitrate during the downwelling periods. The seasonal cycle of the chlorophyll a (CHLa) concentration follows that of nitrate, confirming that the primary production in this region is mainly driven by nitrate availability. The study also highlights the role of the Angola Current in transporting low-nitrate waters from the Equatorial Undercurrent, which influences the nitrate and CHLa balance in the Gabon-Congo upwelling system. These findings provide new insights into the mechanisms governing nutrient dynamics and biological productivity in the Gabon-Congo upwelling system. Understanding these processes is crucial for assessing the impact of climate variability on the regional marine ecosystems and fisheries.

Seasonal impact of submesoscale eddies on the ocean heat budget near the sea ice edge - Lily Greig; David Ferreira (Ocean Science, 2026)
Oceanic submesoscale mixed layer eddies (SMLEs), with horizontal scales of 0.1–10 km, are not captured in climate models. SMLEs energized in the marginal ice zone (MIZ) have been shown to be of importance to sea ice melt rates in summer and to sea ice transport through notably a dynamical coupling with sea ice. Here our focus is on the thermodynamical coupling, which has received comparatively little attention. We aim to quantify, for the first time, the impact of eddies on both sea ice and the heat budget in the MIZ, contrasting different seasons and different background stratifications. To this end, we set up SMLE-resolving simulations of the ocean mixed layer (ML) near the ice edge using the MITgcm, representing a lead or the MIZ. We isolate the effect of eddies by comparing 3D simulations with eddies to 2D (latitude-depth) simulations without eddies. In summer (i.e., melting conditions) and regardless of the background stratification, SMLEs act as a heat pump from the atmosphere over the open ocean to the sea ice. On average over a season, SMLEs triple the meridional heat transport to the ice covered region, increase melting over their meridional extent, and trigger a positive radiative feedback by increasing shortwave absorption over the thinner ice. These changes are in the range 20 %–60 % for reasonable choices of shortwave forcing and initial ice thickness. In winter (i.e., freezing conditions), SMLEs have a relatively small impact on sea ice growth due to compensation between vertical and horizontal eddy heat transports. However, they reduce ML deepening by 80 %/50 % in the open/ice-covered ocean. Overall, our results reveal up to order one impacts of SMLEs on the heat and sea ice budgets in the MIZ, which will require the development of a SMLE parameterization tailored for polar regions.

Mechanism of storm surge induced by low-pressure systems along the northern coast of Kyushu, Japan - Yoshihiko Ide; Shinichiro Ozaki; Masaru Yamashiro (Ocean Science, 2026)
This study aims to clarify the mechanism of storm surge caused by low-pressure systems, which differs from typical storm surges associated with typhoons and has been prominently observed along the northern coast of Kyushu, Japan. An integrated approach was employed by combining the analysis of observational data with numerical simulations using an ocean circulation model. First, based on tide gauge and wind data collected at multiple locations along the Northern Coast of Kyushu, the temporal relationship between storm surge anomalies and wind variations was evaluated. A strong correlation was identified between the rotational component of wind direction and the amplitude of the anomalies. Subsequently, numerical simulations using realistic coastal topography successfully captured the characteristics of coastal wave propagation (i.e. coastal-trapped gravity waves) generated by wind rotation and coastal geometry, and accurately reproduced both the timing and amplitude of the storm surge anomalies in the gauge. Furthermore, numerical experiments using idealized topographic meshes and simplified wind conditions quantitatively assessed how the rotation period of wind and the presence or absence of topographic features affect the resonant amplification or suppression of storm surges. In particular, it was revealed that the Goto Islands contribute to the earlier formation and amplification of storm surges, while the Korean Peninsula suppresses wave amplitude by limiting the spatial extent of wave propagation. These findings contribute to a better understanding of storm surge mechanisms, not only along the northern coast of Kyushu, but also in other regions around the world with similar geographic settings.

Sensitivity of marine heatwaves metrics to SST products, focusing on the Tropical Pacific - Carla Chevillard; Romain Le Gendre; Christophe Menkes; Takeshi Izumo; Bastien Pagli; Simon Van Wynsberge; Sophie Cravatte (Ocean Science, 2026)
Marine heatwaves (MHWs) are increasingly studied in climate sciences for their ecological impacts, for which accurate real-time bulletins and forecasts are essential. Yet, methodological choices in their detection affect metric estimates, underlining the need to better assess these sensitivities. This study provides a thorough assessment of the impact of sea surface temperature (SST) product choice on MHW statistics, focusing on the tropical Pacific. MHW detection was performed on six daily gridded SST datasets: four widely used blended satellite observational products, one ocean reanalysis, and a multi-dataset ensemble mean computed from the four observational products. Sensitivity to SST products was evaluated for six MHW metrics (MHW days per year, number of events per year, duration, maximum intensity, cumulative intensity and onset rate) and for the degree heating weeks (DHW), a widely used index for coral bleaching risk. Inter-product comparisons revealed a significant dispersion among MHW metric estimates. The reanalysis GLORYS12v1 detected fewer, longer and less intense MHWs while OISST detected more MHWs of shorter duration and higher intensity. likely related to the respectively weak and strong high-frequency SST variability (periods shorter than 2 weeks) of the two products. The sensitivity analysis showed that the onset rate was the most sensitive metric to SST product choice and the maximum intensity the most robust one. Metrics uncertainties were quantified inside seven regions of the basin and were largest in the western Pacific Warm Pool. Co-occurrence analyses of MHWs revealed that, over the basin, 10 % to 80 % of MHW days were detected simultaneously by all products, with the western Pacific Warm Pool showing the lowest agreement (10 %–40 %). Filtering MHWs by size also revealed that the detection of large-scale MHWs (>5°lon×5°lat) was more consistent across products than smaller-scale ones. Finally, over the studied period, inter-product differences tended to decrease with time. The DHW also revealed to be sensitive to SST products, with inter-product differences on DHW annual maximum reaching more than 1 °C weeks and percentages of bleaching alert days (DHW≥4 °C weeks) in common across products reaching 70 % at most across much of the basin. These findings contribute to a better understanding of how SST product choice affects the characterization of MHWs and DHW, and their associated uncertainties.

Nordic overturning increases as AMOC weakens in response to global warming - Sasha Roewer; Lukas Fiedler; Marius Årthun; Willem Huiskamp; Stefan Rahmstorf (Ocean Science, 2026)
The Atlantic Meridional Overturning Circulation (AMOC) is weakening in response to global warming, while the Nordic Seas Overturning Circulation (NOC) is projected to strengthen. So far, the causal link between these two opposing trends has been little explored. Using an idealized box model of the Atlantic Ocean and the Nordic Seas and a high resolution climate model, we propose that a density reduction in the subpolar North Atlantic might weaken the AMOC by reducing the density difference with lighter waters further south, while at the same time strengthening the NOC by increasing the density difference with the heavier waters further north. The box model shows that the NOC initially increases moderately as the AMOC weakens in response to combined global warming and freshwater input, while a tipping point may be reached later if deep convection in the Nordic Seas shuts down and the NOC collapses together with the AMOC. These results are supported by GCM simulations.

Temperature-based diagnosis of the Gulf Stream path overestimates its northward shift in a warming ocean - Lina Garcia-Suarez; Katja Fennel; Neha Mehendale; Tronje Kemena; David P. Keller (Ocean Science, 2026)
Temperature-based criteria for diagnosing the location of the Gulf Stream (GS) are widely used and suggest a northward shift of the GS is underway. The apparent northward shift in the GS path has been causally linked to the recent rapid warming in the northwest Atlantic Ocean. This study uses high-resolution climate models to show that temperature-based criteria for diagnosing the GS path overestimate its northward shift in warming scenarios and, for one of the models, indicate a northward shift that does not occur. In contrast, a sea surface height (SSH)-based criterion for diagnosing the GS path remains closely aligned with the true GS core in a warming ocean and thus provides a more reliable estimate of shifts in its path. The temperature-based assessments are biased by the rising background temperature in the ocean, thus creating a misleading indication of a GS migration. These results call into question the notion that warming in the northwest North Atlantic is causally related to a northward migration of the GS and emphasize the need for more robust indicators of its position.

Year-round characterization of upwelling along the Uruguayan coast using a high-resolution regional ocean model - Camila de Mello; Marcelo Barreiro; Madeleine Renom (Ocean Science, 2026)
Upwelling along the Uruguayan coast has largely been interpreted as a summertime process, primarily based on surface temperature signals. Here, we investigate coastal upwelling year-round using a high-resolution regional ocean model (Coastal and Regional Ocean Community, CROCO), with the aim of reassessing how upwelling manifests across seasons under contrasting thermohaline conditions. The seasonal covariability between daily winds and sea surface salinity anomalies was analyzed through a Maximum Covariance Analysis, revealing upwelling signatures during spring, autumn, and summer, marked by positive sea surface salinity anomalies (SSSa) along the Uruguayan coast. Summer upwelling events were characterized by positive SSSa and negative sea surface temperature anomalies (SSTa), indicating that both are reliable upwelling proxies during this season. In contrast, during spring and autumn, SSSa resulted in a better proxy. In this regard, upwellings were clearly marked by increases in SSSa, but the SSTa pattern in the coast was variable due to distinct vertical temperature stratification patterns. Based on the thermal properties of the upwelled waters, three upwelling regimes can be identified: cold-water upwellings associated with warm-season stratified conditions, transitional upwellings occurring under weak or absent thermal stratification, and warm-water upwellings linked to cold-season conditions, characterized by inverse thermal stratification. Overall, our results highlight that upwelling events along the Uruguayan coast cannot be described by a single thermal signature, but rather by seasonally dependent thermohaline regimes.

Distributed optical fibre sensing in physical oceanography: emergence and future prospects - Alberto C. Naveira Garabato; Carl P. Spingys; Andrew J. Lucas; Tiago S. Dotto; Christian T. Wild; Scott W. Tyler; Ted A. Scambos; Christopher B. Kratt; Giuseppe Cappelli; Ethan F. Williams; Mariona Claret; Hannah E. Glover; et al. (Ocean Science, 2026)
Distributed optical fibre sensing (DOFS) is a technology that enables continuous, real-time measurements of a range of environmental parameters along the length of a fibre optic cable. In this article, we review the recently emerged applications of DOFS techniques in physical oceanography and offer a perspective on the technology's potential for future growth within the field. The introduction of DOFS to physical oceanography is contextualised with a brief history of the technology, which spun off primarily from the use of seafloor-laid optical fibres for telecommunications purposes. The key components and underpinning physics of a DOFS system are outlined and, on their basis, the suite of variables that are observable with DOFS are discussed. The implementation factors of DOFS, which include trade-offs between measurement accuracy and spatio-temporal resolutions and ranges, are also examined. The physical oceanographic applications of DOFS to date are then illustrated with case examples of four distinct DOFS techniques: distributed temperature sensing (DTS), which can provide ocean temperature observations; distributed static strain sensing (DSS) and distributed acoustic sensing (DAS), which are sensitive to temperature, cable strain and strain-associated variables, such as pressure and ocean velocity; and ultra-long-range observations of ocean currents with optical interferometry. The forthcoming prospects of DOFS in physical oceanography are considered, and are proposed to include new fibre optic-based approaches to sense ocean salinity and measure through the water column. We conclude with reflections on the future role of DOFS within the Global Ocean Observing System, and highlight the opportunities provided by the existing world-wide network of seafloor-laid optical fibres.

Coastal circulation and dispersion of passive tracers in the Red River plume region: unveiling seasonal- and intra-seasonal variability - Thanh Huyen Tran; Alexei Sentchev; Dylan Dumas; Charles-Antoine Guerin; Sylvain Ouillon; Kim Cuong Nguyen (Ocean Science, 2026)
The Red River (RR) plume region of the Gulf of Tonkin (GoT) plays an important role in driving coastal dynamic variability and regulating sediment and nutrient transport and budgets, and is therefore vital for coastal ecosystems and maritime activities. High-frequency radar (HFR) surface current measurements were used to characterize surface circulation and assess passive-tracer dispersion from August to December 2024, improving understanding of particle transport, dispersal, and fate in this region. The coastal circulation in the region, found to be strongly influenced by winds, tidal forcing, riverine input, and coastal bathymetry, exhibited a large spatio-temporal variability during the analysis period with the occurrence of small-scale structures, i.e., submesoscale eddies. The dispersion under varying forcing conditions and an extreme event – the typhoon Yagi – was analyzed by particle tracking and Lagrangian diagnostics. The results revealed that the dispersion within the RR plume region predominantly approached a Richardson super-diffusive regime after 24 h of tracking. Under the influence of typhoon Yagi, the dispersion was approximated by a ballistic regime after 12 h of tracking, with the spreading rate 10 times faster than that during normal conditions. In addition, the presence of Lagrangian Coherent Structures (LCSs), i.e., eddies next to the river outflow jets, coastal plume fronts, and zones of surface current convergence and divergence in the vicinity of river outlets, significantly influenced the dispersion behavior of tracers in the RR plume region. Overall, this study provides new insights into how coastal circulation and material dispersal in the RR plume respond across a wide range of weather conditions, including extreme events.

Polarity and direction dependence of energetic cross-frontal eddy transport in the Southern Ocean's Pacific sector - Huimin Wang; Lingqiao Cheng; Erik Behrens; Zhuang Chen; Jennifer Devine; Guoping Zhu (Ocean Science, 2026)
Mesoscale eddies play a critical role in mediating meridional transport across the Antarctic Circumpolar Current (ACC), yet the dynamics of cross-frontal eddies (CFEs) and their energy exchanges with frontal jets remain inadequately quantified. This study presents a systematic analysis of CFE characteristics, kinetic energy evolution, and thermohaline transport effects in the Pacific sector of the Southern Ocean, utilizing 23 years (2000–2022) of satellite altimetry and Argo float data. Our results reveal a fundamental polarity- and direction-dependent asymmetry in CFE dynamics. Equatorward-propagating cyclonic eddies (CEs) dominate CFE activity, followed by poleward-moving anticyclonic eddies (AEs). These dominant CFE types exhibit superior energetic characteristics, including significantly higher eddy kinetic energy (EKE) and stronger nonlinearity compared to their counterparts. Complete CFEs experience polarity- and direction-selective energy gains during frontal crossing, with equatorward CEs and poleward AEs extracting energy from eastward frontal jets, while their counterparts lose energy. This energization mechanism has intensified over the past two decades, with both polarity CFEs showing substantial EKE increases that substantially exceed previous basin-scale estimates. Hydrographic analysis demonstrates that CEs and AEs transport distinct water masses across frontal boundaries, creating sharp thermohaline contrasts within interfrontal zones. Our findings establish CFEs as crucial regulators that buffer wind- and warming-induced baroclinicity increases through compensatory heat transport, thereby maintaining the Southern Ocean's thermal equilibrium and modulating the ACC's response to external forcing in a changing climate.

Increased ocean heat transport to the central Arctic despite a well working Barents Sea Cooling Machine - Shaun A. Eisner; James A. Carton; Leon Chafik; Lars H. Smedsrud (Ocean Science, 2026)
The Barents Sea is a primary gateway for Atlantic Water entering the central Arctic Ocean and ubiquitous water-mass transformation on the Barents shelf is key for mitigating increases in heat transport to the central Arctic through the St. Anna Trough. Using a mesoscale-permitting reanalysis spanning 40 years, we derive the first long-term estimate of heat transport through the St. Anna trough, finding that it has increased by 0.11 TW yr−1 since 1980. However, this is only half of the 0.23 TW yr−1 trend in the heat transport into the Barents Sea through the Barents Sea Opening. Decomposing the heat transports reveals that these trends are entirely due to warming temperatures at the sections with no discernible trend in the volume transports. We find that a northward migration of the largest heat fluxes from the ocean to the atmosphere have partially mitigated the increased temperature of Atlantic Water entering through the Barents Sea Opening, resulting in the less dramatic temperature trend in the waters exiting through the St. Anna Trough. Finally, we present evidence for a revised version of the “ocean feedback” hypothesis, which modulates volume transport into and out of the Barents Sea on sub-decadal timescales. The revised “ocean feedback” hypothesis relies on denser waters in the northeastern Barents Sea associated with increases in salinity and sufficient cooling from inflowing Atlantic Water. We find that these denser, more saline waters create lower sea surface height relative to the permanently elevated Kara Sea. The resulting gradient enhances downslope flow into the St. Anna Trough, driving increased inflow of saline Atlantic Water through the Barents Sea Opening and sustaining the feedback loop.

Advances in monitoring the Black Sea: a new regional multidecadal ocean reanalysis at 1∕40° resolution - Leonardo Lima; Diana Azevedo; Mehmet Ilicak; Eric Jansen; Filipe Costa; Adil Sozer; Pietro Miraglio; Emanuela Clementi (Ocean Science, 2026)
The Black Sea regional reanalysis serves as an essential tool for understanding the Black Sea's response to climate variability and advancing regional ocean monitoring efforts. In particular, the Black Sea reanalysis (BLK-REA) is built with high spatial resolution, 1/40° horizontal grid and incorporating 121 vertical levels. The model implementation includes lateral open boundary conditions (LOBC) at the Marmara Sea, allowing more accurate inflow/outflow dynamics through the Bosphorus Strait. BLK-REA assimilates sea level anomaly (SLA) and in-situ observations and applies a heat flux correction via sea surface temperature relaxation. The data assimilation system uses a background error covariance matrix evaluated through monthly EOFs over decadal periods to capture seasonal and decadal variability, and an observation-based mean dynamic topography is used for SLA assimilation. When compared to available observations, the numerical results show high accuracy, with the largest temperature errors observed in the upper layers, primarily linked to the formation of the seasonal thermocline during the summer months. The SLA anomaly error is consistently around 0.02 m from the year 2000 onwards, and regions with elevated SLA errors are closely associated with the Rim Current and its mesoscale variability. These results highlight the added value of a dedicated high-resolution regional reanalysis, as BLK-REA shows superior skill compared to a state-of-the-art global reanalysis in representing sea level, temperature, and salinity, particularly in the upper and intermediate layers of the Black Sea. Furthermore, BLK-REA plays a crucial role in generating Ocean Monitoring Indicators, which are essential for tracking and assessing long-term physical changes in the Black Sea. For example, temperature data indicate ongoing warming in the 25 to 150 m layer, where the Cold Intermediate Layer is located. The system is regularly updated, with the next version expected to improve both the model and DA components. For a future perspective, the next BLK-REA will expand the domain to include the Azov Sea and will feature an enhanced Bosphorus LOBC.

Characterisation of past marine heatwaves around South Pacific Island countries: what really matters? - Shilpa Lal; Sophie Cravatte; Christophe Menkes; Jed Macdonald; Romain Le Gendre; Ines Mangolte; Cyril Dutheil; Neil J. Holbrook; Simon Nicol (Ocean Science, 2026)
Marine heatwaves (MHWs) can have devastating and lasting impacts on marine ecosystems. We investigated past MHW characteristics around 12 southwestern Pacific Island countries and territories (PICTs) using two observed sea surface temperature products and an ocean reanalysis product. PICTs are highly dependent on their marine resources for their livelihoods: a better understanding of MHW characteristics is needed for planning and adaptation to risks associated with MHWs. Our research builds on previous studies where MHWs have been detected and described using a point-based definition. We first revisit past MHW characteristics based on their spatial extent, vertical extent and seasonality. We show that filtering MHWs by size (spatial extent) and seasonality can greatly affect their characterisation and help trace their physical drivers. We then characterise past events inside each Exclusive Economic Zone (EEZ) and at the coast with MHW indices tailored to benefit Pacific Island stakeholders. We consider two types of events: large-scale events, covering a large part of the EEZ, likely to affect pelagic fisheries, and events affecting coastal zones and ecosystems. We distinguish between events occurring in the hot season (November to April), and in the cold season (May to October). We show that all 12 PICTs experienced MHWs in the past 30 years that are getting more frequent with greater spatial extents, longer durations, explained by the long-term warming trend in sea surface temperature, but with lower maximum intensity. New Caledonia, Vanuatu, Fiji and Tonga appear to be more exposed to MHWs with longer duration, higher maximum intensity, and deeper extent compared to other countries.

Atlantic Water flow through Fram Strait to the Arctic Ocean measured by repeated glider transects - Vår Dundas; Ilker Fer (Ocean Science, 2026)
We present transport estimates of Atlantic Water (AW) and Recirculating Atlantic Water (RAW) across a zonal transect at 77°15′ N using repeated ocean glider observations. Over three missions during autumn and winter of 2020 to 2022, 22 high-resolution sections were collected, enabling detailed characterization of circulation branches and volume transport. On average, the West Spitsbergen Current (WSC) and the Front Current each transport approximately 2.5 Sv of AW (Θ>2 °C, SA>35.06gkg-1) northward, yielding a combined net flux of about 5 Sv toward the Arctic. Variability in transport and current structure is substantial and appears linked to atmospheric forcing. Case studies reveal that anomalous northward wind stress coincides with peak AW transport, roughly twice the seasonal mean, consistent with Ekman dynamics and elevated sea surface height along the coast. Conversely, strong southward wind stress weakens the WSC and nearly eliminates the Front Current. Transport of RAW (Θ>0 °C, SA>35.06gkg-1) west of the Front Current is estimated to be about 1 Sv, but this does not capture the expected stronger recirculation transport further west, beyond the glider's target transect. These results highlight the capability of gliders to resolve spatial variability in boundary currents that mooring arrays cannot capture. Extended seasonal coverage, including summer, is needed to assess transport variability under peak wind forcing.

Comparative mesoscale eddy dynamics under geostrophic versus cyclogeostrophic balance from satellite altimetry - Xinman Zhu; Yuhan Cao; Linxiao Liu; Yigang Deng; Ruixiang Liu; Zhiwei You (Ocean Science, 2026)
The curvature of streamlines plays a significant dynamical and structural role in meandering currents. At scales comparable to the deformation radius, the motion of eddies is governed by a balance between the pressure gradient force, the Coriolis force, and the centrifugal force. For mesoscale eddies, the nonlinear term induced by the local curvature of streamlines is non-negligible. This study compares the statistical and dynamical parameters of mesoscale eddies under geostrophic and cyclogeostrophic balances by examining five energetic North Pacific regions: the Aleutian Islands, Kuroshio Extension, South China Sea, California Coastal Current, and Hawaiian Islands. The comparison shows that cyclogeostrophic EKE is lower than geostrophic EKE for cyclonic eddies, whereas it is higher for anticyclonic eddies, particularly in energetic, low-latitude regions. The total number of eddies detected under the cyclogeostrophic balance is 35.65 % lower than under the geostrophic balance. However, the frequency distributions of eddy radii in both frameworks show a right-skewed normal distribution. Detection under the cyclogeostrophic balance tends to eddies with larger radii and shorter lifespans. The velocity difference between the two balances for eddies increases with decreasing latitude. Similarly, case studies indicate that anticyclonic eddies exhibit more pronounced variability in their energy evolution under the influence of streamline curvature, making them more prone to dissipation in low-latitude seas.

Marine heatwaves variability and trends in the Patagonian Shelf - Ana L. Delgado; Vincent Combes; Gotzon Basterretxea (Ocean Science, 2026)
Marine heatwaves (MHWs), have doubled in frequency globally in recent decades and are becoming longer, more intense, and increasingly disruptive to marine ecosystems. However, despite their growing ecological and biogeochemical importance, major productive coastal systems remain understudied, particularly in the Southern Hemisphere. Here, we provide the first comprehensive characterization of MHWs across the Patagonian Shelf (PS), one of the most biologically productive marine regions on Earth, using 40 years of satellite-derived daily sea surface temperature (SST) data. We first assess how the choice of MHW detection method (fixed versus moving climatology) and SST-dataset selection affect MHW metrics. Then we quantify MHW frequency, intensity, duration, and long-term trends, revealing that the PS experiences on average 1.9 ± 2 MHWs yr−1 with a mean cumulative duration of 23–28 d yr−1 and an average intensity of 1.36 ± 0.3 °C. We show that MHW activity varies substantially across the region, with the northern sector and the outer shelf experiencing the most frequent and intense events (>2 events yr−1 and >2 °C). A notable increase in MHW days (+5–10 d per decade) is observed in the northern PS, whereas no significant trends are observed to the south (i.e., south of 48° S). These trends are consistent with background warming of the ocean in this region, suggesting a mechanistic link, whereby long-term warming enhances the likelihood of MHWs occurrence and duration. We further demonstrate that a component of MHW variability can be attributed to the El Niño Southern Oscillation, which exerts a stronger influence on the intensity of thermal anomalies than on the cumulative duration of the events. Together, these findings constitute the first comprehensive assessment of MHWs on the PS and provide essential insight for anticipating their ecological and climatic impacts in one of the Southern Hemisphere's key marine ecosystems.

Thermodynamic concepts used in physical oceanography - Trevor J. McDougall (Ocean Science, 2026)
The thermodynamic concepts that are used in physical oceanography are reviewed, including how the First Law of Thermodynamics is derived, and introducing the several different types of salinity. Different temperature-like variables are discussed, leading to potential enthalpy and Conservative Temperature because of the need to accurately quantify the ocean's role in transporting heat. A key aspect of a thermodynamic variable is the extent of its non-conservation when mixing occurs at a given pressure. Methods are presented that quantify the amount of this non-conservation of several thermodynamic variables, and these are illustrated in the context of the global ocean. There has been confusion in the literature about the meaning of the salinity and temperature variables carried by ocean models, and here we explain why even in older ocean models that use the EOS-80 equation of state (rather than TEOS-10), the model's salinity is Preformed Salinity and the model's temperature variable is Conservative Temperature. The thermodynamic reasoning that leads to the concept of neutral surfaces is reviewed, along with thermobaricity, cabbeling, the dianeutral motion caused by the ill-defined nature of neutral surfaces, and Neutral Surface Planetary Potential Vorticity.

Best practices for estimating turbulent dissipation from oceanic single-point velocity timeseries observations - Cynthia E. Bluteau; Danielle J. Wain; Julia C. Mullarney; Craig L. Stevens (Ocean Science, 2026)
We provide best practices for estimating the dissipation rate of turbulent kinetic energy, ε, from velocity measurements in an oceanographic context. These recommendations were developed as part of the Scientific Committee on Oceanographic Research (SCOR) Working Group #160 “Analyzing ocean turbulence observations to quantify mixing”. The recommendations here focus on velocity measurements that enable fitting the inertial subrange of wavenumber velocity spectra. The method examines the measurable range for this method of dissipation rates in the ocean, seas, and other natural waters. The recommendations are intended to be platform-independent since the velocities may be measured using bottom-mounted platforms, platforms mounted beneath the ice, or platforms directly on mooring lines once the data is motion-decontaminated. The procedure for preparing the data for spectral estimation is discussed in detail, along with the quality control metrics that should accompany each estimate of ε during data archiving. The methods are applied to four “benchmark” datasets covering different flow regimes and two instrument types (acoustic-Doppler and time of travel). Problems associated with velocity data quality, such as phase-wrapping, spikes, measurement noise, and frame interference, are illustrated with examples drawn from the benchmarks. Difficulties in resolving and identifying the inertial subrange are also discussed, and recommendations on how these issues should be identified and flagged during data archiving are provided.

Tidal signatures on surface chlorophyll a concentration in the Brazilian Equatorial Margin - Carina Regina de Macedo; Ariane Koch-Larrouy; José Carlos Bastos da Silva; Jorge Manuel Magalhães; Fernand Assene; Manh Duy Tran; Isabelle Dadou; Amine M'Hamdi; Trung Kien Tran; Vincent Vantrepotte (Ocean Science, 2026)
This study investigates the influence of tides on chlorophyll a (CHL) variability in the Brazilian Equatorial Margin using daily GlobColour and MODIS-Aqua CHL data from 2005 to 2021. The impact of the tides is assessed by comparing the spring with the neap tide signals (fortnightly signal, 14.7 d). Results show that, on the shallow Amazon shelf, significant fortnightly CHL variability is likely primarily driven by barotropic tide-induced friction on the shelf that produces significant vertical mixing. On the northwestern shelf, where the Amazon River plume prevails, CHL levels are higher during neap tides, resulting in a negative spring–neap tide CHL difference (GlobColour: −50 %; MODIS-Aqua: −84 %). Conversely, on the northeastern shelf, characterized by low-turbidity waters, CHL levels are higher during spring tides, leading to a positive spring–neap tide CHL difference (GlobColour: +30 %; MODIS-Aqua: +70 %). Offshore, baroclinic tides, also known as internal tides (ITs), seem to enhance the CHL along their pathways with a spatial structure of a wave-like pattern. The positive CHL peaks are spaced by mode-2 wavelengths (about 68 km), with peak values reaching +3.3 % (GlobColour) and +9.0 % (MODIS-Aqua). Analysis shows that the CHL wave-like pattern suggests contributions from mode-1 and mode-2 internal tides, with mode-2 components having higher spectral coherence with the original signal. A 1–3 d lag between higher CHL variability and tidal potential may indicate delayed nutrient mixing post-spring–neap tides. The effects of ITs on CHL are more pronounced than on sea surface temperature.

A first predictive mechanistic model of cold-water coral biomass and respiration based on physiology, hydrodynamics, and organic matter transport - Evert de Froe; Christian Mohn; Karline Soetaert; Anna-Selma van der Kaaden; Gert-Jan Reichart; Laurence H. De Clippele; Sandra R. Maier; Dick van Oevelen (Ocean Science, 2026)
Cold-water corals form complex three-dimensional structures on the seafloor, providing habitat for numerous species, and act as a carbon cycling hotspot in the deep-sea. The distribution of these important ecosystems is often predicted by statistical habitat suitability models, using variables such as terrain characteristics, temperature, salinity, and surface productivity. While useful, these models do not provide a mechanistic understanding of the processes that facilitate cold-water coral occurrence, and how this may change in the future. Here, we present the results of a mechanistic process-based model in which coral biomass and respiration are predicted based on hydrodynamics, organic matter transport and coral physiology. The model domain comprises the cold-water coral mounds of south-east Rockall Bank in the north-east Atlantic Ocean. Hydrodynamic forcing is provided by a high-resolution Regional Ocean Modelling System (ROMS) model, which drives the transport of reactive suspended particulate organic matter in the region. The physiological cold-water coral model, with coral food uptake, assimilation, and respiration as key variables and with model parameters estimated from available experimental reports, is coupled to the reactive transport model of suspended particulate organic matter. Cold-water coral biomass was mainly predicted on coral mounds and ridges in the area. Model predictions agree with coral reef biomass and respiration observations in the study area and coral occurrences agree with predictions from previously published habitat suitability models. Filter feeding activity by cold-water corals proved to strongly deplete food particles in the bottom waters. Replenishment of food particles by tidal currents was therefore vital for cold-water coral growth. This mechanistic modelling approach has the advantage over statistical and machine learning-based predictions that it can be used to obtain an understanding of the effect of changing environmental conditions such as ocean temperature, surface production export, or ocean currents on cold-water coral biomass distribution and can be applied to other study areas and/or species.

The role of cyclonic eddies in the detachment and separation of Loop Current eddies - Marco Larrañaga; Julien Jouanno; Eric P. Chassignet; Giovanni Durante; Ilkyeong Ma; Julio Sheinbaum; Lionel Renault (Ocean Science, 2026)
The Loop Current (LC) and its associated eddies, known as Loop Current Eddies (LCEs), are key oceanic features in the Gulf of Mexico. Using a statistical analysis of 29 years of satellite altimeter data (1993–2021), we show that more than half of the LCEs that detach from the LC reattach within 30 d and that only 42 % truly separate from the LC and move westward in the Gulf. Our observational analysis also shows that (i) before a detachment can occur, the LC needs to extend far enough north in the Gulf to reach the Mississippi fan (∼27.5° N); and (ii) the ratio of separations to reattachments depends on latitude, with detachments being more prone to reattach if they occur south of 25° N and to separate if they occur north of 25° N. In case (iii), cyclonic eddies are consistently present during the detachment process, with one cyclonic eddy on the eastern side of the LC if the LCE is to reattach, and one on each side if the LCE is to separate. When cyclonic eddies occur on both sides of the LC, their co-occurrence in the LC bottleneck zone forms a large cyclonic structure. This large cyclonic structure is often observed during separation events, and when it is absent, LCEs are more likely to reattach, indicating a potential role in modulating the LC extension into the Gulf of Mexico. Sometimes, it can restrict LC growth on time scales of several months. The observed associations between cyclonic eddies and LCE detachments provide a statistical framework that could help anticipate separation events.

Iron isotope insights into equatorial Pacific biogeochemistry - Capucine Camin; Marie Labatut; Catherine Pradoux; James W. Murray; François Lacan (Ocean Science, 2026)
The EUCFe cruise (RV Kilo Moana, 2006) was designed to characterize sources of Fe to the western equatorial Pacific and its transport by the Equatorial Undercurrent (EUC), a narrow and fast eastward current flowing along the equator, to the eastern equatorial Pacific High Nutrient Low Chlorophyll (HNLC) region. This study presents seawater dissolved (DFe) and particulate (PFe) iron concentrations and isotopic compositions (δ56DFe and δ56PFe) from 15 stations in the equatorial band (2° N–2° S) between Papua New Guinea and 140° W, over more than 8500 km along the equator and in the upper 1000 m of the water column. δ56DFe and δ56PFe ranged from −0.22 ‰ to +0.79 ± 0.07 ‰ and from −0.52 ‰ to +0.43 ± 0.07 ‰, respectively (relative to IRMM-14, 95 % confidence interval). Source signatures, biogeochemical processes and transport all contribute to these observations. Two distinct areas, one under continental influence (the western equatorial Pacific) and an open ocean region (the central equatorial Pacific), emerged from the data. In the area under continental influence, high PFe concentrations along with δ56DFe values systematically heavier than that of δ56PFe indicated an equilibrium fractionation and the co-occurrence of chemical fluxes from both phases toward the other. This exchange occurs through non-reductive processes, as previously proposed from three of the eight stations of this area (Labatut et al., 2014) and extends up to 1200 km from the coast. In the open ocean area, preservation of a DFe isotopic signature of ∼ +0.36 ‰ within the EUC, from Papua New Guinea to the central equatorial Pacific (7800 km), confirmed the origin of the DFe carried within this current toward the HNLC region. At the same depth, bordering the EUC at 2° N and 2° S at 140° W, light isotopic signatures suggested that iron was originating from the eastern Pacific oxygen minimum zones. These light signatures were also observed in deeper central waters, between 200 and 500 m. Our data did not allow conclusions about fractionation during uptake by phytoplankton, but indicated that any fractionation, if present, must be small, no larger than a few tenths of a per mil.

Tracking marine debris in Northwest Spain: assessing wind influence with a Lagrangian transport model - Martiño Rial-Osorio; Vicente Pérez-Muñuzuri; Sara Cloux (Ocean Science, 2026)
Marine debris is responsible for major problems in our oceans, causing serious environmental degradation, detrimental health effects and economic losses in sectors related to the marine environment. In this work, we examine how plastics released by the Ulla river at the estuary's extreme affect the transport, accumulation, and beaching of floating particles in the Ría de Arousa, an estuary on the northwest coast of the Iberian Peninsula, as a result of wind force. Using Lagrangian simulations of particle tracking under different wind drag coefficients (1 %, 3 % and 5 %), we evaluate the spatial and seasonal patterns of particle concentration, residence time and deposition on the coast. Our results show that wind plays a crucial role in modulating particle behavior. Low wind-driven conditions favor greater near-shore accumulation and longer residence times, especially in the northern and inner regions of the estuary. As wind influence increases, particle dispersion intensifies, leading to lower overall accumulation and weakening of correlations between river discharge and coastal deposition. Seasonal differences are also studied, with higher concentrations observed in the north during winter and in the south during summer.

Response of a semi-enclosed sea to perturbed freshwater and open ocean salinity forcing - Lars Arneborg; Magnus Hieronymus; Per Pemberton; Ye Liu; Sam T. Fredriksson (Ocean Science, 2026)
The sensitivity of Baltic Sea salinities to changed freshwater forcing and other forcing factors have been debated during the last decades, since changed salinities would have large impacts on the marine ecosystems, and since this parameter still shows a high degree of uncertainty in regional climate projections. In this study, we performed a sensitivity experiment where freshwater forcing and salinities at the outer boundaries of the North Sea were perturbed in a systematic way in order to obtain a second-order Taylor polynomial of the statistical steady state mean salinity. The polynomial was constructed based on perturbations of a 57 year long hindcast run for the period 1961–2017 with a regional ocean model covering the North Sea and the Baltic Sea. The results show that the Baltic Sea is highly sensitive to freshwater forcing, and that about one third of the boundary salinity change propagates into the Baltic Sea. The results are also analysed in terms of a total exchange flow analysis in the entrance region, and it is found that the Baltic Sea salinity sensitivity to freshwater forcing to a large degree can be explained by increased freshwater input causing (1) dilution inside the Baltic Sea, (2) decreased inflows caused by changes to the mean sea level gradient in the entrance region, and (3) reduced inflow salinities due to recirculation of outflowing Baltic Sea water in the entrance region where the inflow water consists of about two parts outflowing Baltic water and one part North Sea water. Besides providing new understanding of the processes that govern the Baltic Sea salinity sensitivity to freshwater forcing, the results of this study provide means of quickly assessing Baltic Sea salinity changes based on changes of North-East Atlantic salinities and Baltic Sea freshwater forcing.

Evaluation of Extreme Sea-Levels and Flood Return Period using Tidal Day Maxima at Coastal Locations in the United Kingdom - Stephen E. Taylor (Ocean Science, 2026)
Tidal storm surges can result in significant inundation and damage if sea defences are insufficiently robust. Coastal planners need to know the risk of flooding so that sea defences and coastal developments can be specified and located appropriately. Since the original work on extreme value statistics by Gumbel (1954), several alternatives have been proposed for evaluating the risk of tidal inundation, with the Skew Surge Joint Probability Method (SSJPM) gaining popularity. However, SSJPM is complex and cannot always be applied generally. Guided by the search for a general method having wide application and amenable to automation, this paper re-examines the original approach of Gumbel and proposes a simple modification for combined peak selection and declustering; it is termed here TMAX, since it selects one maximum per tidal day. In comparison with the method of Gumbel (1954) using annual maxima (later termed AMAX), the TMAX method offers more efficient use of extreme data events and in addition, simpler handling of missing data. The results of the TMAX method are compared with those of a recent UK study using the SSJPM method at the same United Kingdom coastal locations. The broadly applicable TMAX method has the potential to offer more widespread calculation of flood return period, thereby improving strategies regarding coastal management and resilience.

Mesoscale variability and water mass transport of the Caribbean Current revealed by high-resolution glider observations - Joseph C. Gradone; William D. Wilson; Scott M. Glenn; Leah N. Hopson; Travis N. Miles (Ocean Science, 2026)
The Caribbean Through-Flow (CTF) provides a key pathway linking the North Atlantic Subtropical Gyre and the upper limb of the Atlantic Meridional Overturning Circulation. Yet, its internal structure and variability remain poorly resolved. Autonomous underwater gliders offer a unique capability to address this gap by collecting high-resolution hydrographic and velocity observations in regions where sampling is sparse. Here, data from a glider that operated for >90 d along 69° W in summer 2024 were analyzed to investigate mesoscale-driven variability in the CTF. Two consecutive occupations of this ∼600 km trans-Caribbean section revealed a sharp decline in zonal transport from −17.64 to −9.22 Sv, coinciding with a shift in mesoscale activity. Magnitude and variability in the vertical shear of the subsurface currents and dynamic height anomaly calculations from the glider data showed a shift from flow largely in geostrophic balance during Transect #1 to increased mesoscale influence during Transect #2. Satellite altimetry spanning the full deployment suggested this shift was driven by a cyclonic eddy that passed through the northern half of the section between the timing of the two transects. Despite the large changes in transport between transect occupations, water mass analysis showed that the relative contributions from North and South Atlantic water masses remained nearly constant. Direct sampling of an anticyclonic eddy during a partial Transect #3 revealed strong temperature and salinity anomalies in the upper 200 m. These findings highlight how glider observations can resolve key features and processes governing variability in this critical inter-basin pathway and improve understanding of mesoscale influences on large-scale circulation.

Horizontal transport on the continental shelf driven by periodic rotary wind stress - Nathan Paldor; Lazar Friedland (Ocean Science, 2026)
Wind driven circulation of a uniform density fluid on a linearly sloping continental shelf is studied by employing the Lagrangian equations of motion forced by periodic rotary wind stress. The analysis yields explicit approximate expressions for the water column trajectories in the longshore and cross-shore directions, and these expressions are verified by numerical integration of the governing nonlinear equations. The periodic rotary wind stress generates a steady longshore drift directed with land to its left when the wind rotates counterclockwise at sub-inertial frequencies and with land to its right in all other frequencies. Counterclockwise rotation of the wind at the local inertial frequency results in a strong resonance manifested in very fast longshore drift.

Mechanisms driving mesoscale latent heat flux variations and mixed layer heat content evaluation in the Northwest Tropical Atlantic - Pablo Fernández; Sabrina Speich; Guillaume Lapeyre; Claudia Pasquero; Carlos Conejero; Lionel Renault; Fabien Desbiolles (Ocean Science, 2026)
In this study, a high-resolution ocean-atmosphere coupled simulation is used to assess the effects of sea surface temperature (SST), surface currents, and ocean vertical stratification on the spatial variability of latent heat flux (LHF) and the stability of the marine atmospheric boundary layer (MABL) in the Northwest Tropical Atlantic during January and February 2020. The analysis focuses on the ocean mesoscale (O(50–250 km)) across the Northwest Tropical Atlantic (referred to as the EURECA region in this study) and within three sub-regions characterized by different ocean dynamical regimes: Amazon, Downstream, and Tradewind. Results indicate that the coupling between SST and wind speed (and specific humidity) is stronger (weaker) in the Amazon and Downstream regions, influenced by the warm coastal North Brazil Current eddy corridor and the Amazon River plume, than in the Tradewind region, representative of the open ocean, consistent with previous remote sensing studies. Overall, warmer SSTs are associated with increased wind speeds and variations in specific humidity, deviating from Clausius–Clapeyron expectations. We interpret this as the result of active ocean processes modifying the near-surface atmosphere, enhancing vertical motion in the MABL, and transporting momentum and drier air from the free troposphere toward the surface. To further investigate the impact of mesoscale SST features on LHF, we apply a linear, SST-based downscaling method. Results show that these mesoscale SST structures induce a substantial increase in LHF, 46.8 Wm-2K-1 on average in the Amazon and Downstream regions (warm eddy corridor). In the Tradewind region, the LHF sensitivity to SST is smaller, at about 35 Wm-2K-1. For the Amazon region, of the 46.7 Wm-2K-1 change in LHF associated with SST, approximately 7.8 Wm-2K-1 is attributed to direct mesoscale SST changes (thermodynamic contribution), while the remainder is linked to mesoscale SST-induced modifications in near-surface atmospheric circulation (dynamic contribution), mainly due to the mesoscale SST-induced humidity undersaturation imbalances. The influence of surface currents on LHF is weaker, with deviations not exceeding 15 W m−2. Finally, we focus on the SST mesoscale anomalies linked to the Amazon freshwater plume. We find them to be persistent throughout the period of study affecting LHF by the mechanisms described above. Lateral advection and heat loss to the atmosphere tend to dilute them with their environment by the end of the period of study. This work underscores the importance of a regionalized approach to mesoscale air-sea interaction studies in the Northwest Tropical Atlantic, as LHF sensitivity to SST and surface currents exhibits strong spatial variability driven by distinct oceanic dynamics. Submesoscale LHF sensitivity to SST and currents is not addressed here and will be the subject of future research.

Internal solitary waves refraction and diffraction from interaction with eddies off the Amazon Shelf from SWOT - Chloé Goret; Ariane Koch-Larrouy; Fabius Kouogang; Carina Regina de Macedo; Amine M'Hamdi; Jorge M. Magalhães; José Carlos Bastos da Silva; Michel Tchilibou; Camila Artana; Isabelle Dadou; Antoine Delepoulle; Simon Barbot; et al. (Ocean Science, 2026)
Off the Amazon shelf, internal solitary waves (ISWs) generated by internal tides interact with mesoscale eddies, leading to significant modifications in their propagation and structure. For the first time, these interactions are directly observed from repeated high-resolution satellite measurements, notably those provided by the newly launched SWOT (Surface Water and Ocean Topography) mission. The study investigates ISWs detection and the modifications in ISW's characteristics resulting from interactions with eddies across three contrasting scenarios: ISW propagation in the absence of mesoscale eddies, ISW refraction by a cyclonic eddy, and ISW diffraction by an anticyclonic eddy. ISW crests are extracted using a band-pass filtering approach, enabling precise tracking of essential geometric and dynamical features, including propagation direction, crest-to-crest spacing, and wavefront shape. Prior to any interaction with eddies, three mode-1 ISW packets propagate in a steady and coherent manner, exhibiting planar wavefronts and consistent direction, with Absolute Dynamic Topography (ADT) amplitudes ranging from 8 to 14 cm. The results highlight the diversity of ISW responses depending on eddy conditions. In the absence of eddies, interaction with a seamount induces energy transfer from mode-1 to mode-3 ISWswaves (13 crests detected), while propagation direction is maintained and the ADT signature weakens to below 8 cm. In the presence of a cyclonic eddy above the seamount, ISW trajectories are refracted westward by roughly 50 degrees (7 crests detected), with increased wavecrest curvature and intensification of mode-3 generation, while ADT amplitudes decrease below 6 cm. Conversely, near the western boundary of an anticyclonic eddy, ISWs split into two distinct pathways: a western branch is refracted with flatter wavefronts, reduced crest spacing, and ADT amplitudes below 0.2 cm (9 crests detected), while an eastern branch follows the eddy's edge, displaying enhanced curvature, recognizable surface signatures of wave packets, and ADT values exceeding 9 cm (6 crests detected). Together, these findings demonstrate the ability of SWOT-based monitoring to capture the complexity of ISW dynamics and provide new insight into their nonlinear response to interactions with mesoscale and submesoscale oceanic structures.

Characteristics of ocean mesoscale eddies in the Canadian Basin from a high resolution pan-Arctic model - Noémie Planat; Carolina Olivia Dufour; Camille Lique; Jan Klaus Rieck; Claude Talandier; Louis Bruno Tremblay (Ocean Science, 2026)
Mesoscale eddies are ubiquitous in the Arctic Ocean and are expected to become more numerous and energetic as sea ice continues to decline. Yet, the spatio-temporal characteristics of these eddies are poorly documented. Here, we apply an eddy detection and tracking method to the output of a high-resolution (1/12°) regional model of the Arctic – North Atlantic in order to investigate mesoscale eddies in the Canadian Basin over the period 1995–2020. Over that period, about 6000 eddies per year are detected in the surface layer, while about 9000 eddies per year are detected in the pycnocline layer, and about 5500 eddies per year are detected in the Atlantic Water layer. The eddy population is generally distributed about equally between cyclones and anticyclones. Yet, within the pycnocline and surface layers, a clear dominance of anticyclones over cyclones is found at the centre of the Beaufort Gyre, in line with observations from Ice Tethered Profilers (ITPs). The observed dominance of anticyclonic eddies reported by ITPs thus likely partially arises from the regional focus of the ITPs. On average, eddies travel 11 km, have a radius of 12.1 km, and last 10 d, although the majority of eddies are short-lived (50 % of eddies last less than 4 d). These statistics hide strong regional and temporal disparities within the eddy population. In the surface layer, the seasonal, interannual, and decadal variability in the number of eddies and in their mean characteristics follow that of the sea ice cover. In contrast, within the pycnocline layer and below, the number and properties of eddies show a weakened seasonality. At all depths, the characteristics and density of the eddy population show a strong asymmetry between the slope and the centre of the Canadian Basin. While the upper 85 m show a greater number of eddies over the slope than over the centre of the basin, this pattern is reversed in the pycnocline layer, where a muted eddy activity is observed along the slope and up to 300 km offshore. Within the Atlantic Water layer, a relatively large number of eddies is generated in the vicinity of the cyclonic boundary current along the slope. The vast majority of eddies have a weak temperature and salinity signature with respect to their environment, although a significant portion of the long-lived eddies, located along the Chukchi shelf break, have a relatively large temperature anomaly and penetrate into the Beaufort Gyre, thus suggesting a mechanism for the penetration of heat into the gyre. Over the 26 years analysed here, the number of eddies generated within the upper 85 m increases by 34 %, with the largest increase occurring in the open ocean and marginal ice zone. Within the pycnocline layer, the number of eddies increases by 45 %, with a strong year-long increase in 2008, presumably in response to the Beaufort Gyre spin-up in 2007–2008 associated with the record low in sea ice extent. The number of eddies in the Atlantic Water layer shows an overall increase of 41 %, but little interannual variability. We suggest that this model-based eddy census can thus help investigate recent changes in the dynamical equilibrium of the Beaufort Gyre by providing a consistent spatio-temporal characterization of mesoscale eddies in the Canadian Basin over the past two decades.

Subpolar Atlantic meridional heat transports from OSNAP and ocean reanalyses – a comparison - Susanna Winkelbauer; Isabella Winterer; Michael Mayer; Yao Fu; Leopold Haimberger (Ocean Science, 2026)
Ocean reanalyses are potentially useful tools to study ocean heat transport (OHT) and its role in climate variability, but their ability to accurately reproduce observed transports remains uncertain, particularly in dynamically complex regions like the subpolar North Atlantic. Here, we evaluate currents, temperatures, and resulting OHT at the OSNAP (Overturning in the Subpolar North Atlantic Program) section by comparing OSNAP observations with outputs from a suite of global ocean reanalyses. While the reanalyses broadly reproduce the spatial structure of currents and heat transport across OSNAP West and East, systematic regional biases persist, especially in the representation of key boundary currents and inflow pathways. Temporal variability is well captured at OSNAP West, but none of the reanalyses reproduce the observed OHT variability at OSNAP East, especially a pronounced peak in 2015. This discrepancy in 2015 is traced to the glider region over the eastern Iceland Basin and Hatton Bank, where OSNAP data show a strong, localized inflow anomaly associated with the North Atlantic Current (NAC). This signal is absent from all reanalyses as well as from independent, indirect heat transport estimates based on surface heat fluxes and heat content. Investigation of sea level anomalies and implied geostrophic currents further confirm that this mismatch is mainly driven by differences in flow structure rather than temperature anomalies alone. Our results highlight both the value and limitations of reanalyses in capturing subpolar heat transport variability. While higher-resolution products such as GLORYS12V1 better represent circulation features, significant mismatches remain, especially in regions with sparse observational coverage. The findings underscore the need for improved observational networks and higher-resolution modeling to more accurately constrain subpolar OHT.

Modelling seawater p CO 2 and pH in the Canary Islands region based on satellite measurements and machine learning techniques - Irene Sánchez-Mendoza; Melchor González-Dávila; David González-Santana; David Curbelo-Hernández; David Estupiñán-Santana; Aridane G. González; J. Magdalena Santana-Casiano (Ocean Science, 2026)
Recent advancements in remote sensing systems, combined with new machine-learning model-fitting algorithms, have enabled the estimation of seawater carbon dioxide partial pressure (pCO2,sw) and pH (pHT,is) in the waters around the Canary Islands (13–19° W; 27–30° N). Continuous time-series data collected from moored buoys and Voluntary Observing Ships (VOS) between 2019 and 2024 were used to train and validate the models, providing a robust observational basis for satellite-derived estimates. Among all models tested, bootstrap aggregation (bagging) performed best, achieving an RMSE of 2.0 µatm (R2>0.99) for pCO2,sw and 0.002 for pHT,is. Multilinear regression (MLR), neural networks (NN) and categorical boosting (CatBoost) also showed good predictive skill, with RMSE values between 5.4 and 10 µatm for pCO2,sw (360–481 µatm) and 0.004–0.008 for pHT,is (7.97–8.07). Using the most reliable model, we identified an increasing trend in pCO2,sw of 3.51±0.31 µatm yr−1, exceeding the atmospheric CO2 growth rate (2.3 µatm yr−1), alongside an acidification trend of −0.003 ± 0.001 yr−1. Over the 2019–2024 period, rising atmospheric CO2 and increasing sea surface temperatures (reaching up to 0.2 °C yr−1 during the unprecedented 2023 marine heatwave) likely contributed to these trends. The Canary Islands region shifted from a weak CO2 source (0.90 Tg CO2 yr−1) in 2019 to 4.5 Tg CO2 yr−1 in 2024. After 2022, eastern sites that previously acted as annual CO2 sinks became net sources.

Characterising marine heatwaves in the Svalbard Archipelago and surrounding seas - Marianne Williams-Kerslake; Helene R. Langehaug; Ragnheid Skogseth; Frank Nilsen; Annette Samuelsen; Silvana Gonzalez; Noel Keenlyside (Ocean Science, 2026)
In the Arctic Ocean, satellite-based sea surface temperature data shows that marine heatwave (MHW) intensity, frequency, duration and coverage have increased significantly in recent decades, raising concern for Arctic ecosystems. A high frequency (more than three events per year) of MHWs has been shown around the Svalbard Archipelago. Based on this, we investigate MHW trends around Svalbard at the surface and subsurface, using a regional reanalysis from TOPAZ (1991–2022). We find an increase in the frequency and duration of MHW events around the Svalbard Archipelago over the last decade. Focussing on a region west of Svalbard, we observe an increase in MHW frequency and duration, associated with a long-term rise in sea surface temperature in the region. Analysis of eight individual summer (June–September) MHW events lasting longer than 10 d west of Svalbard, indicated the presence of four shallow (≤50 m) and four deep (>50 m) MHWs after 2010, with a mean duration of 29 d. Some events extended into the Barents Sea. Heat budget analysis demonstrated a greater contribution of ocean heat transport compared to air-sea heat fluxes in driving the MHW events. Deep and shallow events were associated with ocean heat transport anomalies of up to 9 TW. This new understanding of MHW characteristics, including their horizontal and vertical distribution, is key to assessing ecological impacts.

The Arctic overturning circulation: transformations, pathways and timescales - Jakob Dörr; Carlo Mans; Marius Årthun; Kristofer Döös; Dafydd Gwyn Evans; Yanchun He (Ocean Science, 2026)
The Arctic is the northernmost terminus of the Atlantic Meridional Overturning Circulation and is an important source of the densest waters feeding its lower limb. However, relatively little is known about the structure and timescales of the Arctic overturning circulation, and which pathways contribute most to the transformation of Atlantic Waters into dense waters and Polar Waters. In this work, we combine a Eulerian water mass transformation framework and Lagrangian tracking to decompose the time-mean Arctic overturning circulation in an eddy-rich (1/12°) global ocean hindcast (1979–2015). We show that the Atlantic Water branch through the Barents Sea dominates dense Arctic overturning, and that a large portion of these transformed waters takes many decades to exit Fram Strait. Furthermore, we show that surface forcing in the Barents Sea and north of Svalbard dominates dense overturning, but local subsurface mixing with shelf waters and between the two Atlantic Water branches plays an important role for the Fram Strait branch. Our work identifies the dominant processes of the Arctic overturning circulation, and can be used as a baseline to understand its future changes and their impact on the stability of Atlantic overturning.

Eddy kinetic energy variability from 30 years of altimetry in the Mediterranean Sea - Paul Hargous; Vincent Combes; Bàrbara Barceló-Llull; Ananda Pascual (Ocean Science, 2026)
Mesoscale activity plays a central role in ocean variability, substantially influencing the mixing of biogeophysical tracers, such as heat and carbon, and driving changes in ecosystems. Eddy Kinetic Energy (EKE), a metric used for studying the intensity of mesoscale processes, has recently been shown to increase in regions of intense EKE worldwide. Strong EKE positive trends are observed, for example, in the principal western boundary current regions, such as the Gulf Stream, Kuroshio Extension, and the Brazil/Malvinas Confluence. In this study, we assess whether the Mediterranean Sea, known to be a hotspot for climate change impacts, also exhibits such intensification. Despite the high number of observational data and modeling experiments, there is a gap in understanding the long-term evolution of mesoscale dynamics and EKE trends in the Mediterranean Sea. This study investigates EKE trends in the Mediterranean Sea using daily geostrophic currents derived from satellite altimetric data. To test the robustness of the results, we compare EKE trends computed from three different gridded altimetric products: a global product derived from a stable two-satellite constellation (two-sat) and two other products (global and European) incorporating all available satellites (all-sat). While all products reveal a general increase in EKE in the Mediterranean Sea over the last three decades, the trends calculated from the two-sat product are significantly smaller than those computed from the all-sat products. We show that this discrepancy is strongly linked to the increasing number of satellites over time used to construct the all-sat data sets, which enhances both spatial and temporal coverage, and hence, their capacity to detect higher energy levels. To evaluate the fidelity of these gridded products in capturing EKE trends, we compare them with along-track data in high-energy regions of the Mediterranean Sea: the Alboran Sea and the Ierapetra area. These regions exhibit contrasting EKE trends: positive in the Alboran Sea and negative in the Ierapetra area. These findings highlight the importance of using altimetric products with a stable number of satellites constructed for climate applications when addressing long-term ocean variability analysis.

Externally-forced and intrinsic variability of the Mediterranean surface and overturning circulations - Damien Héron; Thierry Penduff; Jean-Michel Brankart; Pierre Brasseur; Samuel Somot; Robin Waldman; Romain Pennel (Ocean Science, 2026)
Part of Mediterranean Sea variability is forced and paced by external drivers (e.g. atmosphere, river runoffs, Atlantic inflow), while the other part has a random phase and spontaneously emerges due to chaotic intrinsic variability (CIV). This study quantifies across time scales the imprints of both variability components on the surface and zonal overturning circulations within the basin, from a 39-year 30-member ensemble ocean 1/12° simulation. We find that most of sea surface height (SSH) variance is intrinsic over 17 % of the basin, in particular in the southern Ionian and Levantine basins, and most notably in the Algerian basin where CIV explains 80 % of the SSH variance at periods greater than 4 months. In contrast, 75 % of the interannual-to-decadal SSH variance of the North Ionian Gyre circulation is paced by the atmosphere, suggesting an external triggering of the Adriatic-Ionian Bimodal Oscillating System (BiOS) reversal. CIV exerts a larger influence on the Rhodes, Bonifacio, and Alboran gyre circulations. Fluctuations of the density-coordinate zonal overturning circulation (ZOCσ) and of associated transports are mostly paced by the forcing over the basin. However, CIV tends to explain a larger fraction of these transports variance in the intermediate and bottom layers near deep convection sites, in particular in the Levantine basin where this fraction exceeds 50 % between 27 and 30° E at submonthly periods, 30 % at subannual periods, and 20 % at longer periods (reaching 20 years locally). This partly random character of the multi-scale Mediterranean variability has implications for evaluating model simulations and designing long-term observation systems, which are finally discussed.

Observation-based quantification of physical processes that impact sea level - Sjoerd Groeskamp (Ocean Science, 2026)
This study provides observationally based estimates of the contributions to sea level rise from individual physical oceanographic processes. The kinematic equation for sea level evolution is used to calculate the spatial distribution of the evolution of sea level rise, and its global integral. Results are separated into impacts from boundary mass fluxes and from non-Boussinesq steric effects. The non-Boussinesq steric effect itself is further decomposed into contributions from boundary buoyancy fluxes and interior buoyancy changes driven by mixing processes. It is neither the intention nor currently possible to close the global mean sea level (GMSL) budget using this approach. Instead, the results quantify the magnitude and uncertainty of the physical oceanographic processes and their relative importance in shaping GMSL rise. This allows for a comparison of the impact on GMSL by single processes or parameterizations. Results indicate large uncertainties associated with boundary heat, mass, and freshwater fluxes and highlight the importance of ocean mixing for GMSL rise. Additionally, GMSL rise is substantially affected by how shortwave radiation is redistributed with depth and by the choice of neutral physics calculation method. This study also finds that nonlinear thermal expansion is offset by mixing-induced densification, and that both processes strongly influence sea level as consequences of the nonlinear equation of state. Many of the results are relevant for observationally based calculations and for modelers making decisions about which methods or parameterizations to use. Understanding the impact of these processes on GMSL rise, and how they change in a transient ocean and climate system, is important because these choices influence projections of future sea level rise, which inform policy decisions.

A realistic physical model of the Gibraltar Strait - Axel Tassigny; Stef L. Bardoel; Thomas Valran; Samuel Viboud; Louis Gostiaux; Joël Sommeria; Lucie Bordois; Xavier Carton; Maria Eletta Negretti (Ocean Science, 2026)
We present a large-scale laboratory model of the Strait of Gibraltar that reproduces realistic topography, tidal forcing, stratification, and rotation, enabling controlled investigation of key exchange processes linking the Mediterranean Sea and Atlantic Ocean. Velocity and density measurements confirm dynamic similarity with ocean observations. Analysis of the flow near Camarinal Sill shows that bottom boundary layers are the primary source of turbulent kinetic energy, exceeding contributions from shear at the interface between Atlantic and Mediterranean waters. The enhanced role of bottom-generated turbulence is linked to separation of the Mediterranean gravity current induced by an adverse pressure gradient during outflow, providing a new explanation for the well-documented detachment of the Mediterranean plume west of the sill. This detachment intensifies during spring tides, driving diluted waters farther into the Atlantic, while during neap tides bottom-generated and interfacial turbulence coincide, offering a consistent explanation for the high dissipation rates reported in field measurements. Overall, tidal forcing promotes full-depth mixing, with up to 20 % density reduction west of CS and oscillatory 20 % variations east, consistent with field data, and simultaneously introducing an important shift between vertical gradients of along-strait velocity and density, with implications for parameterizing turbulent exchange and definition of the composite internal Froude number for reliable diagnostics of hydraulic control. During spring tides, hydraulic control is intermittently lost during inflow (for about 50 % of the tidal cycle) and this loss propagates eastward, while additional control points arise west of the sill. Neap tides exhibit signatures of control which persist much longer during a tidal cycle (up to 88 %) as compared to spring tides, but does not propagate to the east when the tide reverses, offering an explanation of the different release of internal solitary waves observed in the literature (Roustan et al., 2024a). Transport and energy budgets reveal strong longitudinal and transverse variability, highlighting the need for fully three-dimensional diagnostics. Volume and buoyancy transports, dominated by transverse topographic variability, exceed tidal and turbulent transports by up to two orders of magnitude, confirming net Atlantic inflow. These results demonstrate that high-fidelity laboratory modeling can capture the essential three-dimensional dynamics of energetic straits and provides a powerful complement to observational and numerical approaches.

Monsoons, plumes, and blooms: intraseasonal variability of subsurface primary productivity in the Bay of Bengal - Tamara L. Schlosser; Andrew J. Lucas; Melissa Omand; J. Thomas Farrar (Ocean Science, 2026)
During the southwest monsoon, seasonal storms bring torrential rainfall to the South Asian subcontinent and the northern Indian Ocean. Dense cloud cover limits the amount of sunlight that reaches the ocean surface, and sediment-laden river runoff limits the depths to which light can penetrate. Changing light availability should affect phytoplankton primary productivity and its dependent biogeochemical processes, yet little is known about how subtropical weather is linked to ecosystem processes below the ocean’s surface. Here, using novel physical and bio-optical measurements from an array of free-drifting, autonomous systems in the Bay of Bengal, we show that the onset of cloudy conditions associated with “active” monsoon conditions led to >50 % reduction in gross chlorophyll productivity (GCP) near the subsurface chlorophyll maximum (SCM) relative to sunny “break” conditions. Optical backscatter measurements confirm chlorophyll fluorescence fluctuations correspond to biomass variability of a similar scale. Simultaneous bioacoustic measurements collected onboard the autonomous platforms suggest this intraseasonal variability in SCM chlorophyll and biomass generated a response in higher trophic levels. Long-term measurements from biogeochemical (BGC) Argo floats in the bay confirm the presence of intraseasonal oscillations in chlorophyll a concentration with days-to-weeks variability in magnitude similar to the regional annual cycle in the region. Our findings demonstrate that intraseasonal subtropical air-sea variability modulates important regional biogeochemical ocean processes in the Northern Indian Ocean with implications for the Indian Ocean carbon cycle.

Revisited heat budget and probability distributions of turbulent heat fluxes in the Mediterranean Sea - Mahmud Hasan Ghani; Nadia Pinardi; Antonio Navarra; Lorenzo Mentaschi; Silvia Bianconcini; Francesco Maicu; Francesco Trotta (Ocean Science, 2026)
Understanding the surface heat budget of the Mediterranean Sea is essential for assessing its role in regional climate and ocean circulation. Under the steady-state heat budget closure hypothesis, the Mediterranean should exhibit a net surface heat loss to balance the heat gained through the inflow of warm Atlantic water at the Gibraltar Strait. However, literature estimates of the net heat flux vary widely, raising questions about the accuracy of existing reanalysis products. In this study, we compute the net surface heat flux over the Mediterranean using two atmospheric datasets: high-resolution (0.125°) ECMWF analysis and lower-resolution (0.25°) ERA5 reanalysis. By applying the same sea surface temperature fields and bulk formulas in both cases, we isolate the impact of atmospheric resolution and data quality. We find that the ECMWF analysis yields a basin-averaged net heat flux of -3.6±1.3 W m−2, consistent with the closure hypothesis, while ERA5 gives a spurious positive flux of +5±1.2 W m−2. Furthermore, beyond simply assessing the net heat budget, this study delves into the probability distributions of air–sea heat fluxes, aiming to gain a deeper understanding of associated uncertainties and extreme values in turbulent heat fluxes. The probability distributions for turbulent heat flux components exhibit characteristics such as skewness and kurtosis, respectively, varying across the basin. To assess the influence of extremes, we apply the Interquartile Range (IQR) method within statistical models that account for the skewed nature of turbulent heat flux distributions, enabling a consistent treatment of outliers. Our results reveal that extreme negative heat flux events play a critical role in determining the net heat flux direction; excluding these extremes leads to a spurious positive heat budget. Only ECMWF fields are consistent with the heat budget closure hypothesis. Furthermore, we demonstrate that the Mediterranean heat budget closure hypothesis is connected to extreme heat loss events occurring in key regions of the basin, such as the Gulf of Lion, the Adriatic Sea, the Aegean Sea, and the southern Turkish coasts.

Phytoplankton blooms affect microscale differences of oxygen and temperature across the sea surface microlayer - Carsten Rauch; Lisa Deyle; Leonie Jaeger; Edgar Fernando Cortés-Espinoza; Mariana Ribas-Ribas; Josefine Karnatz; Anja Engel; Oliver Wurl (Ocean Science, 2026)
The sea surface microlayer (SML) is the thin layer on top of the ocean that is in direct contact with the atmosphere and is crucial for air–sea interactions. Its properties are influenced in particular by surface-active substances (surfactants), mainly produced by phytoplankton and bacteria. Thus, phytoplankton blooms and their decay can have a considerable influence on the SML. A mesocosm study was conducted to assess the impact of a phytoplankton bloom on the SML using a multidisciplinary approach, which enabled in situ measurements under controlled yet natural conditions. A phytoplankton bloom was induced within a mesocosm facility filled with seawater, resulting in three phases of the study: the pre-bloom, bloom, and post-bloom phases. During all phases, microsensors measured in situ microprofiles of oxygen and temperature with a 125 µm vertical resolution through the air, SML, and underlying water. Microscale oxygen and temperature differences were determined from the profiles, as well as the thicknesses of the oxygen diffusion boundary layer (DBL) and thermal boundary layer (TBL). The night-time oxygen differences (ØΔO2,pre-bloom = −2.16 ± 5.53 µmol L−1, ØΔO2,bloom = +24.90 ± 14.51 µmol L−1, ØΔO2,post-bloom = +2.07 ± 4.82 µmol L−1) correlated highly with the chlorophyll a concentration (r = 0.755, p

Spatiotemporal scales of mode water transformation in the Sea of Oman - Estel Font; Esther Portela; Sebastiaan Swart; Mauro Pinto-Juica; Bastien Y. Queste (Ocean Science, 2026)
In the Sea of Oman, mode water forms at the surface and is trapped under a warm stratified layer in summer. This capped and well-mixed oxygenated layer decouples the oxygen minimum zone from ocean surface processes. It also provides a space for remineralisation, reducing oxygen demand in the oxygen minimum zone. Several physical processes, from isopycnal and diapycnal mixing to advection, transform mode water and change its properties. We perform a volume budget analysis to investigate the mechanisms driving mode water volume change in the Sea of Oman from monthly to 3 d temporal scales using monthly climatologies derived from profiling floats and high-resolution underwater glider observations. Isopycnal and diapycnal water-mass transformations are estimated in a density-spice framework. Our study shows that mode water predominantly transforms along isopycnals, yet strong but transient diapycnal transformation occurs at shorter timescales. Moreover, fluxes between the mode water layer and its surroundings are highly sensitive to the presence of mesoscale eddies. Across eddies, diapycnal and isopycnal transformations intensify by 61 % and 45 % respectively, compared to non-eddy conditions, indicating that eddies are drivers of both lateral and vertical water mass exchanges. This study provides a novel application of the water mass transformation framework to high-resolution underwater gliders, and shows that this methodology can be used at higher resolution than traditional climatological products or models. By comparing monthly climatological products to the high-resolution glider data, we suggest that the climatological estimates are outside of the high-resolution glider mean ± standard error 40 % of the time for diapycnal and 60 % of the time for isopycnal transformation. These results highlight the intense variability occurring at small scales and can serve to inform future estimates of water mass transformation uncertainty from coarser products.

Rogue wave indicators from global models and buoy data - Laura Azevedo; Gabriel Marcon; Steven Meyers; Mark Luther (Ocean Science, 2026)
Rogue waves pose substantial risks to maritime operations and offshore infrastructure, yet their formation mechanisms and predictability remain poorly understood. This study analyses real rogue wave occurrences using in situ observations from CDIP wave buoys from the Free Ocean Wave Data (FOWD) dataset and model-based estimates from ERA5 reanalysis and the ECMWF CY47R1 high-resolution hindcast. Seasonal distributions, wave height comparisons, and spectral analyses reveal that models systematically underestimate extreme wave events due to spectral smoothing and spatial averaging. A key finding is that rogue waves are usually preceded by a sharp decrease in crest-trough correlation below 0.5, followed by a rapid increase usually above 0.6, indicating a transition to a more structured wave field. This pattern, accompanied by spectral bandwidth narrowing and increased relative energy in the 0.25–1.5 Hz range, suggests energy focusing mechanisms play a critical role. Analysis of rogue wave events at four CDIP buoy stations show that the crest-trough correlation parameter alone is not a good rogue wave indicator, but its temporal variability is. These results highlight the need for improved modelling by integrating dynamic wave field specific parameters and high-resolution numerical models to enhance rogue wave risk assessments on a global scale.

Impact of waves on phytoplankton activity on the Northwest European Shelf: insights from observations and km-scale coupled models - Dale Partridge; Ségolène Berthou; Rebecca Millington; James R. Clark; Lucy Bricheno; Juan Manuel Castillo; Julia Rulent; Huw Lewis (Ocean Science, 2026)
The spring bloom is an annual event in temperate regions of the North Atlantic Ocean where the abundance of photosynthetic plankton increases dramatically. The timing and intensity of the spring bloom is dependent on underlying physical conditions that control ocean stratification and mixing. Although surface waves can be an important source of turbulent kinetic energy to the surface mixed layer, they have seldom been considered explicitly in studies of bloom formation. Here, we investigate the role of surface waves in bloom formation using a combination of satellite observations and numerical models. Satellite observations show a positive correlation between wave activity and chlorophyll concentration in the Northwest European shelf (May–September). In the deeper Northeast Atlantic, increased wave activity correlates with lower chlorophyll during periods of high phytoplankton activity (March-May) and higher chlorophyll when activity is low (below 54° N, July–September). We use a first-of-its-kind, km-scale, two-way coupled ocean-wave model system to investigate both the relationship between wave-driven mixing and bloom formation, and the sensitivity of model results to the method by which wave-driven mixing is parameterised. In deep regions, during the spring bloom, a wave-driven mixing event is likely to mix surface chlorophyll to deeper layers, away from light. In contrast, when phytoplankton activity is low in deep regions, wave-driven mixing can entrain nutrients, fueling the growth of nutrient starved phytoplankton near the surface. In June–October, in shallow but weakly stratified regions of the shelf, surface chlorophyll tends to be elevated following a wave-driven mixing event, which can bring to the surface both phytoplankton and nutrients from deeper layers. When contrasted with ocean-only runs, the two way-coupled ocean-wave model tends to produce greater vertical mixing and a delay in bloom onset. These results indicate bloom dynamics are sensitive to the way in which waves are modelled, and that the role of waves in bloom formation should be considered in future studies.

Coastal-to-offshore submesoscale horizontal stirring enhances wintertime phytoplankton blooms in the ultra-oligotrophic Eastern Mediterranean Sea - Yotam Fadida; Vicky Verma; Roy Barkan; Eli Biton; Aviv Solodoch; Yoav Lehahn (Ocean Science, 2026)
The large seasonal increases in marine photosynthetic organisms – i.e., phytoplankton blooms – are a ubiquitous oceanic phenomenon that contributes to the removal of carbon dioxide from the atmosphere and supports the growth of larger marine organisms. The underlying mechanisms controlling the intensity and timing of these blooms have been proposed to be dominated by vertical transport and mixing processes that are enhanced at fine-scale frontal and filamental circulations, commonly known as submesoscale currents. Here we show that the winter blooms characteristic of the ultra-oligotrophic waters of the Eastern Mediterranean Sea, which manifest as a seasonal increase in satellite-derived levels of surface chlorophyll, are intensified by enhanced horizontal stirring induced by the submesoscale currents. Using ocean color remote sensing data and high-resolution numerical simulations, we demonstrate that the intensification of submesoscale currents in winter efficiently connect the coastal waters and the ultra-oligotrophic open-sea waters, thereby enriching the latter with chlorophyll-rich waters. A climatological chlorophyll time series comparison between two different regions equidistant to the Nile River Delta indicates that this submesoscale horizontal stirring mechanism accounts for the ∼ 24.8 % larger wintertime increase in surface chlorophyll observed downstream of the Nile Delta. These results shed new light on the processes governing phytoplankton bloom intensity and emphasize the important role of submesoscale horizontal stirring in modulating the marine ecosystem.

Intrahalocline eddies in the Amundsen Basin observed in the distributed network from the MOSAiC expedition - Alejandra Quintanilla-Zurita; Benjamin Rabe; Claudia Wekerle; Torsten Kanzow; Ivan Kuznetsov; Sinhue Torres-Valdes; Enric Pallàs-Sanz; Ying-Chih Fang (Ocean Science, 2026)
Hydrographic and velocity observations from the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition (2019–2020) reveal the presence of nine intrahalocline eddies (IHEs) in the Amundsen Basin during the winter drift of the Distributed Network (DN). Despite their relevance for Arctic stratification and mixing, IHEs in the Amundsen Basin remain poorly documented. Our study addresses this gap by providing the first detailed characterisation based on coordinated in situ hydrographic and velocity observations during wintertime. Eddies were identified as isopycnal displacements in Ice-Tethered Profiler (ITP) data. Additionally, by assessing rotational velocity signatures from Acoustic Doppler Current Profiler (ADCP) measurements, we applied a centre-detection method based on maximum swirl velocity (MSV). Nine anticyclonic eddies were observed, with radii ranging from 3.7 to 8.4 km and vertical extents between 23 and 80 m. Most eddies exhibited solid-body rotation in their cores, with maximum azimuthal velocities of up to 0.28 m s−1 and localised shallowing of the mixed layer by over 10 m. Water mass analysis showed that the eddy cores contained Eurasian halocline waters with consistent anomalies in temperature, salinity, and density relative to surrounding profiles, allowing us to infer pre-existing stratification conditions and offering clues to their origin. The observed eddy scales lie close to or slightly below the first baroclinic Rossby deformation radius of approximately 6.9 km, placing them in the (sub)mesoscale dynamical regime and suggesting a transitional balance where both geostrophic and cyclogeostrophic effects may be relevant. The MSV method yields systematically larger eddy radius estimates up to 25 % greater than traditional detection techniques that rely on velocity profiles or isopycnal displacements alone. This correction to the radius is essential, as it provides a more realistic measure of eddy size and dynamics under ice-covered conditions and could improve comparability across under-ice eddy studies. Although specific generation mechanisms remain uncertain, thermohaline signatures suggest that shallow local convection and baroclinic instability play a role in their formation. Our results provide new insights into the dynamics of under-ice eddies and their potential impact on Arctic oceanography and climate processes, addressing essential gaps in understanding polar mesoscale dynamics.

Impact of assimilated observations on the Corsica Channel transport in a 4D-Var system for the northwestern Mediterranean Sea - Michele Bendoni; Andrew Michael Moore; Roberta Sciascia; Carlo Brandini; Katrin Schroeder; Mireno Borghini; Marcello Gatimu Magaldi (Ocean Science, 2026)
We present a 4D-Var data assimilation (DA) system covering the North-Western Mediterranean Sea implemented with the Regional Ocean Modeling System (ROMS). We study, throughout the year 2022, its ability to improve the description of the overall circulation and the capability to constrain the transport across the Corsica Channel (CC), the dynamics of which are crucial in determining the circulation throughout the region. The system assimilates Sea Surface Temperature (SST) and Sea Level Anomaly (SLA) observations from satellites, surface velocity data from High-Frequency Radars (HFRs), and in situ temperature, salinity and velocity observations, the latter from a mooring located in the CC. For all the observed state variables, DA is able to improve the forecast and the analysis compared to a free run without DA, with root mean squared error reduction up to 60 % and correlation increase up to 0.4. The general circulation after DA is characterized by a reduction of the Eastern Corsica Current (ECC) and an increase of the Western Corsica Current (WCC). An adjoint sensitivity-based method was used to evaluate the impact of observations on the CC transport state estimates. The net reduction in transport induced by DA, changed the annual average value of 0.49 Sv for the free run, to 0.31 and 0.28 Sv for the forecast and analysis, respectively. The observations that contribute most to the transport changes are the in situ velocity data and those from HFRs. The observation impacts were found to vary seasonally, and sometimes act in competition to shape the circulation pathways across the CC. The sensitivity of the transport to SST and in situ temperature and salinity observations indicates that remote measurements (e.g. those from the Gulf of Lion) can potentially play a significant role in constraining the CC transport. Transport variations are largely affected by free surface gradients contained in the increment, and promoted by modifications to the open boundary conditions. This indicates that the CC dynamic is controlled by mechanisms operating at basin scales.

Bottom mixed layer derivation and spatial variability over the central and eastern abyssal Pacific Ocean - Jessica Kolbusz; Devin Harrison; Nicole Jones; Joanne O'Callaghan; Taimoor Sohail; Todd Bond; Heather Stewart; Alan Jamieson (Ocean Science, 2026)
The bottom mixed layer (BML) of the abyssal ocean regulates heat exchange between the deep interior and seafloor, driving water–mass transformation and influencing global circulation. Spatial variability of the BML was examined in the under-sampled abyssal Pacific Ocean using surface-to-seafloor temperature and pressure observations over 4 months in 2023–2024. Given the typical decadal repeat rate of global hydrographic sections, subdecadal variability in the abyssal ocean has remained poorly resolved. Our observations contribute towards filling this gap for the central and eastern abyssal Pacific Ocean. Four methods were used to determine the BML thickness, with the threshold method providing the most reliable estimates. The mean BML thickness was (226 ± 172 m) with added repeat hydrographic sections providing context and additional data points. At each BML data point we determined the slope, the terrain roughness and the extracted predicted internal tide energy dissipation (over five different low-mode processes and high-mode local processes) at 50 km scales from publicly available datasets. These factors were input into a Random Forest Regressor (RF) model, the first time machine learning techniques have been applied to investigate BML thickness. The RF feature importance scores identified bottom depth, total internal tide energy dissipation, followed by slope, as the strongest predictors of BML thickness, revealing the importance of low-mode internal wave energy losses in this abyssal setting. Targeted and sustained observations near the seafloor at gateway regions of abyssal pathways are vital for understanding energy exchange that influences meridional overturning circulation. Our results highlight a regime where sustained low-mode internal tide energy loss, modulated by topographic slope and depth, governs the BML thickness in the abyssal Pacific. However, the rate at which BML thickness changes over time and the processes that cause these changes remain key unresolved factors.

A robust minimization-based framework for cyclogeostrophic ocean surface current retrieval - Vadim Bertrand; Julien Le Sommer; Victor Vianna Zaia De Almeida; Adeline Samson; Emmanuel Cosme (Ocean Science, 2026)
Estimations of surface currents at submesoscales (1–50 km) are crucial for operational applications and environmental monitoring, yet accurately deriving them from satellite observations remains a challenge. While the geostrophic approximation has long been used to infer ocean surface currents from Sea Surface Height (SSH), it neglects nonlinear advection, which can become significant at submesoscales. To address this limitation, we present a robust and efficient minimization-based method for inverting the cyclogeostrophic balance equation, implemented in the open-source Python library jaxparrow. Unlike the traditional fixed-point approach, our method reformulates the inversion as a minimization problem, providing stable estimates even in regions where a cyclogeostrophic solution may not exist. Using a submesoscale-permitting model simulation and both DUACS and the high-resolution NeurOST SSH products, we demonstrate that cyclogeostrophic corrections become increasingly relevant at finer spatial scales. Validation against drifter-derived velocities shows that our approach consistently improves current estimates in energetic regions, reducing errors by up to 20 % compared to geostrophy alone in energetic regions of the global ocean. These results support the systematic inclusion of cyclogeostrophic inversion in the analysis of high-resolution SSH fields.

Remineralisation changes dominate oxygen variability in the North Atlantic - Rachael N. C. Sanders; Elaine L. McDonagh; Siv K. Lauvset; Charles E. Turner; Thomas W. N. Haine; Nadine Goris; Richard Sanders (Ocean Science, 2026)
Oxygen is fundamental to ocean biogeochemical processes, with deoxygenation potentially reducing biodiversity, and disrupting biogeochemical cycles. In recent decades, the global ocean oxygen concentration has been decreasing, but this decrease is underestimated in numerical ocean models by as much as 50 %. Mechanisms responsible for this deoxygenation include (i) solubility-driven deoxygenation due to ocean warming, and (ii) changes in the remineralised signal due to either a change in the supply of biological material to depth or a change in circulation leading to change in the residence time of water, and hence the accumulation of the remineralised oxygen deficit, or a combination of both. The magnitude of oxygen change due to each process is currently unclear. Here, we describe and implement a new method to decompose oxygen change into its constituent parts by linking each process to concomitant changes in temperature and dissolved inorganic carbon. Using observations on a repeated section of the North Atlantic at 24.5° N, we show that the consistent oxygen decrease observed since 1992 in the upper 2000 m has been dominated by an increase in remineralisation-related oxygen-consumption. While warming-driven solubility changes have a much smaller impact on the upper ocean in comparison, their impact has trebled in the past twenty years, suggesting they will become an increasingly significant driver of deoxygenation with future warming. Remineralisation-related oxygen consumption peaks at a depth of approximately 600 m, where it is responsible for up to 70 % of the total deoxygenation. While this study does not determine the exact cause of the remineralisation-driven change, little change in primary productivity has been observed in the region, suggesting that a change in ocean circulation is indirectly driving the majority of deoxygenation in the Subtropical North Atlantic, via a non-local change in remineralisation.

Observations of tracer ventilation in the Cape Basin, Agulhas Current Retroflection - Renske Koets; Sebastiaan Swart; Kathleen Donohue; Marcel du Plessis (Ocean Science, 2026)
The Cape Basin is a highly dynamic region, strongly influenced by the Agulhas Retroflection and its associated ring shedding. The region is characterized by high eddy kinetic energy, amplified mixing and water mass transformation. While model studies have shown that meso- to submesoscale features enhance water mass formation and tracer stirring, there have been limited observations made at the required spatiotemporal scales to capture such stirring and mixing processes. This study integrates high-resolution glider observations with satellite data to indicate the presence of shear-driven instabilities occurring at submesoscale fronts that enhance vertical diapycnal transport, leading to low apparent oxygen utilization and high levels of particulate organic carbon in the deeper ocean. These tracers are then distributed within the ocean interior via mesoscale advection and stirring along isopycnals, providing observational evidence for the role of the meso- to submesoscale strain field in surface to ocean interior water mass transformation and their broader implications on ocean circulation.

Ocean circulation, sea ice, and productivity simulated in Jones Sound, Canadian Arctic Archipelago, between 2003–2016 - Tyler Pelle; Paul G. Myers; Andrew Hamilton; Matthew Mazloff; Krista M. Soderlund; Lucas Beem; Donald D. Blankenship; Cyril Grima; Feras Habbal; Mark Skidmore; Jamin S. Greenbaum (Ocean Science, 2026)
Jones Sound is one of three critical waterways in the Canadian Arctic Archipelago that regulate liquid exchange between the Arctic Ocean and northern Atlantic Ocean. However, to date, no high-resolution ocean circulation model exists to study the recent evolution of Jones Sound, meaning that our understanding of circulation within the sound is based either on temporally and spatially sparse oceanographic observations or on extrapolating conditions within Baffin Bay, which has a more dense observational record. To address this, we develop a high-resolution (1/120°, 0.9 km) Jones Sound configuration of the Massachusetts Institute of Technology general circulation model and perform coupled ocean–sea ice–biological productivity simulations between 2003–2016. We find that circulation through Lady Ann Strait, Fram Sound, and Glacier Strait comprises 71 %, 14 %, and 15 % of the volumetric transport into and out of Jones Sound, with tidal flushing enhancing the magnitude of volumetric transport through Fram Sound. Warming Atlantic Water within western Baffin Bay flows into Jones Sound through Lady Ann Strait, becomes well-mixed, and circulates counterclockwise, encroaching on the terminus of most tidewater glaciers that line the eastern periphery of the sound. Furthermore, we find that sustained atmospheric and oceanic warming drives an 11 % reduction in the 2003–2016 mean summertime sea ice area, decreased wintertime sea ice thickness, and delayed onset of sea ice refreeze in the fall (thus lengthening the amount of time during which Jones Sound is ice-free). Tidal flushing through Cardigan Strait is critical in triggering melt-back of sea ice across northern Jones Sound. Lastly, this decline in sea ice increases light availability and, when coupled with warming of the subsurface waters in Jones Sound, facilitates enhanced primary productivity down to ∼ 21 m depth. While we note that the modeled warming signal in Baffin Bay is overestimated relative to observations, the results presented here improve our general understanding of how this critical waterway might change under continued polar-amplified global warming and underscores the need for sustained oceanographic observations in this region.

A decade of continuous Rockall Trough transport observations using moorings and gliders - Kristin Burmeister; Sam C. Jones; Neil J. Fraser; Alan D. Fox; Stuart A. Cunningham; Lewis A. Drysdale; Mark E. Inall; Tiago S. Dotto; N. Penny Holliday (Ocean Science, 2026)
The Rockall Trough, northwest of Scotland and Ireland, is a key conduit for the North Atlantic Current (NAC) and European Slope Current (ESC) transporting heat and salt toward the Nordic Seas and Arctic Ocean while mediating exchanges between the open ocean and the European shelf. We present a decade-long record of Rockall Trough circulation from the Ellett Array providing the first continuous estimates of heat and freshwater transport between 2014 and 2024. We develop a methodology that combines the high spatial resolution of gliders with the high temporal resolution of moorings and ocean reanalysis output producing continuous eastern boundary velocity fields of the ESC for integration into the full Rockall Trough transport product. This approach improves the mean structure of the ESC, capturing the southward undercurrent previously unresolved and enhancing the ability to reproduce extreme, likely mesoscale, transport events. The Rockall Trough transport is dominated by the NAC flowing through the mid basin, exhibiting multi-year variability consistent with changes in the subpolar gyre and the mid-2010s cold freshwater anomaly. The ESC acts as a secondary driver, is not correlated with the NAC and is influenced by along-slope wind stress. Since 2022, warmer and saltier conditions, amplified by the 2023 extreme North Atlantic marine heatwave, have strengthened northward volume, heat, and salt transport through the Ellett Array. Our results highlight the value of sustained glider-based boundary current observations for Atlantic climate monitoring and demonstrate that the combined mooring–glider framework provides a robust and transferable approach for long-term ocean transport monitoring.

Compounded effects of long-term warming and the exceptional 2023 marine heatwave on North Atlantic coccolithophore bloom dynamics - Thibault Guinaldo; Griet Neukermans (Ocean Science, 2026)
The North Atlantic is undergoing rapid ecological evolution under the influence of both long-term warming and the increasing frequency of marine heatwaves. In 2023, the North Atlantic experienced record-breaking sea surface temperature anomalies, exceeding +5 °C regionally and lasting several months. Using 25 years of satellite-derived particulate inorganic carbon data (1998–2023), we assess the response of coccolithophore blooms across two biogeographical boundaries: the Celtic Sea and the Barents Sea. We show that the 2023 marine heatwave led to reduced bloom intensity and fragmentation in the Celtic Sea, while leading to record-high intensity and extent in the Barents Sea. These contrasting responses are modulated by long-term sea surface temperature trends, upper-ocean stratification, and polar front shifts. Our findings suggest a spatial shift of coccolithophore blooms with potential implications for the carbon cycle under long-term warming and stratification.

Hidden vortices: near-equatorial low-oxygen extremes driven by high-baroclinic-mode vortices - Florian Schütte; Johannes Hahn; Ivy Frenger; Arne Bendinger; Ahmad Fehmi Dilmahamod; Marco Schulz; Peter Brandt (Ocean Science, 2026)
Long-term time series of dissolved oxygen (DO) measurements from the upper 500 m of the eastern tropical North Atlantic (ETNA), collected over a period of up to 15 years at three different mooring sites, reveal recurring extreme low-oxygen events lasting for several weeks. Similarly, observations from 15 individual meridional ship sections between 6 and 12° N along 23° W show DO concentrations far below 60 µmol kg−1 in the upper 200 m – significantly lower than the climatological values in this depth range (>80 µmol kg−1). Two-third of these low-oxygen events could be related with high-baroclinic-mode vorticies (HBVs) with their cores located well below the mixed layer. Despite the energetic equatorial circulation and the expected dominance of wave-like structures in the near-equatorial region, these HBVs persist as relatively long-lived and coherent features. Based on moored and shipboard observations from the ETNA, and supported by an eddy-resolving ocean-biogeochemistry model, we characterize their dynamics and DO distribution. Observed water mass properties and model analyses suggest that most HBVs originate from the eastern boundary and can persist for more than six months. As they propagate westward into regions of higher potential vorticity (PV), anticyclonic HBVs with low-PV cores remain more effectively isolated and have longer lifespans compared to cyclonic HBVs with high-PV core. The vertical structure of the dominant anticyclonic HBVs corresponds to baroclinic modes 4–10, with associated Rossby radii ranging from 34 to 13 km, respectively. This is consistent with observed eddy sizes and is well below the corresponding 1st baroclinic Rossby radius of deformation (>100 km). Since none of the observed HBVs exhibit a surface signature, a substantial portion of the near-equatorial eddy field may remain undetected by satellites, yet still exert significant influence on local ocean ecosystems and biogeochemical cycles.

Passive acoustic monitoring from profiling floats as a pathway to scalable autonomous observations of global surface wind - Louise Delaigue; Pierre Cauchy; Dorian Cazau; Julien Bonnel; Sara Pensieri; Roberto Bozzano; Anatole Gros-Martial; Christophe Schaeffer; Arnaud David; Paco Stil; Antoine Poteau; Catherine Schmechtig; et al. (Ocean Science, 2026)
Wind forcing plays a pivotal role in driving upper-ocean physical and biogeochemical processes, yet direct wind observations remain sparse in many regions of the global ocean. While passive acoustics have been used to estimate wind speed from moored and mobile platforms, their application to profiling floats has been demonstrated only in limited cases. Here we report the first deployment of a biogeochemical profiling float equipped with a passive acoustic sensor explicitly designed for wind retrieval, aimed at detecting wind-driven surface signals from depth. The float was deployed in the northwestern Mediterranean Sea near the DYFAMED (DYnamique des Flux Atmosphériques en MEDiterranée) meteorological buoy from February to April 2025 and operated at parking depths of 500–1000 m. We demonstrate that wind speed can be successfully retrieved from subsurface ambient noise using established acoustic algorithms, with float-derived estimates showing good agreement with collocated surface observations. To evaluate scalability to remote regions, we simulate a remote deployment scenario by refitting the acoustic model of Nystuen et al. (2015) using ERA5 reanalysis as a reference for surface wind. The ERA5-based calibration performs well under moderate winds but exhibits systematic high-wind bias (≥ 10 m s−1). Finally, we apply a residual learning framework to correct these estimates using a limited subset of DYFAMED wind data, simulating conditions where only brief surface observations are available. The corrected wind time series achieved a 38.6 % reduction in RMSE, demonstrating the effectiveness of combining reanalysis with sparse in-situ calibration. This framework improves agreement with in-situ wind observations relative to reanalysis alone, supporting a scalable strategy for float-based wind monitoring in data-sparse ocean regions. Such capability has direct implications for improving estimates of air–sea exchanges, interpreting biogeochemical fluxes, and advancing climate-relevant ocean observing.

Metrological concepts applied to Total Alkalinity measurements in seawater: reference materials, inter-laboratory comparison and uncertainty budget - Gaëlle Capitaine; Samir Alliouane; Thierry Cariou; Jonathan Fin; Paola Fisicaro; Thibaut Wagener (Ocean Science, 2026)
Total alkalinity (TA) measurements in seawater are crucial for characterizing and monitoring the oceanic carbonate system. While international best practices and guidelines exist, the field still lacks widely available traceable reference materials and a well-established uncertainty budget of the measurement method. In this study, we applied key metrological principles – development of reference materials, inter-laboratory comparison and uncertainty quantification – to TA measurements. We developed two reference materials, including an artificial material with a rigorously characterized reference value and an associated uncertainty budget, being traceable to the International System of units (SI). These materials were tested in an inter-laboratory comparison involving five laboratories and demonstrated the applicability of the reference materials developed for quality control. Additionally, we established an uncertainty budget for the TA measurement method using two metrological approaches. The resulting expanded uncertainty was 5 µmol kg−1 (with a coverage factor k=2) in TA, approaching the 4 µmol kg−1 target set by the Global Ocean Acidification Observing Network for climate monitoring. These findings mark a significant step toward improving the quality and comparability of TA measurements, thereby strengthening long-term ocean carbonate system monitoring.

Using surface drifters to characterise near-surface ocean dynamics in the southern North Sea: a data-driven approach - Jimena Medina-Rubio; Madlene Nussbaum; Ton S. van den Bremer; Erik van Sebille (Ocean Science, 2026)
The large size of traditional drifters limits their ability to mimic the transport of buoyant objects at the ocean surface, which are subject to complex interactions among direct wind drag, fast-moving surface currents, and wave-induced transport. To better capture these dynamics, we track the trajectories of 12 novel, ultra-thin surface drifters deployed in the southern North Sea over 68 d. We adopt a data-driven approach to model drifter velocity using hydrodynamic and atmospheric data, applying both a linear leeway parameterisation and two machine learning models: random forest and support vector regression. Machine learning model-agnostic interpretation techniques reveal that tidal forcing predominantly drives zonal motion, whereas wind is the main driver in the meridional direction in this region. Notably, the wind exhibits a saturation effect, and its contribution to explaining the variance of the drifter velocity decreases at higher speeds. In trajectory prediction experiments, we find that machine learning models, particularly random forest, outperform linear models, with the latter achieving comparable accuracy only at short time scales. Using a hybrid approach and deriving a non-linear function of the wind from machine learning interpretable methods to include in the leeway parameterisation significantly improves the model prediction of the drifter trajectory. Finally, we test the generalisability of the North Sea-trained models using an independent drifter dataset from the Tyrrhenian Sea. Despite the differences in ocean dynamics between the regions, the machine learning models reproduce the observed trajectories with comparable accuracy to state-of-the-art studies, demonstrating robust explanatory skill and a low degree of overfitting in this instance.

Internal tides on the Al-Batinah shelf: evolution, structure and predictability - Gerd A. Bruss; Estel Font; Bastien Y. Queste; Rob A. Hall (Ocean Science, 2026)
Internal tides are a key mechanism of energy transfer on continental shelves. We present observations of internal tides on the northern Oman shelf based on moored temperature and velocity records collected during 2021–2022. The regional shelf exhibits strong shallow summer stratification (Npeak2∼10-2s-2), supporting shoreward-propagating internal tides with pronounced fortnightly modulation in amplitude and energy fluxes. Despite semidiurnal dominance in barotropic forcing, the internal tides appear predominantly in the diurnal band. During summer, the incoming internal tide energy flux at our mooring on the central shelf (23 m depth, 10 km from shore), ranges around FEin∼5–15 W m−1 and we estimate that the onset of main internal-tide dissipation occurs further inshore over the shallow inner shelf. As the thermocline deepens in fall, the onset of the saturation zone migrates seaward from its summer location to reach our central-shelf mooring by early October. There, waveform structures undergo seasonal transition from quasi-linear depression waves in summer to increasingly nonlinear features in fall, including steepening, asymmetry, polarity reversal and a shift toward first-mode dominance . In October FEin increases to ∼ 30 W m−1 primarily driven by the increasing nonlinearity as the internal tides reach their finite-depth shoaling regime over the central shelf. Cross-shelf coherence and phase-speed estimates also confirm that the observed internal tides maintain spatial coherence from the shelf edge inshore beyond the typical internal surf zone. Skill scores indicate that the predictability of the local internal tides decreases by 50 % after around 18 d, which is comparable to high-predictability sites globally. Inshore-directed energy flux, diurnal dominance and multi day phase lags to barotropic forcing still indicate remote generation. For the local barotropic tidal currents, variability in bulk KE and predictability-scores suggest seasonal modulation by the internal tides.

Reduced cooling in the Norwegian Atlantic Slope Current: investigating mechanisms of change from 30 years of observations - Till M. Baumann; Øystein Skagseth; Randi B. Ingvaldsen; Kjell Arne Mork (Ocean Science, 2026)
The Norwegian Atlantic Current (NwAC) is a principal conduit for poleward heat and salt transport within the Atlantic Meridional Overturning Circulation (AMOC) and plays a key role of water mass transformation in the Nordic Seas. Its variability exerts a critical influence on high-latitude climate, Arctic Ocean inflows, and deep-water formation in the Nordic Seas. This study presents a comprehensive analysis of a 30-year (1993–2022) hydrographic dataset from four repeat sections across the NwAC, spanning from the southern Norwegian Sea (62.8° N) to Bjørnøya (74.5° N). Hydrographic measurements of temperature and salinity, along with derived relative geostrophic velocities, were combined with surface geostrophic currents from satellite altimetry to obtain absolute geostrophic velocities throughout the water column at each section. This allows us to robustly define the current core of the NwAC and assess its properties. The data reveal substantial variability in water properties and transport across seasonal to multiannual timescales, alongside significant warming trends. Atlantic Water (AW) cools and freshens during its journey along the Norwegian coast, but our analysis shows that north of Lofoten (69° N), the cooling has reduced by 0.11–0.13 °C per decade. We examine three potential drivers of this reduced cooling: (1) decreased air-sea heat fluxes, (2) reduced lateral heat loss due to decreasing eddy-activity, and (3) increased advection speed within the current core. We find no evidence for any changes in eddy kinetic energy, but both increased advection speed and reduced air-sea heat loss may contribute to the observed decline in cooling. Simple box model estimates suggest that while neither of the two factors can explain all variability observed in the cooling north of Lofoten, changed heat fluxes can quantitatively account for the long term trends. Our results imply a northward amplification of AW warming along the northern rim of the Atlantic Overturning Circulation.

Internal tide loss of coherence in a realistic simulation of the North Atlantic - Adrien Bella; Noé Lahaye; Gilles Tissot (Ocean Science, 2026)
The loss of coherence of the semidiurnal internal tide is investigated using a high-resolution realistic numerical simulation over the North Atlantic. The analysis focuses on processes resulting from the interaction between the internal tide and the mesoscale background flow at time scales typically shorter than one month. To this end, a theoretical framework based on vertical mode decomposition and the splitting of the internal tide signal into coherent and incoherent components is developed and applied to the outputs of the numerical simulation. This framework enables the transfer terms between the coherent and incoherent parts, and between the different vertical modes – and therefore horizontal scales – of the internal tides to be evaluated. By focusing on three subdomains with contrasting dynamics, we demonstrate that coherent-to-incoherent energy transfers significantly impact the internal tide energy budget. These transfers are dominated by advection by slowly varying flows and mainly occur without changing the vertical mode of the internal tide involved. This is attributed to the dominance of the barotropic and first baroclinic modes in the mesoscale flow combined with the structure of the mesoscale flow/internal tide interaction terms. Typical energy transfer rates are of the order of a few tens of days in the Gulf Stream region and a few hundred days in the Azores for the mode 1 internal tide.

Tracing suspended sediment fluxes using a glider: observations in a tidal shelf environment - Sabrina Homrani; Orens Pasqueron de Fommervault; Mathieu Gentil; Frédéric Jourdin; Xavier Durrieu de Madron; François Bourrin (Ocean Science, 2025)
Underwater gliders equipped with current profilers and optical turbidity sensors offer a low-energy solution for high-resolution measurements of currents, suspended particle properties, and sediment transport in coastal waters. Because the spatial structure of hydrosedimentary processes often changes on short time scales (hours to weeks), especially in coastal areas, validating the distribution of glider observations is required to assess our capacity to represent hydrosedimentary processes. Here we propose to validate in a shelf tide-dominated environment, both (i) glider-based currents, and (ii) glider-based acoustic backscatters and optical turbidities in full resolution delayed mode, using in situ collocated and synchronous ancillary observations. The deployed glider system correctly measures the periodic pattern of the tidal current, with a RMSD of O(3 cm s−1), demonstrating the system's ability to accurately capture tidal variability. Glider optical turbidities highly correlate with the ancillary observations (R2 up to 0.83). They also correlate well with their glider acoustic counterpart for most of the campaign period (R2=0.76), allowing an estimation of suspended particulate matter concentrations from acoustic measurements. Hence, the glider could observe not only the presence of bottom nepheloid layers of several mg L−1 but also residual fluxes of the order of 1 gm-1s-1 on the shelf. These results highlight the potential of gliders for quantifying sediment fluxes and advancing our understanding of coastal hydrosedimentary processes.

Estimating the AMOC from Argo profiles with machine learning trained on ocean simulations - Yannick Wölker; Willi Rath; Matthias Renz; Arne Biastoch (Ocean Science, 2025)
The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in our climate system, continuous monitoring is important and could be enhanced by combining all available information. Moored measuring arrays like RAPID divide the AMOC in near-surface contributions, western-boundary currents, and the deep ocean in the interior of the basin. For the deep-ocean component, moorings measure density and focus on the calculation through geostrophy. These moored devices come with a high maintenance effort. Existing reconstruction studies show success with near-surface variables on monthly time scales, but do not focus on the interior transport. For interannual to decadal time scales, the geostrophic contribution becomes an important contribution. Argo floats could provide required information about the geostrophic circulation as they continuously and cost-effective deliver hydrographic profiles. But they are spatially unstructured and only report instantaneous values. Here we show that the geostrophic part of the AMOC can be data-drivenly reconstructed by Argo profiles. To demonstrate this, we use a realistic and physically consistent high-resolution model VIKING20X. By simulating virtual Argo floats, we demonstrate that a learnable binning method to process the spatially variable Argo float distribution is able to reconstruct the geostrophic part of the VIKING20X AMOC by up to 80 % explained variance and a mean error of less than one Sverdrup for the geostrophic transport. Using methods of explainable AI we investigate the importance of our input components showing an increasing importance of the Argo profiles on seasonal and interannual timescales, validating the usefulness of the Argo floats for the reconstruction. Our results demonstrate how an AMOC reconstruction from unstructured Argo profiles could replace estimates of the geostrophic deep-ocean component of the AMOC from the RAPID Array in the context of high-resolution ocean and climate models.

The North Atlantic mean state in mesoscale eddy-resolving coupled models: a multimodel study - Amanda Frigola; Eneko Martin-Martinez; Eduardo Moreno-Chamarro; Margarida Samsó; Saskia Loosvelt-Tomas; Pierre-Antoine Bretonnière; Daria Kuznetsova; Xia Lin; Pablo Ortega (Ocean Science, 2025)
Ocean mesoscale structures, which are parameterized in models with standard resolutions on the order of 1° or coarser, have an impact at larger scales, affecting the ocean mean state and circulation. Here we study the effects of increasing model ocean resolution to mesoscale eddy-resolving scales on the representation of the North Atlantic mean state, by comparing an ensemble of four HighResMIP coupled historical simulations with nominal ocean resolutions of at least 1/10° – corresponding to the models CESM1-CAM5-SE-HR, EC-Earth3P-VHR, HadGEM3-GC31-HH, and MPI-ESM1-2-ER – to a baseline of 39 Coupled Model Intercomparison Project phase 6 (CMIP6) simulations at coarser resolution. We find an improved representation of the Gulf Stream (GS) structure and position in the mesoscale-resolving ensemble, which leads to significantly reduced surface temperature and salinity biases north of Cape Hatteras (NCH). While higher resolution lessens the mean cold–fresh surface biases in the Central North Atlantic (CNA), the improvement is not statistically significant, as some mesoscale-resolving models still present an overly weak North Atlantic Current (NAC). Important differences also occur in the Labrador (LS) and western Irminger Seas (IS). Although the mesoscale-resolving ensemble exhibits larger warm and salty local biases at the surface compared to the low-resolution one, its full-depth profile reveals significantly weaker vertical stratification in the area, closer to observations. This reduced stratification in the high-resolution ensemble is consistent with the presence of stronger (although not significantly stronger) deep water convection in the region. While in the LS the wide range of MLD observational estimates makes model assessment challenging, in the Nordic Seas and along the East Greenland Current, convection in the high-resolution model ensemble is in better agreement with observational records, compared to the low-resolution ensemble. Another clear improvement in the mesoscale-resolving ensemble is found for the representation of the Atlantic overturning in depth-space, which is significantly closer to RAPID observations at 26.5° N than in the low-resolution counterpart; however, it still remains too shallow compared to observations and reanalyses. The subpolar gyre (SPG), as characterized by the barotropic streamfunction, is not significantly stronger in the higher resolution ensemble, although it presents a narrower and locally stronger boundary current.

Drivers of seasonal hydrography in Disko Bay, Greenland - Linda Latuta; Lars H. Smedsrud; Elin Darelius; Per Juel Hansen; Josh K. Willis (Ocean Science, 2025)
This study investigates the seasonal dynamics of Disko Bay (Qeqertarsuup Tunua) in west Greenland. On the eastern side, the bay's hydrography is influenced by ice-ocean interactions and exchange with Ilulissat Icefjord (Kangiata Sullua), while on the western side, the bay exchanges waters with Baffin Bay. Since the mid-1990s, this region has experienced ocean warming, sea-ice decline, and the retreat of Greenland's fastest-flowing marine-terminating glacier. Although West Greenland Irminger Water (WGIW) is known to be a significant heat source behind these changes, the timing and pathways of its entry into Disko Bay remain poorly understood. We present a two-year (2022–2024) observational record of Disko Bay hydrography, providing new insights into the seasonal evolution and spatial structure of Polar Water (PW) and WGIW. Each spring, dense WGIW crosses the topographic barrier between Baffin Bay and Disko Bay, filling the Disko Bay basin and producing the highest observed temperature and density at depth. The PW–WGIW boundary shoals to depths shallow enough for WGIW to renew the Ilulissat Icefjord basin. In autumn/winter 2022, an additional episodic renewal coincided with strong upwelling-favourable winds along the west Greenland shelf. While WGIW renewal dominates winter and spring seasonality (a period also marked by sea-ice presence), summer and autumn hydrography are shaped by PW. With the onset of the melt season, a fresh stratified layer forms in the upper 50 m and progressively thickens and extends downward, continuing to freshen and cool through autumn. Beneath this layer, denser PW warms steadily along isopycnals, with spatial analyses indicating an advective pathway transporting this warming signal along the bay's periphery.

Diel variability affects the inorganic carbon system in the sea-surface microlayer and influences air-sea CO 2 flux estimates - Ander López-Puertas; Oliver Wurl; Sanja Frka; Mariana Ribas-Ribas (Ocean Science, 2025)
The ocean plays a crucial role in the global carbon cycle by absorbing and storing about one-third of anthropogenic carbon dioxide (CO2). It is estimated that the ocean has sequestered approximately 26 % of CO2 emissions over the last decade, resulting in significant changes in the marine carbon system and impacting the marine environment. The sea-surface microlayer (SML) plays a crucial role in these processes, facilitating the transfer of matter and energy between the ocean and the atmosphere. However, most studies on the carbon cycle in the SML have primarily addressed daily variability and overlooked nocturnal processes, which may lead to inaccurate global carbon estimates. We analysed temperature, salinity, pHT25, and pCO2 using data collected over three complete diel cycles during an oceanographic campaign along the Croatian coast near Šibenik in the Middle Adriatic. Our analysis revealed statistically significant differences (p

Silicification in the ocean: from molecular pathways to silicifiers' ecology and biogeochemical cycles - Ivia Closset; J. Jotautas Baronas; Fiorenza Torricella; Félix de Tombeur; Bianca T. P. Liguori; Alessandra Petrucciani; Natasha Bryan; María López-Acosta; Yelena Churakova; Antonia U. Thielecke; Zhouling Zhang; Natalia Llopis Monferrer; et al. (Ocean Science, 2025)
The oceanic silicon (Si) cycle has undergone a profound transformation from an abiotic system in the Precambrian to a biologically regulated cycle driven by siliceous organisms such as diatoms, Rhizaria, and sponges. These organisms actively uptake Si using specialized proteins to transport and polymerize it into amorphous silica through the process of biosilicification. This biological control varies depending on environmental conditions, influencing both the rate of silicification and its ecological function, including structural support, defence, and stress mitigation. Evidence suggests that silicification has evolved multiple times independently across different taxa, each developing distinct molecular mechanisms for Si handling. This review identifies major gaps in our understanding of biosilicification, particularly among lesser-known silicifiers beyond traditional model organisms like diatoms. It emphasizes the ecological significance of these underexplored taxa and synthesizes current knowledge of molecular pathways involved in Si uptake and polymerization. By comparing biosilicification strategies across taxa, this review calls for expanding the repertoire of model organisms and leveraging new advanced tools to uncover Si transport mechanisms, efflux regulation, and environmental responses. It also emphasizes the need to integrate biological and geological perspectives, both to refine palaeoceanographic proxies and to improve the interpretation of microfossil records and present-day biogeochemical models. On a global scale, Si enters the ocean primarily via terrestrial weathering and is removed through burial in sediments and/or authigenic clay formation. While open-ocean processes are relatively well studied, dynamic boundary zones – where land, sediments, and ice interact with seawater – are increasingly recognized as key interfaces regulating global Si fluxes, though they remain poorly understood. Therefore, special attention is given to the role of dynamic boundary zones such as the interfaces between land and ocean, the benthic zone, and the cryosphere, which are often overlooked yet play critical roles in controlling Si cycling. By bringing together cross-discipline insights, this review proposes a new integrated framework for understanding the complex biological and biogeochemical dimensions of the oceanic Si cycle. This integrated perspective is essential for improving global Si budget estimates, predicting climate-driven changes in marine productivity, and assessing the role of Si in modulating Earth's long-term carbon balance.

Microstructure observations and mixing parameterizations along an Atlantic transect in very weak turbulence - Niek Kusters; Sjoerd Groeskamp; Bieito Fernandez Castro; Hans van Haren (Ocean Science, 2025)
Microstructure measurements of shear and temperature can be used to calculate ocean turbulent dissipation rates and diffusivities. Here microstructure observations are taken along an transect in the North Atlantic, that includes observations of very weak deep ocean turbulence. In this paper we show the necessity of using the thermistor probes, instead of the more common shear probes, to calculate dissipation rates when they are smaller than 1×10-10 W kg−1. Profiles of combined dissipation rates from the shear and thermistor probes are then compared to the finescale strain parameterization and Thorpe sorting method. Based on this comparison, recommendations and restrictions are suggested for applying both parameterizations in a weakly turbulent environment. The results indicate that temperature-based strain provides improved estimates of dissipation rates in the deep ocean where density gradients are small, while density-based strain provides better results otherwise. We find that Thorpe based estimates are very accurate when pre-existing knowledge of the turbulent kinetic energy dissipation rate ε is used. When this knowledge is not available, using climatological mean estimates of ε can allow for more detailed estimates of dissipation by applying the Thorpe resorting method. Finally, we employ the triple decomposition framework to get more insights in the relative roles of dianeutral and isoneutral mixing processes, and use this to calculate the dianeutral and isoneutral diffusivities. It turns out that the triple decomposition is generally not a good predictor of the isoneutral diffusivity. Overall, this paper has assessed the potential of direct observations and parameterizations of dissipation and showed that dissipation rates can be estimated quite well within a factor 5 between different methods, but it becomes difficult to achieve higher accuracy.

Observations of cross-shelf transport due to internal wave pumping on the Bay of Biscay shelf - Adèle Moncuquet; Nicole L. Jones; Lucie Bordois; François Dufois; Pascal Lazure (Ocean Science, 2025)
Coastal cross-shelf transport drives the redistribution of sediment, nutrients and pollutants on continental shelves. Here, the cross-shelf volume flux is quantified from in situ measurements at two coastal sites on the Bay of Biscay (BoB) shelf. At both sites, a semidiurnal internal tide propagates onshore, and mode 1 nonlinear internal wave packets are observed. The Eulerian and Stokes drift contributions to the subtidal cross-shelf transport are estimated along density layers from ADCP, temperature sensors, and CTD measurements at 62 m water depth on the Landes plateau or SE-BoB (44° N) and 47 m water depth on the Armorican shelf or N-BoB (47° N). The Stokes drift transport has possible contributions from the internal tides, nonlinear internal waves and the surface waves and tide. At both sites, the vertical profile of the month-long averaged Stokes drift volume flux matches the shape of the semi-analytical Stokes drift volume flux due to a linear mode-1 internal tide. We demonstrate that nonlinear internal wave events can also contribute to the Stokes drift volume flux. We thereby attribute the Stokes drift volume flux at the study sites to internal wave pumping (IWP). At both sites, the IWP is responsible for a near-seabed onshore volume flux during stratified conditions and spring internal tides, that is equivalent to a wind-driven upwelling event generated by a 4 m s−1 wind. At N-BoB (47° N), IWP is the main contributor to the total cross-shelf volume flux under stratified conditions and a spring internal tide. At SE-BoB (44° N), IWP augments the near-seabed onshore volume flux during upwelling events and maintains a near-seabed onshore volume flux even during downwelling events.

Interannual variability of summertime cross-isobath exchanges in the northern South China Sea: ENSO and riverine influences - Yunping Song; Yuxin Lin; Peng Zhan; Zhiqiang Liu; Zhongya Cai (Ocean Science, 2025)
This study, combining long-term observations and high-resolution simulations, investigates the interannual variability of summer shelf circulation in the Northern South China Sea (NSCS) from 2000 to 2022. We elucidate the responses of NSCS shelf circulation to ENSO and Pearl River Estuary (PRE) freshwater runoff, revealing distinct spatial and mechanistic signatures. During El Niño years, a pronounced sea level anomaly dipole forms between the central and southern South China Sea, intensifying northward geostrophic currents in the southern basin and modulating Kuroshio intrusions. Simultaneously, an amplified PRE plume extends eastward to the 100 m isobath, markedly reducing nearshore salinity. Analysis of depth-integrated vorticity equations indicates that the pressure gradient force – driven by the joint effect of baroclinicity and bottom relief (JEBAR) and bottom pressure gradients – governs NSCS circulation variability. In coastal regions, cross-isobath velocity anomalies are primarily controlled by bottom stress curl and nonlinear vorticity advection, whereas JEBAR dominates offshore dynamics beyond the 100 m isobath. During El Niño summers, bottom density anomalies generate positive cross-isobath velocity anomalies through JEBAR, partially offset by negative anomalies from altered vertical stratification, sustaining a meandering shelf current. These results highlight the interplay of regional and remote forcings, advancing understanding of NSCS hydrographic dynamics.

Observations of turbulent mixing in the Dotson Ice Shelf cavity - Maren Elisabeth Richter; Karen J. Heywood; Rob A. Hall; Peter E. D. Davis (Ocean Science, 2025)
Dotson Ice Shelf (DIS) is located in the Amundsen Sea sector of Antarctica, an area of rapid glacial mass loss due to ocean-driven basal melting. Here, warm Circumpolar Deep Water is transported onto the continental shelf and can access ice shelf cavities and deep grounding lines, causing melting, glacial retreat and thus sea level rise. The circulation of this warm water, and the heat transport within ice shelf cavities, remains mostly unknown. We collected data from over 100 km of dive tracks along the seabed under DIS using an autonomous vehicle, AutoSub Long Range. This study presents observations of ocean velocity, turbulent kinetic energy dissipation rate (ε) from microstructure measurements, and heat flux calculations. Rates of background mixing are ε≈10-10 W kg−1 with patches of higher mixing of ε≈10-8 W kg−1. Higher turbulent kinetic energy dissipation rate is associated with stronger along-slope currents, high vertical current shear, steeper bathymetry, and positive temperature anomalies. Average vertical heat fluxes are on the order of 0.1 W m−2 and maximum heat fluxes reach 52 W m−2. This is compared to the 59 to 176 W m−2 needed to maintain observed average basal melt rates at DIS. Turbulent mixing is higher in the fast-flowing inflow region and over rough topography. We show a highly complex spatial pattern of turbulent mixing and of bottom topography. The bottom topography is currently not resolved in bathymetry products and both the topography and turbulent mixing are currently not resolved in models of ice-shelf–ocean interactions. The levels of turbulent mixing experienced by the warm mCDW inflow to the DIS will lead to negligible loss of heat during its path to the grounding line, leaving plenty of heat available to melt the ice shelf base there. Higher average vertical heat fluxes than observed here must occur in areas of the cavity not accessed in this study.

Changes in water mass composition and circulation in the central Arctic Ocean between 2011 and 2021 inferred from tracer observations - Anne-Marie Wefing; Annabel Payne; Marcel Scheiwiller; Christof Vockenhuber; Marcus Christl; Toste Tanhua; Núria Casacuberta (Ocean Science, 2025)
The Arctic Ocean is changing rapidly and Atlantic Water circulation plays a key role in the warming, sea-ice decline, and ecosystem changes observed in the Arctic. Still, we only have limited understanding of the pathways and circulation times of Atlantic-derived water both at surface and mid-depth layers in the Arctic Ocean, and their evolution over time. Here, we investigate the water mass composition and circulation in the central Arctic Ocean in 2021 and assess temporal changes thereof between 2011 and 2021 by using the long-lived anthropogenic radionuclides 129I and 236U. This study is based on radionuclide data collected in the central Arctic Ocean and for the first time north of Greenland as part of one of the ocean expeditions of the Synoptic Arctic Survey (SAS-Oden 2021), and available historic data across the Arctic Ocean between 2011 and 2021. We obtain tracer ages as well as the mixing of different endmembers in the surface layer using a tracer-based mixing model. Atlantic Water circulation times and mixing in the mid-depth Atlantic layer are obtained from the Transit Time Distribution (TTD) model. For 2021, we find a sharp decrease in surface 129I and 236U concentrations between the Amundsen and Makarov Basins, pointing to substantial fractions of Pacific Water reaching the Lomonosov Ridge from the Amerasian side. In the halocline layer, similar 129I and 236U concentrations on both sides of the Lomonosov Ridge suggest a common formation region of halocline waters with a clear Atlantic Water signal. North of Greenland, we find a mixture of waters that originate from the Canada and Amundsen Basins, both in the surface and the mid-depth layer. Circulation times of Atlantic Water in the mid-depth layer point to a longer transport route on the Makarov Basin side of the Lomonosov Ridge compared to the Amundsen Basin. When looking at the temporal variability between 2011 and 2021, we observe a shift of the Atlantic-Pacific Water front from the Makarov Basin towards the Lomonosov Ridge from 2011/2012 to 2015 and 2021. In the mid-depth Atlantic layer, we find an increase in mean and mode ages from 2015 to 2021, suggesting a slowdown or changes in the pathways of the Arctic Ocean Boundary Current, which is in line with recent studies based on gas tracers.

Modelling river-sea continuum: the case of the Danube Delta - Christian Ferrarin; Debora Bellafiore; Alejandro Paladio Hernandez; Irina Dinu; Adrian Stanica (Ocean Science, 2025)
Understanding water transport and circulation in coastal seas and transitional environments is a key focus of oceanographic and climate research, particularly in recognizing the role of the land-sea interface. The Danube Delta serves as a natural laboratory for river-sea hydrodynamic modelling due to its complex morphology, composed of multiple river branches, channels, and lagoons. Moreover, this coastal environment is subjected to various natural and anthropogenic stressors, and numerical modelling can provide a scientific basis for assessing the impact of human activities. In this work, the SHYFEM finite element hydrodynamic model was applied to the entire river-sea continuum of the Danube Delta region to describe the transport and mixing processes within and between the interconnected water bodies forming the delta. The model was run for the period 2015-2019 and enabled the characterization of: (1) water discharge distribution among the river branches; (2) general hydrodynamic characteristics of the coastal region of freshwater influence; (3) transport time scale of the Razelm Sinoie Lagoon System. Finally, the Danube Delta modelling tool was used to evaluate the potential effects of hydrological reconnection (restoration) measures in the Razelm Sinoie Lagoon System aimed at improving connectivity and water renewal.

Enhancing coastal winds and surface ocean currents with deep learning for short-term wave forecasting - Manuel García-León; José María García-Valdecasas; Lotfi Aouf; Alice Dalphinet; Juan Asensio; Stefania Angela Ciliberti; Breogán Gómez; Víctor Aquino; Roland Aznar; Marcos Sotillo (Ocean Science, 2025)
Accurate short-term wave forecasts are crucial for numerous maritime activities. Wind and surface currents, the primary forcings for spectral wave models, directly influence forecast accuracy. While remote sensing technologies like Satellite Synthetic Aperture Radar (SAR) and High Frequency Radar (HFR) provide high-resolution spatio-temporal data, their integration into operational ocean forecasting remains challenging. This contribution proposes a methodology for improving these operational forcings by correcting them with Artificial Neural Networks (ANNs). These ANNs leverage remote sensing data as targets, learning complex spatial patterns from the existing forcing fields used as predictors. The methodology has been tested at three pilot sites in the Iberian–Biscay–Ireland region: (i) Galicia, (ii) Tarragona and (iii) Gran Canaria. Using SAR as a reference, the ANN corrected winds present Root Mean Square Deviation (RMSD) reductions close to 35 % respect to ECMWF-IFS, and improvements close to 3 % for the scatter-index. Surface currents are also improved with ANNs, reaching speed and directional biases close to 2 cm s−1 and 6° and correlation close to 35 % and 50 %, respectively. Using these ANN forcings in a regional spectral wave model (Copernicus Marine IBI-WAV NRT) leads to improvements in the Wave Height (Hm0) bias and RMSD around 10 % and 5 % at the NE Atlantic. Mean wave period (Tm02) also improves, with reductions of 17 % and 5 % in bias and RMSD. Preliminary moderate improvements were also present in extreme events (e.g. storm Arwen at Galicia, November 2021), as the Hm0 was corrected close to 0.5 m and Tm02 by around 0.4 s. However, properly quantifying this impact requires further assessment.

Local and remote climatic drivers of extreme summer sea surface temperatures in the Arabian Gulf - Zouhair Lachkar; Olivier Pauluis; Francesco Paparella; Basit Khan; John A. Burt (Ocean Science, 2025)
The Arabian Gulf (also known as the Persian Gulf; hereafter simply the Gulf) is a shallow, subtropical, semi-enclosed sea that experiences the highest average summer sea surface temperatures (SSTs) in the global ocean. While local marine organisms have adapted to these extreme conditions, interannual temperature fluctuations result in occasional marine heatwaves (MHWs) that trigger mass coral bleaching and other ecological impacts. However, the climatic drivers of this variability remain poorly understood. In this study, we investigate the sources and mechanisms behind extreme summer SSTs in the Gulf. Analyzing a regional eddy-resolving ocean hindcast simulation combined with ERA5 reanalysis data, we find that extreme summer surface temperatures are tightly linked to lower-than-normal surface pressure over the Arabian Peninsula and higher-than-normal surface pressure over Iran and Pakistan. These pressure anomalies are typically associated with stronger monsoonal winds in the western Arabian Sea and weakened local winds. Enhanced monsoon circulation leads to (i) increased evaporation over the Arabian Sea and greater moisture transport into the Gulf in the lower troposphere, trapping heat near the surface, and (ii) enhanced subsidence over the Arabian Peninsula and the Gulf in the upper troposphere, inducing further surface heat retention. Meanwhile, weakened local Shamal winds – particularly in the northern Gulf – reduce evaporative cooling and facilitate the accumulation of moist, warm air, amplifying local warming. These regional atmospheric changes are strongly modulated by large-scale climate variability modes, with El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) together explaining over 50 % of the observed interannual SST variability in the Gulf. La Niña (El Niño) and the negative (positive) phase of the NAO both favor weaker (stronger) Shamal winds and warmer (cooler) SSTs during the peak summer months over the Gulf. Additionally, La Niña (El Niño) is associated with stronger (weaker) monsoon winds in the southern and western Arabian Sea, leading to stronger and more persistent warming (cooling) anomalies in the Gulf. The influence of these teleconnections is additive, with the warmest summers occurring when La Niña and negative NAO phases coincide. These findings have implications for the predictability of summer MHWs and associated ecosystem risks in the Gulf.

Surface evolution and wind effects during a cyclonic eddy splitting event in the Balearic Sea - Sebastien Donnet; Helga S. Huntley; Maristella Berta; Luca Centurioni; Leo Middleton; Tamay Özgökmen; Pierre-Marie Poulain; Alex Kinsella; Annalisa Griffa (Ocean Science, 2025)
During the period of 23–28 February 2022, a cyclonic eddy in the Balearic Sea was observed to split into smaller eddies. Serendipitously, a wealth of data was collected of the event, including satellite chlorophyll maps, Lagrangian drifters at several depths, hydrographic sections intersecting the splitting eddy, and wind speed and direction. Sufficiently many drifters were in the area to estimate kinematic properties (divergence, vorticity, and strain rate) from clusters along the edge of the eddy before and during the elongation period that led to the splitting. The vertical velocity w can be computed from colocated divergence values from surface drifters (CARTHE and CODE, within the top meter) and near-surface drifters (SVP, at 15 m depth). Together, the observations delineate the process of eddy elongation, leading to vorticity and strain-rate intensification on 25 February, followed by the collapse of the ridge in the center of the eddy and the emergence of smaller eddies on 26 February, terminating with the splitting into submesoscale cyclones on 28 February. Dense drifter observations and daily hydrography supplement the remotely sensed descriptive view of the eddy splitting process. In particular, they confirm dominant internal dynamics, consistent with isopycnal doming, but also point to a role played by the winds, which shifted from predominantly southwesterly to predominantly northeasterly and strengthened significantly before weakening again in the area of interest during the eddy splitting period. Nonlinear Ekman pumping wEknl is estimated from the wind data and drifter-derived vorticities to capture the contribution of the wind effects to the patterns of up- and downwelling accompanying the eddy splitting. The wEknl patterns are consistent with the drifter-based w and divergence estimates. Moreover, the nonlinear Ekman pumping is found to be of the same order of magnitude as (though generally less than) w, suggesting that the wind likely influences the observed surface processes.

Dense shelf water and associated sediment transport in the Cap de Creus Canyon and adjacent shelf under mild winter regimes: insights from the 2021–2022 winter - Marta Arjona-Camas; Xavier Durrieu de Madron; François Bourrin; Helena Fos; Anna Sanchez-Vidal; David Amblas (Ocean Science, 2025)
Dense shelf water cascading (DSWC) is a key oceanographic process in transferring energy and matter from continental shelves to deep ocean areas. Although intense DSWC (IDSWC) events have received most attention due to their large impacts, mild DSWC (MDSWC) events are the most frequent in the northwestern Mediterranean and are expected to become more common under climate change. However, their dynamics, particularly in the Cap de Creus Canyon, have been less comprehensively described and compared to strong-winter events. This study investigates MDSWC in the Cap de Creus Canyon during the mild winter of 2021–2022, examining shelf-canyon transports of both dense shelf waters and suspended particulate matter (SPM). Observations from the FARDWO-CCC1 multiplatform cruise in March 2022 revealed the presence of cold, dense, and turbid shelf waters, enriched in dissolved oxygen and chlorophyll a, on the continental shelf adjacent to the canyon. These waters cascaded into the canyon head and progressed further into the canyon along its southern flank to ∼ 390 m depth. Estimated water and SPM transports during this event were 0.7 Sv and 105 metric tons (t), respectively, at the continental shelf. Within the canyon, transports were 0.3 Sv and 105 metric tons in the upper section, while mid-canyon transports were lower (0.05 Sv and 104, respectively), indicating that most dense shelf waters likely remained confined to the shelf area and upper canyon. During this event, dense shelf waters were transported ∼ 30 km from the shelf into the canyon. Our results show that significant transport of dense shelf waters (260 km3) and suspended sediment can occur in the Cap de Creus Canyon during MDSWC events under mild winters, also contributing significantly to the formation of Western Intermediate Water (WIW) in the canyon. The Mediterranean Sea Physics reanalysis data indicate that the cascading season lasted from late-January to mid-March 2022, with several shallow cascading pulses throughout this period. Peak transport occurred in mid-March associated with an eastern storm, which likely intensified MDSWC in the canyon. Our study reinforces the idea that dense shelf water transports exhibit marked interannual variability, even under mild winters.

Deriving hourly diagnostic surface velocity fields considering inertia and an application in the Yellow Sea - Sung-Won Cho; Jang-Geun Choi; Deoksu Kim; Wenfang Lu; Young-Heon Jo (Ocean Science, 2025)
Surface currents play an important role in the transport of floating materials in the Yellow Sea, a region strongly influenced by tidal forcing and seasonal wind variability driven by the East Asian monsoon. While diagnostic models have been widely used to estimate surface currents, due to their steady-state assumption, high frequency variations such as tides and inertial oscillations cannot be resolved. To address this limitation, a time-dependent diagnostic model incorporating inertial terms into the governing equations is proposed. The performance of the proposed method is evaluated using buoy and drifter observations from 2015 to 2023. The time-dependent model in this study captures not only low frequency components (geostrophic and Ekman currents) but also high frequency variability (inertial oscillations and tides). Compared to the traditional model assuming steady-state, it shows significant improvement, achieving a correlation of 0.76 and Root-Mean-Square Error of 0.18 m s−1 (compared to −0.08 and 0.43 m s−1 for the steady model, that caused by wrong governing equation ignoring inertia to describe tides) because of successful consideration of high frequency variability. The decay rate of inertial oscillations is analytically derived, providing insight into the time scale for past signals in surface currents to dissipate. We expect that this study offers a practical framework for surface current estimation considering both high and low frequency signals and can be applied for quick assessments of material transport in other coastal oceans.

Wave-driven amplification of surf-zone bottom stress on rough seabeds - Damien Sous; Marc Pezerat; Solène Dealbera; Héloïse Michaud; Denis Morichon (Ocean Science, 2025)
The present paper proposes a unified view of the wave-driven amplification of the wave-averaged bottom shear stress in rough seabed contexts, covering both co- and opposing wave/current cases. The analysis is first based on a series of field observations performed over the Flysch rocky shore platform of Socoa. The momentum balance is examined locally, separating the net effect of the waves on the depth- and wave-averaged momentum budget, based on velocity and pressure measurements. The present observations confirm that, in the presence of complex seabed topography, the bed shear stress is an important component of the momentum balance. The results highlight two distinct regimes depending on the breaking activity due to the intricate composition between waves and mean currents in the wave averaged shear stress. In moderately developed undertow conditions, the bottom stress brings a negative contribution to the wave momentum balance, and acts to to promote wave setdown, while in conditions of depth-limited wave breaking saturation the bed friction acts to increase the wave setup. A novel empirical parameterization of the mean bottom stress under combined waves and current is proposed. The in-situ findings are complemented by a series of wave-resolving simulations on idealized closed and open beaches, confirming the complex effect of waves on the time-averaged water circulation.

An overview of the ocean data ecosystem - Maya Bloch Haimson; Yoav Lehahn; Tomer Sagi (Ocean Science, 2025)
The oceans, covering approximately 70 % of Earth's surface, play a pivotal role in climate regulation, biodiversity, and biogeochemical processes. The large and growing volume and complexity of ocean data, spanning diverse disciplines and formats, and dispersed across a wide range of sources, presents opportunities and challenges for advancing scientific research, informing policy, and addressing societal needs. In this review paper we aim to create an easy-to-navigate map of the field of ocean data, enabling the reader to establish a broad understanding of the ocean data sector, and bridging gaps between different disciplines and levels of familiarity with ocean data. This is done through the concept of the “data ecosystem”, which is used to describe the actors, organizations, and infrastructures involved in all aspects of the data value chain. We propose a structured ocean data ecosystem model as a method for comprehensive mapping of the ocean data market landscape. The proposed model consists of five key elements: stakeholders, societal elements, interoperability tools (such as standards and best practices), data sources and product offering, and emerging solutions. We provide an up-to-date analysis of ocean data sources and emerging solutions and a summary of relevant data standardization efforts such as marine standards, vocabularies, and ontologies. All this will promote the development of needs-based solutions, components, products, services, and technologies, thus contributing to the evolution of the ocean data ecosystem and promoting data-based ocean research.

Blank variability in coulometric measurements of dissolved inorganic carbon - Matthew P. Humphreys; Sharyn Ossebaar (Ocean Science, 2025)
Marine dissolved inorganic carbon (DIC) is by far the largest pool of carbon in the Earth surface system that exchanges with the atmosphere on human-relevant timescales. Measurements of DIC are therefore necessary to study the changing marine carbon cycle. The most accurate routine DIC measurement method is coulometry. In this method, the signal detected by a coulometer for each measurement must be corrected for background noise, which is termed the blank. The current best practice recommendation is to measure the blank once per analysis session and use this constant value to correct all measurements. However, calculating the blank for each measurement separately shows that the blank sometimes changes during analysis sessions. Correcting measurements to a constant blank when the blank is actually changing leads to an apparent drift in DIC results and therefore lower accuracy. Here, we propose an alternative method for coulometer blank corrections in which the blank is calculated on a per-measurement basis. The per-measurement blank values are then fitted to a smoothing function to determine a set of fitted blank values with which the measurements are corrected. We test the three different approaches (constant, per-measurement and fitted) by applying them to 263 measurements of a laboratory internal standard conducted during 89 analysis sessions over ∼ 7 years. Switching from the constant blank to either the per-measurement or fitted blank improves the precision from 1.85 to 1.31 µmol kg−1. This improvement is statistically significant and important relative to the climate-quality uncertainty target for DIC measurements of ±2 µmol kg−1. Using the fitted blank rather than per-measurement blank eliminates a number of outliers, notably reducing the total range and kurtosis of the residuals. A free and open source Python package (koolstof) has been made available to perform fitted blank corrections for some common coulometer data types. We recommend that in future coulometric DIC analyses, per-measurement blanks should be routinely calculated as part of the quality control process and the fitted blank method applied either as standard or when a changing blank is observed.

Role of sea ice, stratification, and near-inertial oscillations in shaping the upper Siberian Arctic Ocean currents - Igor V. Polyakov; Andrey V. Pnyushkov; Eddy C. Carmack; Matthew Charette; Kyoung-Ho Cho; Steven Dykstra; Jari Haapala; Jinyoung Jung; Lauren Kipp; Eun Jin Yang; Sergey Molodtsov (Ocean Science, 2025)
The Siberian Arctic Ocean (SAO) is the largest integrator and redistributor of Siberian freshwater resources and acts to significantly influence the Arctic climate system. Moreover, the SAO is experiencing some of the most notable climate changes in the Arctic, and advection of anomalous Atlantic- (atlantification) and Pacific-origin (pacification) inflow waters and biota continue to play a major role in reshaping the SAO in recent decades. In this study, we use a large collection of mooring data to create a coherent picture of the spatiotemporal patterns and variability of currents and shear in the upper SAO during the past decade. Although there was no noticeable trend in the upper SAO's current speed and shear from 2013 to 2023, their seasonal cycle has significantly strengthened. The cycle reveals a strong relationship between upper ocean currents and their shear with sea ice conditions – particularly during transitional seasons – evidenced by a strong negative correlation (−0.94) between seasonal sea ice concentration and current shear. In the shallow ( 100 m) layers. These findings are critical to understanding the ramifications for mixing and halocline weakening, as well as the rate of atlantification in the region.

Seasonal interplay of water mass mixing and nutrient dynamics in an Arctic fjord: a case study of Kongsfjorden, Svalbard - Hyebin Kim; Dukki Han; Sang Rul Park; Tae-Hoon Kim (Ocean Science, 2025)
This study examined seasonal variations in water mass structure and nutrient dynamics in Kongsfjorden, a high Arctic fjord where water mass composition varies seasonally due to mixing among Atlantic Water, Polar Surface Water, and glacial meltwater. In spring, the dominance of Modified Atlantic Water (MAW) facilitated active vertical mixing, leading to relatively high, uniform nutrient concentrations throughout the water column. In summer, the enhanced influence of glacial meltwater and warmer Polar Surface Waters (PSWw) resulted in strong surface stratification and significant nutrient depletion in the upper layer. To disentangle the effects of physical mixing from biological consumption, theoretical nutrient concentrations were calculated based on a four-component water mass mixing model. The positive differences between theoretical and observed concentrations (ΔNutrient) were indicative of significant biological uptake, which accounted for substantial nutrient reductions in observed surface concentrations from spring to summer: approximately 69 ± 18 % for NOx (sum of nitrate and nitrite; NO3- + NO2-), 74 ± 15 % for phosphate, and 47 ± 18 % for silicate. Crucially, ΔNutrient values served as a “biogeochemical memory”, reflecting the cumulative net biological consumption since the spring bloom rather than just instantaneous phytoplankton biomass. These biological processes also altered nutrient stoichiometry, as reflected by an increase in the surface dissolved inorganic nitrogen to phosphorus (DIN/DIP) ratio (DIN = NO2? + NO3- + NH4+; DIP = PO43-) from 15.0 in spring to 18.8 in summer, indicating a shift in nutrient limitation patterns. Consequently, summer surface waters transitioned toward potential co-limitation, with concentrations of phosphate (∼ 0.13 ± 0.07 µM) and silicate (∼ 1.66 ± 0.39 µM) approaching their respective limitation thresholds. These findings highlight a clear seasonal transition from a physically controlled, nutrient-replete spring to a biologically regulated, nutrient-limited summer. This understanding is crucial for predicting how Arctic fjord ecosystems, and their primary productivity, will respond to ongoing Atlantification and increased freshwater input under climate change.

The effect of storms on the Antarctic Slope Current and the warm inflow onto the southeastern Weddell Sea continental shelf - Vår Dundas; Kjersti Daae; Elin Darelius; Markus Janout; Jean-Baptiste Sallée; Svein Østerhus (Ocean Science, 2025)
Storms have been suggested to drive enhanced southward transport of modified Warm Deep Water (mWDW) towards the Filchner Ice Front in the southern Weddell Sea. This region is a known location of dense bottom water production and is thus tightly linked to the global climate system. However, increased heat transport could lead to higher ice shelf melt rates and disrupt dense water production. The role of storms and wind forcing in enhancing the southward heat transport is therefore of interest. We utilize observational records spanning up to four years of data from a network of moorings deployed in the Filchner Trough region to investigate how the regional ocean circulation responds to storm events. We find that about 70 % of the storm events that (i) last sufficiently long (longer than 5.7 d), (ii) have a large enough accumulation of ocean surface stress anomaly throughout the storm (larger than 0.9 N m−2 d−1), and (iii) are severe enough at their peak intensity (maximum stress above 0.5 N m−2) lead to a significant increase in the speed of the Antarctic Slope Current (ASC) just upstream of Filchner Trough while roughly 25 % of the identified events also cause increased southward current speed on the shelf at depths where mWDW is expected to be present during the summer and autumn. At the southernmost mooring (76° S) storm-driven responses are observed mainly during the latter part of the record (mid-2019 to early 2021). This interannual variability in storm response indicates a potential dependency on background hydrography and circulation that remains to be fully explained. This study highlights the potential importance of storms for southward heat transport: an accelerated circulation on the shelf increases the likelihood for warm summer inflow to reach the ice shelf front and cavity before the heat is lost to the atmosphere through winter convection.

Contribution of dark inorganic carbon fixation to bacterial carbon demand in the oligotrophic Southeastern Mediterranean Sea - Tom Reich; Natalia Belkin; Guy Sisma-Ventura; Hagar Hauzer; Maxim Rubin-Blum; Ilana Berman-Frank; Eyal Rahav (Ocean Science, 2025)
Photosynthetically derived organic matter sinking to depth from the illuminated layers is often not sufficient to meet the energy demands of microbes in the dark ocean. This “mismatch” is especially notable in the warm and oligotrophic eastern Mediterranean Sea where the annual primary production is one of the lowest in the world's oceans. Yet its aphotic zone is considered a hotspot for microbial activity. Here, we investigated the role of photic and aphotic dark inorganic carbon fixation rates (DCF) and their contribution to bacterial carbon demand in the southeastern Mediterranean Sea during the mixed and stratified periods. Our results demonstrate that DCF rates are measurable throughout the water column (0–1750 m) and are the same order of magnitude as photosynthesis (34 vs. 45 g C m−2 yr−1, respectively). Using a carbon mass balance that considers photosynthesis, DCF and bacterial production, we show that chemoautotrophy provides ∼ 35 % of the “missing carbon” supply needed for microbial growth and activity in the aphotic layer, while other sources of dissolved organic carbon remain to be elucidated. These findings underscore the need for further research into the factors affecting DCF, its role in global carbon budgets, and its potential to enhance atmospheric inorganic carbon sequestration.

Modeling water column gas transformation, migration and atmospheric flux from seafloor seepage - Knut Ola Dølven; Håvard Espenes; Alfred Hanssen; Muhammed Fatih Sert; Magnus Drivdal; Achim Randelhoff; Bénédicte Ferré (Ocean Science, 2025)
Understanding the fate of gas seeping from the seafloor is crucial for assessing the environmental impacts of both natural and anthropogenic seep systems, such as CH4 cold seeps, leaking gas wells, and future carbon capture projects. We present a comprehensive modeling framework that integrates physical, chemical, and biological processes to estimate the 3-dimensional water column dissolved gas concentration field and 2-dimensional atmospheric flux field resulting from seafloor seeps. The framework consists of two main components: (1) a gas-phase model that calculates free gas dissolution and direct atmospheric release at the seep site, and (2) a concentration model that combines particle dispersion modeling with an adaptive-bandwidth kernel density estimator and customizable process modules. Applying the framework to a natural CH4 seep at 200 m depth offshore northwestern Norway (20 May–20 June 2018), we found that dissolved methane was advected northeastward along the coast, spreading across shelves, reefs, and into fjord systems. Within days, the vertical CH4 concentration profile was near inverted, with near-surface maxima, facilitating atmospheric exchange. Diffusive emissions covered large areas (>105 km2) and was almost 3 times the local free gas flux. Around 0.7 % of dissolved CH4 reached the atmosphere during a 4 week period, microbial oxidation removed around 65 %, while ∼ 34 % remained in the water column. Uncertainties caused by a range of model framework elements remain substantial, e.g. can estimates of microbial oxidation removal change from 65 % to as low as 5.5 % or as high as 91.4 % depending on rate coefficient assumptions. Our framework provides a globally applicable tool that integrates free and dissolved gas dynamics and accommodates advanced hydrodynamic modeling. Its ability to explicitly resolve spatiotemporal fields enables the inclusion of complex physical and biogeochemical process modules and supports not only the quantification of atmospheric fluxes but also applications that require explicit field representations, such as assessing impacts on local ecosystems.

ENSO and the Temperature of the North Equatorial Counter Current - David John Webb (Ocean Science, 2025)
A study using the CESM climate model shows that when the sea surface temperatures are above 28.9 °C, increases in the temperature of the North Equatorial Counter Current result in significant increases in deep atmospheric convection above the Inter-Tropical Convergence Zone. This changes the longitudinal structure of the Hadley Circulation and so may have a global impact on weather and climate. At the same time, sea level atmospheric pressure increases over the Indian Ocean and decreases in the central and south-west Pacific Ocean, in a pattern similar to that of the Southern Oscillation. The longitudinal pressure gradient in the Central Pacific drops leading to weaker surface easterlies, reduced ocean upwelling along the Equator and increases in some near surface ocean temperatures. Together the results imply that the monthly and annual variations in the temperature of the North Equatorial Counter Current are responsible, at least in part, for many of the atmospheric and oceanic features associated with the El Niño and the Southern Oscillation.

Catalogue of extreme sea levels recorded at tide-gauge station Bakar in the northeastern Adriatic Sea (Mediterranean) - Iva Međugorac; Karla Jambrošić; Domagoj Dolički; Josipa Kuzmić; Jadranka Šepić; Iva Vrkić Seidl; Goran Gašparac (Ocean Science, 2025)
Extreme sea levels in the northern Adriatic Sea occasionally occur in late autumn and winter as a result of storm surges that combine with other sea-level processes of different spatial and temporal scales. This paper presents an (empirical) analysis of the 27 most intense episodes recorded at the Croatian tide-gauge station Bakar on the northeastern coast of the Adriatic Sea in the period from 1929 to 2022. Extreme sea levels were defined as events in which the hourly sea level exceeded 89 cm (99.99th percentile threshold) above the long-term average. The study examines the following: (i) the evolution of sea level, analysed through five components – high-frequency oscillations, tide, synoptic component (storm surge and basin-wide seiche), planetary-scale variability, and long-term sea-level variability; (ii) the meteorological conditions, based on reanalysis series and fields (mean sea-level pressure, 10 m wind, and 500 hPa surface geopotential heights); (iii) the impact of these episodes on the natural and built environments along the Croatian coastline; and (iv) the relevant scientific literature addressing these episodes. The study is complemented by an online catalogue, which contains supplementary information and is continuously updated with the latest extreme episodes (https://projekti.pmfst.unist.hr/floods/storm-surges/, Međugorac et al., 2024b).

A data-driven wind-to-current response function and application to ocean surface current estimates - Clément Ubelmann; J. Thomas Farrar; Bertrand Chapron; Lucile Gaultier; Laura Gomez-Navarro; Marie-Hélène Rio; Gérald Dibarboure (Ocean Science, 2025)
This study investigates the reconstruction of wind-driven currents based on an empirical impulse response function. Surface current observations derived from drifting buoy data and wind-stress from the ERA5 reanalyses are used to derive the response function. The function is expected to be sensitive to the ocean mixed-layer depth and more generally the turbulent viscosity profile which can display strong spatio-temporal variability. In this work, however, only seasonal and meridional variations are considered. Despite this crude approximation, the simplified response function can explain a significant portion of the current variability in independent observations. A practical application is the release of a new total surface current product (denoted WOC). Compared to existing products based on the same input datasets, such as the CMEMS MOB-TAC (Guinehut, 2021) surface current product, the WOC estimates are designed to include higher frequency content, in particular in the inertial band. Beyond successful validation, the characteristics of the response function (amplitudes and phases) reveal interesting properties of the upper-ocean variability. The function shows some similarities to one derived theoretically from a simple 1-layer (slab) model, but also differences that highlight the value of fitting the function to the data without the use of an explicit dynamical model. These results open perspectives for studying some dependencies between subsurface variables and the response function, particularly interesting in the context of future spaceborne Doppler scatterometers such as ODYSEA (Rodríguez et al., 2019), expected to provide simultaneous wind and current observations. This instrument could indirectly probe subsurface properties through the synoptically-observed response function.

Diagnosing the Atlantic Meridional Overturning Circulation in density space is critical in warmer climates - Fernanda DI Alzira Oliveira Matos; Dmitry Sidorenko; Xiaoxu Shi; Lars Ackermann; Janini Pereira; Gerrit Lohmann; Christian Stepanek (Ocean Science, 2025)
The Atlantic Meridional Overturning Circulation (AMOC) plays a crucial role in shaping the global climate system by redistributing heat and influencing large-scale climate patterns. Utilizing the AWI-CM3 model, we investigate the AMOC sensitivity to an abrupt climate change scenario (abrupt-4xCO2) with respect to pre-industrial climate (PI), comparing the AMOC diagnosed in depth (z-AMOC) and density (ρ-AMOC) space. Water mass transformations are assessed to analyze the impact of background climate on surface-forced and interior-mixing-induced transformations. We find that both the location and magnitude of AMOC maximum are directly affected by the framework choice. In PI, the ρ-AMOC maximum is substantially stronger than that of the z-AMOC, while at 26° N the two diagnostics are nearly equivalent. Consequently, the variability of the z-AMOC maximum correlates only with that at 26° N, reflecting isopycnal flattening into constant depth levels in the subpolar North Atlantic inherent to these diagnostics. Strong AMOC weakening is observed under 4xCO2 forcing in both frameworks until simulation year 75. Subsequently, both diagnostics reveal a weaker AMOC with an approximate strength of 7.1 Sv, although with z-AMOC displaying a slight recover towards the end of the simulation and ρ-AMOC oscillating steadily around 5 Sv. At 26° N, variability patterns remain comparable to PI, albeit with an additional ρ-AMOC weakening of approximately 2 Sv, indicating divergence between the representation of AMOC dynamics in the subtropical Atlantic within both frameworks in comparison to PI. The diagnostics in density space allow for the attribution of this further ρ-AMOC weakening to increased entrainment of fresher overflows from amplified GIN seas overturning and reduced deep convection in the Labrador and Irminger Seas. In contrast, the diagnostics in depth space only reveals reduced downwelling around the southwestern Greenland coast and along the path of the Gulf Stream, features that are more challenging to evaluate against available observations. Thus, the comparison between z-AMOC and ρ-AMOC indicates that diagnosing the AMOC in density space provides more physically meaningful information regarding the state of the water mass transformations and their contribution to ocean circulation regimes across the entire Atlantic basin, not only the subpolar North Atlantic, and especially as the climate continues to warm. These findings emphasize the importance of diagnosing AMOC in density space to better understand water mass transformations, which are concealed in depth space and to capture AMOC variability in warmer climates, across all latitudes.

Impact of internal tides on chlorophyll a distribution and primary production off the Amazon shelf from glider measurements and satellite observations - Amine M'hamdi; Ariane Koch-Larrouy; Alex Costa da Silva; Isabelle Dadou; Carina Regina de Macedo; Anthony Bosse; Vincent Vantrepotte; Habib Micaël Aguedjou; Trung-Kien Tran; Pierre Testor; Laurent Mortier; Arnaud Bertrand; et al. (Ocean Science, 2025)
The ocean region off the Amazon shelf including the shelf break presents a hotspot for internal tide (IT) generation, yet its impact on phytoplankton distribution remains poorly understood. While previous studies have extensively examined the physical characteristics and dynamics of ITs, their biological implications – particularly in nutrient-limited environments – remain underexplored. To address this question, we analyzed a 26 d glider mission deployed over September–October 2021 sampling hydrographic and optical properties (chlorophyll a) at high resolution along an IT pathway as well as satellite chlorophyll a and altimetry data to assess mesoscale interactions. Chlorophyll a dynamics were analyzed under varying IT intensities, comparing strong (HT) and weak (LT) internal tide conditions. Results reveal that ITs drive vertical displacements of the deep chlorophyll maximum (DCM) from 15 to 45 m, accompanied by 50 % expansion in its thickness during HT events. This expansion is observed with a dilution of the chlorophyll a maximum concentration within the DCM depth. While direct turbulence measurements were not collected, the observed vertical redistribution of chlorophyll a is indicative of tidally driven cross-isopycnal exchanges, the only physical mechanism explaining the transfer of biomass above and below the DCM. At the surface, turbulent fluxes provide 38 % of the chlorophyll a input, while the remainder is supplied by in situ biological activity. Notably, total chlorophyll a in the water column increases by 14 %–29 % during high internal tide phases, indicating a net enhancement of primary productivity driven by the combined effects of vertical mixing and stimulated surface-layer biological activity. These findings indicate that internal tides can be an important driver of chlorophyll a distribution and short-term biological variability in our study region. By reshaping the vertical chlorophyll a profile through vertical mixing, active internal tides influence primary productivity and may contribute to carbon cycling, particularly in oligotrophic oceanic environments where both a deep chlorophyll maximum and strong internal tides are present.

When storms stir the Mediterranean depths: chlorophyll a response to Mediterranean cyclones - Giovanni Scardino; Alok Kushabaha; Mario Marcello Miglietta; Davide Bonaldo; Giovanni Scicchitano (Ocean Science, 2025)
Mediterranean cyclones induce significant biogeochemical perturbations in the Mediterranean Sea, with particularly notable effects on chlorophyll a (Chl a) dynamics. This study examines Chl a variability during twenty Mediterranean cyclones, comparing offshore and nearshore responses. Through an integrated investigation of reanalysis products, ARGO float observations, and Sentinel-2 multispectral satellite imagery, we quantified vertical and surface Chl a concentrations, while concurrently assessing nitrate distribution, currents, and mixed layer depth (MLD) variability. Our results revealed that both tropical-like cyclones and extratropical cyclones generated a pronounced uplift of the deep chlorophyll maximum (DCM) in cyclones exhibiting slow-moving phases. Notably, wind-driven upwelling and air–sea heat exchange critically govern DCM uplift for these cyclones. We demonstrated that these physical mechanisms collectively drive DCM uplift along the trajectories of intense, slow-moving Mediterranean cyclones, with significant implications for nutrient cycling and primary productivity across the Mediterranean basin.

Estimating oceanic physics-driven vertical velocities in a wind-influenced coastal environment - Maxime Arnaud; Anne Petrenko; Jean-Luc Fuda; Caroline Comby; Anthony Bosse; Yann Ourmières; Stéphanie Barrillon (Ocean Science, 2025)
Despite the challenge of measuring them due to their small intensities, oceanic vertical velocities (W) constitute essential key variables in understanding ocean dynamics, and ocean-atmosphere interactions and biogeochemical processes. Coastal events and fine-scale processes (1–100 km and days to weeks) can lead to high-intensity vertical velocities. Such processes can be observed in the Northwestern Mediterranean Sea. In particular, the Gulf of Lion is a region prone to intense north-westerly and easterly wind episodes that strongly impact the oceanic circulation. This work presents mooring ADCPs as reliable tools for physics-driven W measurements, with an adaptive algorithm which is applicable anywhere offshore in the ocean to detect W in fine-scale processes. The JULIO mooring (JUdicious Location for Intrusion Observation) is located on the boundary of the eastern side of the Gulf of Lion's shelf at the 100 m isobath. JULIO provides Eulerian measurements of three-dimensional current velocities over two main time-periods: 2012–2015, and since 2020. Vertical velocity measurements from JULIO show a good agreement with two independent methods: a Free-Fall Acoustic Doppler Current Profiler and an innovative Vertical Velocity Profiler. To measure physics-driven vertical velocities, we developed a method to identify and filter out biology-induced vertical velocities. Combining satellite and in situ observations with wind model outputs, we identify wind-induced downwelling and upwelling events at JULIO associated with physics-driven vertical velocities with maximum amplitudes of −465/127 m d−1. Hence, this analysis underlines the need for long term multimethod observations in such coastal areas forced by intense wind episodes.

On the applicability of linear wave theories to simulations on the mid-latitude β -plane - Itamar Yacoby; Hezi Gildor; Nathan Paldor (Ocean Science, 2025)
The applicability of one-dimensional (zonally invariant) harmonic and trapped wave theories for Inertia-Gravity waves to simulations on the mid-latitude β-plane is examined by comparing the analytical estimates in the geostrophic adjustment and Ekman adjustment problems with numerical simulations of the linearized rotating shallow water equations. The spatial average of the absolute differences between the theoretical solutions and the simulations, ϵ(t), is calculated for values of the domain's north-south extent, L, ranging from L=4 to L=60 (where L is measured in units of the deformation radius). The comparisons show that: (i) though ϵ oscillates with time, its low-pass filter, ϵLP(t), increases with time. (ii) In small domains, ϵLP(t) in harmonic theory is significantly smaller than in trapped wave theory, while the opposite occurs in large domains. (iii) The accuracy of the harmonic wave theory decreases with L for 0 20 the trend changes with time. (iv) The accuracy of the trapped wave theory increases with L in the geostrophic adjustment problem, while in the Ekman adjustment problem, its best accuracy is obtained when L≈30. (v) There is a range of L and t values for which no theory provides reasonable approximations, and this range is wider in the Ekman adjustment problem than in the geostrophic adjustment problem. Non-linear simulations of a multilayered stratified ocean show that internal inertia-gravity waves exhibit the same characteristics as the wave solutions of the linearized rotating shallow water equations in a single layer ocean.

Atmospheric cold pools abruptly reverse thermohaline features in the ocean skin layer - Lisa Gassen; Samuel M. Ayim; Leonie Jaeger; Jens Meyerjürgens; Mariana Ribas-Ribas; Oliver Wurl (Ocean Science, 2025)
The ocean skin layer, which covers the upper millimetre of the sea surface, regulates the exchange of heat, gases, and freshwater between the atmosphere and the ocean. However, there is a lack of small-scale mechanistic understanding of these fluxes, especially under abrupt meteorological shifts, due to observational challenges during stormy conditions in the open sea. This study provides unique data on temperature and salinity anomalies between the skin layer and a depth of 100 cm during atmospheric cold pools, which induce abrupt shifts in air temperature, wind speed, precipitation, and heat fluxes. We determined how these abrupt meteorological shifts forced the anomalies and altered the conditions at the air–sea boundary layer during three events monitored by an autonomous surface vehicle. Two cold pool events were observed in the harbour of Bremerhaven and one event in the North Sea. Here, we show that the skin layer instantly reacts to abrupt meteorological shifts forced by cold pools. The average temperature change in the skin layer was twice as much as at a depth of 100 cm. An abrupt change in meteorological conditions, shifting the net heat flux from positive to negative, can turn a warm skin layer into a cooler layer compared with the 100 cm depth. Salinity anomalies in the harbour were less affected by abrupt meteorological shifts, including freshwater fluxes, than those in the North Sea event. The current velocities showed that changes in wind direction could alter the surface current direction, and that the backscatter signal consistently reflects wind-induced mixing, with higher backscatter observed during increased wind conditions. This study reveals the complex relationships between atmospheric conditions and oceanic responses and provides valuable information for understanding air–sea interactions and their implications for climate dynamics.

Merging of a mesoscale eddy into the Lofoten Vortex in the Norwegian Sea captured by an ocean glider and SWOT observations - Gillian M. Damerell; Anthony Bosse; Ilker Fer (Ocean Science, 2025)
The Lofoten Vortex (LV) is an intense, apparently permanent anticyclone in the Lofoten Basin of the Norwegian Sea. It is characterised by a 1200 m thick core of Atlantic Water, with a radius of 15–20 km, in nearly solid-body rotation, reaching speeds up to 0.8 m s−1. Potential vorticity in the core is nearly 2 orders of magnitude lower than the surroundings, creating a barrier to lateral mixing. It has previously been postulated that anticyclonic eddies in the Lofoten Basin, shed from the eastern branch of the Norwegian Atlantic Current along the Lofoten Escarpment, merge into the LV, contributing to maintaining its large heat and salt content and energetics, but such merging events have proven to be difficult to observe directly due to their transient and unpredictable nature. In April 2023, an eddy merger event was successfully observed using a combination of in situ data from an autonomous ocean glider and absolute dynamic topography (and derived velocities) from the fast sampling calibration phase of the Surface Water Ocean Topography (SWOT) satellite altimeter. During the observed merging process, an incoming eddy gradually approaches the LV and then elongates as the two begin to co-rotate and then merge, with a corresponding spin-up of vorticity and eddy kinetic energy and possible ejection of water of low potential vorticity from the merged LV core. The incoming eddy had a smaller radius and higher Rossby number than the LV. It has a similar density range to the LV, and, therefore, a double-core vertical structure did not form after the merger. During the observed period, merging eddies were the dominant process affecting the evolution of the LV, clearly outweighing vertical 1D processes due to atmospheric forcing and lateral mixing between the LV core and the outer rim. Through the influx of buoyant waters, spin-up of eddy kinetic energy and increasingly anticyclonic vorticity, eddy mergers contribute to the longevity of the LV.

Estimates of Atlantic meridional heat transport from spatiotemporal fusion of Argo, altimetry, and gravimetry data - Francisco M. Calafat; Parvathi Vallivattathillam; Eleanor Frajka-Williams (Ocean Science, 2025)
Understanding how changes in Atlantic meridional heat transport (MHT) and the Earth's climate relate to one another is crucial to our ability to predict the future climate response to anthropogenic forcing. Attaining this understanding requires continuous and accurate records of MHT across the whole Atlantic. While such records can be obtained through direct ocean observing systems, these systems are expensive to install and maintain, and thus, in practice, records of MHT derived in this way are restricted to a few latitudes. An alternative approach, based on hydrographic and satellite components of the global ocean observing system, consists of inferring heat transport convergence as a residual from the difference between ocean heat content (OHC) changes and surface heat flux. In its simplest form, this approach derives the OHC from hydrographic observations alone; however, these observations are spatially sparse and unevenly distributed, which can introduce significant errors and biases into the MHT estimates. Here, we combine data from hydrography, satellite altimetry, and satellite gravimetry through joint spatiotemporal modelling to generate probabilistic estimates of MHT for the period 2004–2020 at 3-month resolution across 12 latitudinal sections of the Atlantic Ocean between 65° N and 35° S. Our approach leverages the higher spatial sampling of the satellite observations to compensate for the sparseness and irregular distribution of the hydrographic data, leading to significantly improved estimates of MHT compared to those derived from hydrographic data alone. The fusion of the various datasets is done using rigorous Bayesian statistical methods that account for the spatial resolution mismatch between datasets and ensure an adequate representation and propagation of uncertainty. Our estimates of MHT at 26° N agree remarkably well with estimates based on direct ocean observations from the RAPID array, both in magnitude and phase of the variability, with a correlation of 0.68 for quarterly (3-monthly) time series and 0.81 after applying a yearly running mean. For the period 2004–2017, the correlation increases to 0.78 and 0.92, respectively. The time-mean MHT at 26° N is also captured by our approach, with a value of 1.14 PW [1.01, 1.27] (5 %–95 % credible interval). Estimates of MHT at other latitudes are also consistent with what we expect based on earlier estimates as well as on our current understanding of MHT in the Atlantic Ocean.

Cold lenses in the Amundsen Sea: impacts of sea ice formation on subsurface pH and carbon - Daisy D. Pickup; Dorothee C. E. Bakker; Karen J. Heywood; Francis Glassup; Emily M. Hammermeister; Sharon E. Stammerjohn; Gareth A. Lee; Socratis Loucaides; Bastien Y. Queste; Benjamin G. M. Webber; Patricia L. Yager (Ocean Science, 2025)
The Amundsen Sea polynya hosts intense sea ice formation, but, due to the presence of relatively warm and salty modified Circumpolar Deep Water, the cold, brine-enriched water is not typically dense enough to sink to the deep ocean. A hydrographic survey of the Dotson Ice Shelf region in the Amundsen Sea using two ocean gliders identified and characterised subsurface lenses containing water with temperatures less than −1.70 °C. These lenses, located at depths between 240 to 500 m, were colder, saltier and denser than the overlying Winter Water (WW) layer. The pH of the lenses was 7.99, lower than WW by 0.02 and the dissolved inorganic carbon concentration was higher in the lenses than WW by approximately 10 µmol kg−1. The lenses were associated with a dissolved oxygen concentration greater than surrounding water at the same depth and density due to the cold temperatures increasing O2 solubility. We hypothesise that these lenses are a product of wintertime surface cooling and brine rejection in areas with intense sea ice formation. They may form in shallow regions, potentially around the Martin Peninsula and Bear Island, where intense upper ocean heat loss occurs, and then spill off into the deeper Dotson-Getz Trough to reach their neutrally-buoyant depth. This is supported by wintertime temperature and salinity observations. This study highlights the importance of shallow parts of shelf seas for generating cold dense water masses in the warm sector of Antarctica. These lenses are widespread in the region of the Dotson-Getz Trough and have the potential to sequester carbon deeper than typical in the region, alongside cooling the water impinging on the Dotson ice shelf base.

Effect of double-diffusion processes in the deep ocean on the distribution and dynamics of particulate and dissolved matter: a case study in the Tyrrhenian Sea - Xavier Durrieu de Madron; Paul Blin; Mireille Pujo-Pay; Vincent Taillandier; Pascal Conan (Ocean Science, 2025)
This study examines conductivity–temperature–depth (CTD), acoustic Doppler current profiler (ADCP) and optical data collected during the PERLE-3 cruise in March 2020 between the surface and 2000 m depth over an east–west section of the Tyrrhenian Sea in the Mediterranean. The focus is on the impact of double-diffusion processes, in particular salt fingering, on the distribution and dynamics of particulate and dissolved matter. Staircases form at the interface between the warm, saline Eastern Intermediate Water (EIW) and the colder, less saline Tyrrhenian Deep Water (TDW) in the centre of the basin in a region of weak hydrodynamic activity. The results show that thermohaline staircases formed by salt fingering significantly influence particle sedimentation and biogeochemical cycling in deep ocean environments by altering vertical flux patterns. These density steps create distinct vertical layers that act as physical barriers, slowing the descent of particles and facilitating their retention and aggregation. The retention of fine particles at density gradients promotes the formation of larger aggregates, thereby affecting the particle size distribution. The staircases also affect dissolved matter by creating pronounced concentration gradients of oxygen and nutrients, which may influence microbial activity and nutrient fluxes.

Flow structure and mixing near a small river plume front: Winyah Bay, SC, USA - Christopher Papageorgiou; George Voulgaris; Alexander E. Yankovsky; Diane Bennett Fribance (Ocean Science, 2025)
This study presents Eulerian data from Winyah Bay, SC, USA collected during the passage of a tidal plume. The data captured the evolution and structure of the plume and include high-resolution velocity and temperature time series, supplemented by temperature – salinity profiles from a MicroCTD profiler. The observations identified a pre-existing plume extending to 4 m, with a water density of 1023.6 kg m−3, laying above denser ambient waters. Upon arrival, the newly discharged tidal plume introduced a fresher layer (1020.7 kg m−3) extending to 2.6 m, gradually thinning due to radial spreading. The plume's frontal propagation measured at 0.36 m s−1 with a calculated Froude number of 1.32, indicating gravity current dynamics. In the across-front direction a return flow developed under the plume that extended throughout the water column, resembling an estuarine-like circulation pattern. Mixing processes were examined using the available overturn potential energy (AOPE) in the water column as described in Smith (2020). The analysis showed that near the bed, bottom boundary layer turbulence is the main mixing mechanism both before and after the passage of the front. In the surface layer, before the arrival of the front, mixing is driven by wind-induced shear and overturning. Within the gravity current, despite the high turbulent kinetic energy dissipation rates in certain regions, shear-induced mixing was minimal. These findings were reflected in the density diffusivity estimates near the surface that varied from 10−6 m2 s−1 prior to the arrival of the front, increasing to 10−5 m2 s−1 very near the front and diminishing to 10−10 m2 s−1 within the plume despite the high velocity shear observed there. The limited mixing within the plume despite the high shear observed can be explained using the concept of a layer-interface structure, where layers of high turbulence and low stratification alternate with regions of low turbulence and higher stratification. Although not resolved by these observations, this structure has been observed in numerical simulations of mixing. The development of the counter-flow under the plume suggests that tide and/or wind induced straining may play an important role in enhancing stratification in plumes like the one studied here.

Can satellite altimetry observe coastally trapped waves on sub-monthly timescales? - Marcello Passaro (Ocean Science, 2025)
Coastally trapped waves (CTWs) are a major cause of sub-seasonal coastal sea level variability. While they have mostly been studied using numerical models, observational evidence is limited due to the sparse spatial coverage of the tide gauge network and the limitations of satellite altimetry gridded maps, which arise from the interpolation of sparse along-track data. The simultaneous operation of multiple altimetry missions, advancements in processing technologies, the advent of wide-swath altimetry, and the development of new interpolation techniques have the potential to significantly improve the monitoring of CTWs. In this study, we analyze three months of sea level data from satellite altimetry to evaluate the new capabilities for detecting sub-monthly variability, comparing the results to tide gauge data and an ocean model in Eastern Australia, an area known for its dominance of CTWs at these time scales. The results demonstrate that in the study area, the correlation between tide gauges and coastal daily sea level grids from satellite altimetry exceeds 0.5, even when considering time series filtered to capture only sub-monthly variability. CTWs are generally well detected, though discrepancies remain, particularly in terms of amplitude, wavelength and period.

Geostrophic circulation and tidal effects in the Gulf of Gabès - Maher Bouzaiene; Antonio Guarnieri; Damiano Delrosso; Ahmad F. Dilmahamod; Simona Simoncelli; Claudia Fratianni (Ocean Science, 2025)
The mean kinematic features in the Gulf of Gabès region are analyzed based on 30 years of altimetry data (1993–2022) and the outputs of a high-resolution ocean model for the year 2022. A comparison of the seasonal variability in three different geographical areas within the gulf is presented. In the northern and southern parts of the gulf, anticyclonic structures prevail, while the central area is dominated by divergence. Similarity in flow topology is found in these three areas of the gulf due to the signature of hyperbolic regions. In winter and fall, the mean flow is oriented northward, while it is reversed in spring and summer. The tidal perturbations influence sea level, kinetic energy and hyperbolic geostrophic structures, leading to the generation of a cyclonic current in the central part of the gulf and to the presence of persistent strain gradients amplifying hyperbolic structures. The finite-time Lyapunov exponent (FTLE) computed using altimetry data highlights the link between physical and biogeochemical dispersion, with the Gulf of Gabès mean circulation features acting as transport barriers for phytoplankton dispersion.

Enhancement of near-inertial waves by cyclonic eddy in the northwestern South China Sea during spring 2022 - Qi'an Chen; Hongzhou Xu; Dongxiao Wang; Bo Hong; Chunlei Liu; Zheyang Zhang; Huichang Jiang; Wei Song; Tong Long; Ling Wang; Sumin Liu; Rongjie Chen (Ocean Science, 2025)
By analyzing datasets derived from four moorings during spring 2022, this study provides direct evidence that near-inertial waves (NIWs) can be largely enhanced by a passing cyclonic eddy (CE) in the northwestern South China Sea. Results show that the enhancement of NIWs mainly occurred at north side of the CE due to asymmetry of eddy structure. In vertical, the enhancement concentrated at above 200 m and reached peaks at around 100 m. Significant energy transfer rates between the CE and NIWs appeared at the same depth of the enhancement can reach a magnitude of 10−8 m2 s−3 at the CE's edge. Under the impact of the CE, power of the first five NIWs modes was promoted significantly and dominated by the second and third modes. Overall, the CE transferred energy to NIWs before near-inertial kinetic energy reaching its peaks, while NIWs gave energy back to the CE after the peaks.

Thwaites Eastern Ice Shelf cavity observations reveal multiyear sea ice dynamics and deepwater warming in Pine Island Bay, West Antarctica - Christian T. Wild; Tasha Snow; Tiago S. Dotto; Peter E. D. Davis; Scott Tyler; Ted A. Scambos; Erin C. Pettit; Karen J. Heywood (Ocean Science, 2025)
Pine Island Bay (PIB), situated in the Amundsen Sea, is renowned for its retreating ice shelves and highly variable sea ice. While brine rejection from sea ice formation and glacial meltwater influence seawater properties, the downstream impacts beneath the region's floating ice shelves remain poorly understood. Here, we exploit an unprecedented multiyear (2020–2023) oceanographic time series from instruments deployed through boreholes beneath the Thwaites Eastern Ice Shelf (TEIS), immediately downstream of PIB, offering new insight into how ice–ocean–atmosphere interactions in PIB shape oceanographic conditions within the subshelf cavity. Our observations reveal a sustained warming and thickening of the modified Circumpolar Deep Water (mCDW) layer near the seabed since January 2020, critical in a region where mCDW drives basal melting beneath West Antarctica's most vulnerable outlet glaciers. Concurrently, the retreat of the multiyear sea ice edge by over 150 km across most of PIB has enhanced the advection of Winter Water, contributing to a cooling of more than 1 °C in the upper 250 m beneath TEIS between July 2021 and January 2023. Superimposed on these trends are episodic temperature and salinity anomalies lasting several weeks, originating in PIB and advecting past the moorings. These events link mobile sea ice cover to subshelf hydrography, as mid-depth waters temporarily warm and increase in salinity, leading to an increase in density, while deeper mCDW simultaneously cools and freshens, reducing its density. Overall, these changes are associated with reduced stratification in the cavity. As sea ice continues to decline in a warming Antarctic climate, our results offer a glimpse into how ocean circulation and basal melting may evolve across the Amundsen Sea Embayment. This dataset provides a critical benchmark for refining process-based models and improving melt rate parameterizations in coupled ice–ocean simulations.

On the global reconstruction of ocean interior variables: a feasibility data-driven study with simulated surface and water column observations - Aina Garcia-Espriu; Cristina González-Haro; Fernando Aguilar-Gómez (Ocean Science, 2025)
This work uses data-driven approaches to study the feasibility of reconstructing ocean interior variables (temperature and salinity) from surface observations provided by satellites and interior observations provided by Argo floats. The feasibility of the approach is based on an Observing System Simulation Experiment (OSSE) in which we use the outputs from an ocean numerical model as the ground truth, and simulate a real observing system of the ocean, taking the surface of the model as a simulation of satellite observations, and vertical profiles in the same locations as the real Argo floats. We implemented different models based on Random Forest Regressors and Long-Short Term Memory networks, which were trained with the simulated observations and validated against the complete numerical model results. We obtain high spatial and temporal correlation using both technologies and an accurate description of the annual variability of the data, accompanied by small biases.

Forcing-dependent submesoscale variability and subduction in a coastal sea area (Gulf of Finland, Baltic Sea) - Kai Salm; Germo Väli; Taavi Liblik; Urmas Lips (Ocean Science, 2025)
Submesoscale (SMS) processes in a stratified coastal environment of the Gulf of Finland (Baltic Sea) were investigated using glider missions and a realistic simulation with a grid spacing of 0.125 nautical miles. The study period covered the transition from developing to established seasonal stratification. SMS variability, defined as small-scale anomalies of temperature and salinity cancelling each other's contribution to density (spice), was concentrated around the base of the upper mixed layer (UML) in spring and within the seasonal thermocline in late summer. We suggest that this shift was caused by the changes in surface heat flux and wind forcing. Both observations and simulations revealed events of high SMS variability, which can be related to background mesoscale dynamics and changes in wind forcing. We examined the event in which elevated spice in the subsurface layer did not coincide with the high Rossby number O(1) in the surface layer but instead appeared offshore of a coastal baroclinic current. Sloping isopycnals and intensified vertical velocities pointed to active SMS subduction. We propose that frontal instabilities, likely related to flow–topography interactions, drive the lateral and downward transport of surface waters, contributing to tracer redistribution below the thermocline.

Insights into mesoscale eddy dynamics: a three-dimensional perspective on potential density anomalies - Yan Barabinot; Sabrina Speich; Xavier Carton (Ocean Science, 2025)
Mesoscale eddies are fundamental components of the global ocean circulation. In situ observations and Lagrangian analyses have shown that some eddies are materially coherent, transporting within their cores a water mass distinct from the surrounding environment. Additionally, laboratory experiments indicate that eddies locally deflect isopycnal surfaces in accordance with thermal wind balance, regardless of whether they trap a water mass. These two mechanisms drive spiciness mode anomalies and heaving mode anomalies, respectively, associated with mesoscale eddies. In this study, we quantitatively assess the physical processes governing mesoscale eddy dynamics by introducing a novel theoretical decomposition of the potential density field within eddy cores that accounts for both effects. We apply this framework to six anticyclonic eddies sampled during the EUREC4A-OA, METEOR 124, and Physindien 2011 oceanographic campaigns. Unlike previous studies, we evaluate not only the amplitude of these anomalies but also their vertical structure. Our results confirm that heaving mode anomalies dominate the total density anomaly. However, contrary to previous assumptions, we demonstrate that their vertical structure is dictated by the local background stratification and often exhibits a nearly Gaussian profile. In contrast, spiciness anomalies provide only a second-order contribution to the total potential density anomaly, making them negligible for most dynamical processes governing mesoscale eddies. By bridging experimental results with observational eddy datasets, our study refines the understanding of the mesoscale eddy vertical structure, providing a more accurate predictive framework for their shape and role in the transport of oceanic properties.

Amplified warming and marine heatwaves in the North Sea under a warming climate and their impacts - Bayoumy Mohamed; Alexander Barth; Dimitry Van der Zande; Aida Alvera-Azcárate (Ocean Science, 2025)
The northeast Atlantic and adjacent regions, such as the North Sea, are among the fastest-warming areas in the world. However, the role of climate change and internal variability in marine heatwaves (MHWs) in this region remains poorly understood. This study aims to quantify the relevant changes in sea surface temperature (SST) and MHWs in the North Sea, as well as to identify the leading patterns of interannual MHW variability over more than 4 decades (1982–2024). Our results indicated a new regime shift in the annual mean SST in the North Sea since 2013. Therefore, we examined the relationships between MHW trends and long-term sea surface warming trends to quantify the role of climate change in the intensification of MHWs. We found that the increase in MHWs is related to the significant decadal change in SST over the North Sea and we have revealed that large-scale climate modes, such as the Atlantic Multidecadal Oscillation and the East Atlantic Pattern, play a crucial role in this decadal change in SST. In particular, the SST trend has doubled in the post-2013 period (0.8 °C per decade), compared with the pre-2013 period (0.4 °C per decade), leading to longer and more frequent MHWs. The SST, MHW frequency, and MHW days increased significantly by 0.38 °C per decade, 1.04 events per decade, and 17.27 d per decade, respectively, over the entire study period. After removing the long-term sea surface warming trend before MHW detection, all MHW features exhibited insignificant trends, indicating that the long-term SST trend is the primary driver of the observed long-term MHW trend in the North Sea region, thereby confirming the crucial role of mean SST changes in MHWs in this region. Furthermore, we found that 80 % of the observed trend in MHW frequency is attributed to long-term warming, while the rest is attributed to internal variability. The SST record in May 2024, manifest by the longest (27 d) and most intense (2.2 °C) MHW event, is attributed to an anomalous anticyclonic atmospheric circulation over the Baltic Sea and southern Norway, which enhanced solar radiation over the North Sea. Finally, we also investigated how the chlorophyll a concentration responded to the MHW, revealing a decrease in the deep and cold-water regions of the northern North Sea and an increase in the shallow and warm-water areas of the southern North Sea.

An Atlantic-wide assessment of marine heatwaves beyond the surface in an eddy-rich ocean model - Tobias Schulzki; Franziska U. Schwarzkopf; Arne Biastoch (Ocean Science, 2025)
Periods of prolonged anomalously high temperatures in the ocean, known as marine heatwaves (MHWs), can have devastating effects on ecosystems. Although MHWs are extensively studied in the near-surface ocean, little is known about MHWs at depth. As continuous observations in space and time are very sparse away from the surface, basin-wide studies on MHWs at depth have to rely on models. This introduces additional challenges due to the long adjustment timescale of the deep ocean, resulting in long-term drift following the model's initialization. This unrealistic model drift dominates the MHW statistics below approximately 100 m when a fixed baseline is used. As a result, MHW studies at depth require a long model spin-up or have to apply a detrended baseline removing temperature trends. Based on a comparison of two model configurations with eddy-permitting and eddy-rich horizontal resolution, we show that the representation of mesoscale dynamics leads to pronounced differences in the characteristics of MHWs, in particular along the boundaries and along pathways of highly variable currents. Although a high horizontal resolution of the model is important, MHW statistics can be calculated on a coarser grid, which largely decreases the amount of data that needs to be processed. Our results highlight the importance of horizontal and vertical heat transport within the ocean to sub-surface, but also near-surface, MHWs. By investigating the vertical coherence of MHWs in an example region – here, the Cabo Verde archipelago – we show that MHWs are coherent over layers of a few hundred to 1000 m thickness, independent of the baseline used. These ranges are closely related to the vertical structure of the temperature field.

Leading dynamical processes of global marine heatwaves in an ocean state estimate - Jacopo Sala; Donata Giglio; Antonietta Capotondi; Thea Sukianto; Mikael Kuusela (Ocean Science, 2025)
Marine heatwaves (MHWs) have emerged as a very active area of research due to the devastating impacts of these events on marine ecosystems across different trophic levels. However, a clear understanding of the local drivers of these extreme ocean conditions is still limited at a global scale. Observations of the terms needed to constrain ocean heat budgets are very sparse, ocean reanalysis products are generally non-conservative and inadequate to conduct accurate heat budget analyses, and the fidelity of climate models with respect to simulating MHWs is still unclear. In this study, we make use of Argo float observations, a satellite-based sea surface temperature product, and the Estimating the Circulation and Climate of the Ocean (ECCO) state estimate to assess MHW characteristics over the global ocean. ECCO is then used to evaluate local MHW drivers. ECCO assimilates observations using the adjoint methodology, which optimizes the system trajectory given the observational constraints in a conservative fashion, making it an ideal product for the estimation of heat budgets. The representation of MHWs in ECCO is generally consistent with observations, although ECCO tends to underestimate MHW frequency and intensity and overestimate duration, relative to the observational products. Atmospheric forcing emerges as the dominant contributor to MHW onset and decline across most regions, while ocean dynamics, including the advective and diffusive convergence of heat, play crucial roles in the equatorial regions, specifically in the extratropical zones (e.g., western boundary currents, such as the Gulf Stream and Kuroshio) and the Southern Ocean. Regional analyses in the northeastern Pacific, southwestern Pacific, and Tasman Sea show diversity in leading dynamical mechanisms for MHW onset and decline both across regions and across events in the same regions: while air–sea exchanges of heat may contribute most frequently to MHW onset and decline, other mechanisms can also often provide dominant contributions and, at times, be the main driver. A more complete understanding of MHWs and their drivers is crucial for predicting their initiation, duration, intensity, and decline, to ultimately inform the development of mitigation and adaptation strategies for affected communities.

Mechanism of delayed storm surges in straits: seiche-induced oscillations triggered by typhoon passage - Shinichiro Ozaki; Yoshihiko Ide; Masaru Yamashiro (Ocean Science, 2025)
Storm surges are typically driven by a combination of low atmospheric pressure and strong winds associated with typhoons, with the maximum surge often occurring near the time of the typhoon's closest approach. However, during Typhoon Maysak (2020), delayed flooding was observed along the northern coast of Kyushu Island (NCKI), approximately 10 h after the typhoon had passed. By that time, both atmospheric pressure and wind intensity had substantially weakened, indicating that conventional mechanisms – such as wind setup and the inverted barometer effect – could not fully account for the phenomenon. This study explores the mechanisms behind delayed storm surges along the NCKI through a combination of observational data analysis, storm surge simulations, and continuous wavelet transform (CWT) analysis. The results reveal that typhoons passing through the western channel of the Tsushima Strait (WCTS) frequently generate a double-peaked storm surge anomaly, with the second peak being both larger and longer-lasting than the first. Spectral analysis identified dominant oscillations with periods of approximately 5 and 10 h, corresponding to natural modes of the Tsushima Strait. These oscillations, interpreted as low-mode seiches, are excited by the release of potential energy trapped within the strait once the external forcing has subsided. Notably, the 5 h mode appears to be strongly influenced by the Coriolis effect. Our findings reveal a previously unrecognized storm surge mechanism in which significant surges occur not during but after a typhoon’s passage through a strait. This delayed response arises from resonant amplification caused by modal oscillations within the strait, particularly in cases where typhoon tracks curve westward. The study underscores the critical roles of strait geometry, natural oscillation modes, and the timing of external forcing in shaping storm surge behavior. These insights extend beyond the NCKI and are relevant to other semi-enclosed strait regions worldwide that exhibit similar meteorological and geographical characteristics, even though the extent of influence from oscillation modes, periods, and amplitudes may vary depending on the strait's horizontal scale, water depth, and latitude.

Mechanisms of warm-water intrusions onto the West Spitsbergen Shelf during winter - Lukas Frank; Jon Albretsen; Ragnheid Skogseth; Frank Nilsen; Marius O. Jonassen (Ocean Science, 2025)
The West Spitsbergen Current, flowing northward along the continental slope in the eastern Fram Strait, represents a key pathway for warm Atlantic Water entering the Arctic Ocean. However, along the west coast of Svalbard, parts of this Atlantic Water frequently diverge from its core, intruding eastward onto the West Spitsbergen Shelf and further toward the adjacent fjords. Here, the associated excess heat has a substantial impact on the regional hydrography, as well as on the regional marine biosphere and cryosphere. This study uses a high-resolution, fully dynamical regional ocean model to investigate the mechanisms driving such warm-water intrusions during winter. Our results show that warming events on the West Spitsbergen Shelf are associated with a variety of cross-shelf exchange processes, including surface Ekman transport, upwelling of Atlantic Water from deeper slope regions across the shelf break, and topographical steering of Atlantic Water onto the shelf along shallower isobaths. The eastward displacement of the West Spitsbergen Current core itself is most frequently involved in triggering shelf warming events. Regardless of the specific mechanism, the intrusion depth on the shelf is governed by the relative density difference between the intruding Atlantic Water and the ambient shelf water. As winter progresses, increased shelf density due to cooling, brine rejection, and previous Atlantic Water intrusions enhances the likelihood of near-surface intrusions, in contrast to intrusions typically penetrating the shelf at depth in early winter. These findings highlight the complexity and seasonality of the cross-shelf intrusions of Atlantic Water from the West Spitsbergen Current onto the West Spitsbergen Shelf. They also emphasize the need for high-resolution modeling and complementary observations to fully capture the dynamics of these intrusions and their broader implications for Atlantic Water heat transport and ecosystem variability in the Svalbard region.

A novel multispecies approach for the detection of regime shifts in a plankton community – a case study in the North Sea - Paul Dees; Friederike Fröb; Beatriz Arellano-Nava; David G. Johns; Christoph Heinze (Ocean Science, 2025)
The physical environment both above and below the ocean surface has changed dramatically during the last century. Changes in the marine environment induced by increased release of greenhouse gases and direct exploitation of resources include increased ocean temperature, decreased salinity and pH, and removal of apex predators. The risk of ecological regime shifts occurring has similarly increased. A variety of methodologies to identify regime shifts have already been used in the North Sea, which has become an important case study for the analysis of regime shifts in a semi-enclosed waterbody. The North Sea is regarded as a case study in part due to the operation of the continuous plankton recorder, which has provided detailed abundance records of phyto- and zooplankton for over 60 years. Here, we propose a new methodology to calculate regime shift likelihood for every month between 1958 and 2020. This unique model produces a single time series of regime shift likelihood, using sequential abundance data of more than 300 plankton species. We show the model's ability to identify when regime shifts occurred in the past by comparing it to previous less automated methodologies. We have validated the model for use in the North Sea by estimating how often false positives and false negatives are generated. Results from the model indicate evidence for three periods of high regime shift likelihood in various parts of the North Sea: between 1962 and 1972, between 1989 and 1999, and from 2002 until 2015. We show that these periods are consistent with previous estimates of North Sea regime shifts, and discuss possible applications of the model's output of a single time series.

Phytoplankton detection study through hyperspectral signalling in the Patagonian fjords - Pilar Aparicio-Rizzo; Dagoberto Poblete-Caballero; Cristian Vera-Bastidas; Iván Pérez-Santos; Daniel Varela (Ocean Science, 2025)
In recent decades, the monitoring of coastal areas has become a priority due to the continued growth of human population pressure. Areas such as these constitute biodiversity hotspots in which the increase in phytoplankton blooms has become a socioecological problem with severe impacts at global and regional levels. One area affected by these blooms is the Patagonian fjords, a complex and intricate coastal system that is highly exposed to climate forces and anthropogenic activities, particularly the aquaculture industry (primarily salmon and mussel farming), which is the main driver of the local economy. In this context, ensuring prompt and accurate monitoring of phytoplankton in the area is crucial. As such, the focus of the present study is on the use of a new technology that combines hyperspectral sensors and unmanned aerial vehicles (UAVs) to detect, identify and differentiate phytoplankton species from optical data. Findings have identified differences not only between diatoms and dinoflagellates through the shape and magnitude of the spectral signal at 440, 470, 500, 520, 550, 570 and 580 nm, but also at the genus level (Rhizosolenia sp., Pseudo-nitzschia sp., Skeletonema sp., Chaetoceros sp. and Leptocylindrus sp.) and species level (Heterocapsa triquetra). Chlorophyll a concentration played a key role in reflectance spectra, demonstrating high variability in the green-red (∼500–750 nm) bands at low concentrations ( 4 µg L−1). Although the present study represents a positive step forward in the use of new tools and offers a novel monitoring methodology with regards to phytoplankton found in complex coastal systems, including the detection of a new identification route, increasingly high-quality imaging and data from a broader range of ecosystems and environments remains a necessity.

Spatiotemporal properties of intrinsic sea level variability along the southeastern United States coastline - Carmine Donatelli; Christopher M. Little; Rui M. Ponte; Stephen G. Yeager (Ocean Science, 2025)
The influence of intrinsic ocean variability on coastal sea level remains largely unexplored but is of potential importance for emerging forecasting efforts. As in weather forecasts, intrinsic variability will amplify uncertainty in initial conditions. However, variability originating from intrinsic processes may be predictable in a forecast system with sufficient resolution and accurate initialization. Here, we examine the spatiotemporal properties of intrinsic sea level variability along the southeastern United States coast using a suite of global ocean/sea-ice simulations at 0.1° horizontal resolution. In model simulations, intrinsic variability is a dominant component of the monthly de-seasonalized and detrended sea level variability in deep waters, but it is damped along continental shelves, where it comprises ∼ 10 %–30 % of the sea level standard deviation. Our analyses demonstrate that US East Coast and Gulf of Mexico shelves exhibit a common intrinsic mode, with maximal amplitude in the South Atlantic Bight and almost no expression north of Cape Hatteras. This intrinsic coastal mode is coherent with sea level along the Gulf Stream axis after detachment from Cape Hatteras. Intrinsic sea level variability in the detached Gulf Stream leads the coastal mode by 2–3 months, suggesting that intrinsic coastal sea level variability may exhibit predictability.

Mechanisms of the overturning circulation in the northern Red Sea other than convective mixing - Lina Eyouni; Zoi Kokkini; Nikolaos D. Zarokanellos; Burton H. Jones (Ocean Science, 2025)
The northern Red Sea (NRS) is where Red Sea Outflow Water (RSOW) and, occasionally, Red Sea Deep Water are formed. Glider observations are used to describe the formation mechanisms and pathways of intermediate waters in the NRS in late winter from 31 January to 18 April 2019. Utilizing glider observations, atmospheric reanalysis products, and satellite datasets, we evaluated the mesoscale activity and atmospheric conditions that contribute to outflow water formation. The cyclonic circulation in the regional surface dense water exposes it to the atmosphere, ventilating the water column and contributing to phytoplankton growth – enhancement of chlorophyll concentration due to nutrients upwelled into the euphotic layer (Zeu). Subduction of this water in the three-dimensional cyclonic circulation transported oxygenated and elevated chlorophyll concentration water to depths between 150 and 250 m along the 28.2 kg m−3 isopycnal. Unlike previous observations, in late February, a strong anticyclonic circulation blocked the inflow of warmer, fresher water into the region. It was accompanied by a negative heat flux and uplifting of dense water to the surface. Net cooling through mid-March cooled the incoming surface waters from the south. At the end of the observational period, the intrusion of warmer, fresher waters from the south coincided with the re-establishment of cyclonic circulation and capped the dense surface water that had formed during March. These observations demonstrate that multiple processes contribute to RSOW formation: convective mixing, cyclonic uplifting of dense water, subduction, and mesoscale or submesoscale processes.

Mesoscale dynamics of an intrathermocline eddy in the Canary Eddy Corridor - Luis P. Valencia; Ángel Rodríguez-Santana; Borja Aguiar-González; Javier Arístegui; Xosé A. Álvarez-Salgado; Josep Coca; María D. Gelado-Caballero; Antonio Martínez-Marrero (Ocean Science, 2025)
High-resolution observations of an intrathermocline eddy were conducted in November 2022 within the Canary Eddy Corridor. Formed in early summer 2022, this mature mesoscale eddy exhibited a vertical extent of 550 m, with its core centered at 110 m depth, and a segmented horizontal structure comprising a 23 km radius solid-body core surrounded by a 47 km wide outer ring. Propagating southwestward at 4.5 km d−1, its motion was consistent with the phase speed of a first-mode baroclinic Rossby wave. The eddy's rotational dynamics featured a 3.9 d inner-core rotation period shaped by stratification, leading to the formation of distinct rotational layers. Rossby number estimates (maximum of −0.7 at the center and −0.5 on average) and low core potential vorticity (∼10-11 m-1s-1, 90 % lower than surrounding values) revealed a regime dominated by planetary rotation, yet with a dynamically significant centripetal contribution – suggestive of a cyclogeostrophic momentum balance – and strong water mass isolation. Burger numbers, ranging from 1.27 to 0.14 (length-scale-based) and from 0.21 to 0.69 (energy-based), underscored the role of stratification and buoyancy forces in shaping the eddy's vertical structure. The eddy carried available heat and salt anomalies of 6.550×1018 J and 0.015×1012 kg, driving heat and salt (freshwater equivalent) fluxes of 4.60×1012 W and 0.42×109 kg s−1 (−0.012 Sv, where 1 Sv=106 m3 s−1), highlighting its role in transporting coastal upwelling waters into the ocean interior. The intrathermocline nature of the Bentayga eddy appears to have developed during its growth phase, likely driven by surface convergence linked to the interaction with upwelling filaments and subsequent isopycnal deepening as it propagated offshore. Low dissolved oxygen concentrations (100–110 µmol kg−1) and low apparent oxygen utilization (20–30 µmol kg−1) within the eddy core support the hypothesis of recent trapping of surface-derived upwelled waters. Over the course of its year-long lifespan, the eddy experienced intrinsic instabilities and eddy-to-eddy interactions, ultimately decaying by early summer 2023. The distinct properties of this eddy, together with the apparent variability among similar features in the Canary Eddy Corridor, underscore the need for expanded quasi-synoptic high-resolution studies. Comprehensive observational programs and advanced numerical simulations are essential to better understand the role of intrathermocline eddies (ITEs) as zonal pathways for heat, salt, and biogeochemical properties within regional ocean circulation.

Contribution of meridional overturning circulation and sea ice changes to large-scale temperature asymmetries in CMIP6 overshoot scenarios - Pedro José Roldán-Gómez; Pablo Ortega; Markus G. Donat (Ocean Science, 2025)
This work analyzes overshoot simulations from 13 models, including the SSP5-3.4OS and SSP1-1.9 experiments from the Coupled Model Intercomparison Project Phase 6 (CMIP6), to assess how regional temperatures after overshoot differ from those of before, and how these differences may be impacted by changes in sea ice, the Atlantic Ocean Heat Transport (OHT) and the Atlantic Meridional Overturning Circulation (AMOC). The overshoot scenarios, characterized by a peak in radiative forcing levels followed by a decline, show that changes during the CO2 increasing phase are not necessarily compensated during the CO2 decreasing phase, particularly at the regional level. Even if the global mean temperature may recover after the overshoot, regional conditions post-overshoot may still differ from those pre-overshoot, with spatial patterns characterized by large-scale temperature asymmetries. These asymmetries are found between Northern (NH) and Southern Hemisphere (SH), between high and mid-latitudes of the NH, and between western and eastern areas of the Southern Ocean. Changes in sea ice and ocean circulation are identified as potential sources of hysteresis, highlighting the impact of oceanic changes in the behavior of atmospheric variables in case of overshoot. The analyses from this work show that the relative contribution of each mechanism strongly depends on the model. Inter-model differences in the contributions of the meridional overturning can be associated with different climatologies of Mixed Layer Depth (MLD) in the northern North Atlantic (NNA) and in certain areas of the Southern Ocean. Despite these differences across models, both ocean circulation and sea ice changes contribute to shaping the regional temperatures after overshoot, with the temperature asymmetries between NH and SH mainly explained by changes in the AMOC, those between high and mid-latitudes of the NH by sea ice changes, and those between western and eastern areas of the Southern Ocean by the Southern Meridional Overturning Circulation (SMOC). These results highlight the importance of model intercomparison and analysis of ocean dynamics to understand the regional impacts of an overshoot, and more generally the responses to forcing changes.

Process-based modelling of nonharmonic internal tides using adjoint, statistical, and stochastic approaches – Part 2: Adjoint frequency response analysis, stochastic models, and synthesis - Kenji Shimizu (Ocean Science, 2025)
Internal tides are known to contain a substantial component that cannot be explained by (deterministic) harmonic analysis, and the remaining nonharmonic component is considered to be caused by random oceanic variability. For nonharmonic internal tides originating from distributed sources, the superposition of many waves with different degrees of randomness unfortunately makes process investigation difficult. This paper develops a new framework for process-based modelling of nonharmonic internal tides by combining adjoint, statistical, and stochastic approaches and uses its implementation to investigate important processes and parameters controlling nonharmonic internal-tide variance. A combination of adjoint sensitivity modelling and the frequency response analysis from Fourier theory is used to calculate distributed deterministic sources of internal tides observed at a fixed location, which enables assignment of different degrees of randomness to waves from different sources. The wave phases are randomized by the statistical model from Part 1 using horizontally varying phase statistics calculated by stochastic models. Essential inputs of the model suite are barotropic tidal currents, background stratification, and the variance and spatial correlation of internal-tide phase speed. An example application to nonharmonic vertical-mode-one semidiurnal internal tides on the Australian North West Shelf shows that (i) phase-speed variability primarily makes internal tides nonharmonic through phase modulation, and (ii) important controlling parameters include the variance and correlation length of phase speed, as well as anisotropy of the horizontal correlation of phase modulation. The model suite also provides a map of nonharmonic internal-tide sources, which is convenient for identifying important remote sources, such as the Lombok Strait in Indonesia. The proposed modelling framework and model suite provide a new tool for process-based studies of nonharmonic internal tides from distributed sources.

Process-based modelling of nonharmonic internal tides using adjoint, statistical, and stochastic approaches – Part 1: Statistical model and analysis of observational data - Kenji Shimizu (Ocean Science, 2025)
A substantial fraction of internal tides cannot be explained by (deterministic) harmonic analysis. The remaining nonharmonic part is considered to be caused by random oceanic variability, which modulates wave amplitudes and phases. The statistical aspects of this stochastic process have not been analysed in detail, although statistical models for similar situations are available in other fields of physics and engineering. This paper aims to develop a statistical model of the nonharmonic, incoherent (or nonstationary) component of internal tides observed at a fixed location and to check the model's applicability using observations. The model shows that the envelope-amplitude distribution approaches a universal form given by a generalization of the Rayleigh distribution, when waves with non-uniformly and non-identically distributed amplitudes and phases from many independent sources are superimposed. Mooring observations on the Australian North West Shelf show the applicability of the generalized Rayleigh distribution to nonharmonic vertical-mode-one to mode-four internal tides in the diurnal, semidiurnal, and quarter-diurnal frequency bands, provided that the power spectra show the corresponding tidal peaks clearly. These results demonstrate the importance of viewing nonharmonic internal tides as the superposition of many random waves. The proposed distribution can be used for many purposes in the future, such as investigating the statistical relationship between random internal-tide amplitude and the occurrence of nonlinear internal waves, as well as assessing the risk of infrequent strong waves for offshore operations. The proposed statistical model also provides the basis for investigating processes and parameters controlling nonharmonic internal-tide variance in Part 2.

Evolutions of the seasonal anticyclonic circulation around the Qingdao cold water mass in the China marginal sea and its mechanism - Lin Lin; Hans von Storch; Yang Ding (Ocean Science, 2025)
The circulation structure surrounding the Qingdao cold water mass in 2019 was investigated using three-dimensional ensembles of numerical simulations. This study reveals that a cold pool appears in early spring and reaches its peak in late May, and this pool is accompanied by local seasonal anticyclonic circulation. Momentum diagnostics reveal that vertical friction cannot be ignored because of the shallow topography and surface wind stress; as a result, the geostrophic balance is inapplicable in the Qingdao cold water mass region. Seasonal circulation mostly results from the balance of the pressure gradient, Coriolis, and vertical friction forces. The no-tide and no-wind numerical simulation results suggest that when the tidal forcing is turned off, unrealistically strong currents appear and are caused by the decrease in vertical friction in the no-tide simulation. Moreover, the direction of the eastern side of the anticyclonic circulation is reversed. Furthermore, the seasonal southwesterly monsoon contributes to the magnitude of the anticyclonic circulation, especially in the western portion of the anticyclonic circulation, by piling up the surface water eastward and further changing barotropic pressure gradient force. Additionally, upwelling occurs vertically around the Qingdao cold water mass and is influenced by tidal and wind forcings. The wind forcing affects upwelling (especially in the western part), which can be explained by Ekman pumping; the tides contribute to upwelling in the eastern part by reshaping the thermocline and further changing the barotropic and baroclinic pressure gradient force. Ensemble simulations are used, and a t test reveals that 51 % (90 %) of points, difference between the control run and the ensemble experiments without tidal forcing (and without wind forcing) is statistically significant.

Influence of wind stress and the Isonzo/Soča River outflow on surface currents in the Gulf of Trieste - Davide Lombardo; Sofia Flora; Fabio Giordano; Emanuele Ingrassia; Milena Menna; Stefano Querin; Laura Ursella (Ocean Science, 2025)
In this study, we analysed the extreme meteo-marine event that occurred between October and November 2023 in the northern Adriatic Sea region, focusing on its effects in the Gulf of Trieste (GoT). This event featured a strong inflow of very humid southerly winds, very intense thundershowers and storm surges on the coast, and caused several damages in the north-eastern part of Italy. The heavy rainfall in the Isonzo/Soča catchment led to two major runoff peaks with significant discharges into the GoT. These events represent an ideal case study to investigate how the interplay between local river discharges and the wind regime influence ocean currents in coastal areas. We adopted an integrated approach to fully understand the dynamics of peculiar environmental processes, clearly emerging from the visual analysis of satellite images (Sentinel-2) collected throughout the event. Therefore, we incorporated in our analysis multiple datasets, including rainfall, wind and surface currents from both HF radars and numerical models. In this way, we could analyse the interaction between river runoff and ocean currents in the GoT, and the importance of prevailing winds such as Bora and Sirocco in modulating current dynamics. The use of rainfall time series allowed us to assess the impact of precipitation on environmental conditions, while the analysis of the wind regime helped to understand the patterns, distribution and intensity of winds in the area of interest. It was observed that the intense runoff of the Isonzo/Soča River triggered by heavy rainfall can override the effects of wind, leading to a dominance of river-induced circulation in the GoT. Such a multi-platform integrated approach proves to be a useful tool for the analysis and, potentially, forecast of surface ocean dynamics and possible extreme events in coastal areas.

The coupled oxygen and carbon dynamics in the subsurface waters of the Gulf and Lower St. Lawrence Estuary and implications for artificial oxygenation - William A. Nesbitt; Samuel W. Stevens; Alfonso O. Mucci; Lennart Gerke; Toste Tanhua; Gwénaëlle Chaillou; Douglas W. R. Wallace (Ocean Science, 2025)
The Gulf and Lower St. Lawrence Estuary have experienced major environmental change over the past century, including the development of hypoxic bottom waters and their simultaneous warming and acidification. Here, we use biogeochemical observations collected during the 2021–2023 TReX project as well as historical data, integrated with a tracer-calibrated 1D Advection-Diffusion model with variable boundary conditions to represent dissolved oxygen (DO) and dissolved inorganic carbon (DIC) dynamics within the core of the oxygen minimum zone (27.15–27.3 kg m−3 isopycnals) of the Laurentian Channel. The rate of in-channel oxygen utilization in the deep layer was nearly invariant from 2003 to 2023 at 21.1 ± 2.5 µmol kg−1 yr−1 and the DIC accumulation rate was estimated to be 18.3 ± 2.5 µmol kg−1 yr−1. Using δ13CDIC data, we assess the effect of microbial organic matter remineralization processes and dilution of the 13CDIC pool (-6.6×10-3 ‰ µmol−1 of added metabolic DIC). This combination of long-term observations and tracer-informed transport modeling helps reconcile differences in prior estimates of biogeochemical transformation rates and improves our ability to predict deepwater DO and DIC cycling. Finally, we apply the model to the mitigation scenario proposed by Wallace et al. (2023) for artificial re-oxygenation of the Laurentian Channel bottom waters using pure oxygen. We estimate that the injection of ∼ 8.3 × 105 t yr−1 of oxygen, equivalent to an additional 55 µmol kg−1 relative to the 2023 boundary concentration proximal to the Cabot Strait, would be required to achieve and maintain above hypoxic levels (>62.5 µmol kg−1) at the head of the Laurentian Channel. Using the model, we estimate the time required to re-establish steady-state along-channel distributions of DO and DIC following a change in offshore boundary conditions to be about 10 years, or twice the along-channel transit time. This study provides new long-term characterizations of deepwater DO and DIC cycling and offers a first-order assessment of the feasibility of large-scale re-oxygenation in the Gulf and Lower St. Lawrence Estuary.

Capturing dense shelf water cascading with a high-resolution ocean reanalysis - Helena Fos; Jesús Peña-Izquierdo; David Amblas; Marta Arjona-Camas; Laia Romero; Víctor Estella-Pérez; Cristian Florindo-Lopez; Antoni Calafat; Marc Cerdà-Domènech; Pere Puig; Xavier Durrieu de Madron; Anna Sanchez-Vidal (Ocean Science, 2025)
Dense shelf water cascading (DSWC) is an oceanographic process that occurs when dense shelf water overflows over the shelf edge downslope toward the deep sea. Monitored in the northwestern Mediterranean by moorings since 1993 in the Lacaze-Duthiers Canyon and since 2005 in the Cap de Creus Canyon, numerical modeling with reanalysis extends this timeline further into the past. This study investigates a regional reanalysis (1987–2021) validated against mooring observations at 750–1000 m depth. The reanalysis successfully reproduces observed intense DSWC (IDSWC) events from 1999, 2000, 2005, 2006, 2012, 2013, and 2018 while identifying one previously unreported event in 1987 and detecting no IDSWC between 1988 and 1998. The reanalysis effectively matches 84 % of observed IDSWC days within the same week and 56 % on the exact date. Instead of assimilating IDSWC events from mooring observations to resolve the cascading process, the model relies solely on the seawater density on the shelf and revealed the seawater properties along the canyon that caused IDSWC. This work highlights the importance of high-resolution reanalyses in investigating the impacts of mesoscale processes on larger scales in the deep ocean.

Assessment of ocean bottom pressure variations in CMIP6 HighResMIP simulations - Le Liu; Michael Schindelegger; Lara Börger; Judith Foth; Junyang Gou (Ocean Science, 2025)
Ocean bottom pressure (pb) variations from high-resolution climate model simulations under the CMIP6 (Coupled Model Intercomparison Project Phase 6) HighResMIP protocol are potentially useful for oceanographic and space-geodetic research, but the overall signal content and accuracy of these pb estimates have hitherto not been assessed. Here, we compute monthly pb fields from five CMIP6 HighResMIP models at 1/4° grid spacing over both historical and future time spans and compare these data, in terms of temporal variance, against observation-based pb estimates from a 1/4° downscaled GRACE (Gravity Recovery and Climate Experiment) product and 23 bottom pressure recorders, mostly in the Pacific. The model results are qualitatively and quantitatively similar to the GRACE-based pb variances, featuring – aside from eddy imprints – elevated amplitudes on continental shelves and in major abyssal plains of the Southern Ocean. Modeled pb variance in these regions is ∼ 10 %–80 % higher and thus overestimated compared to GRACE, whereas underestimation relative to GRACE and the bottom pressure recorders prevails in more quiescent deep-ocean regions. We also form variance ratios of detrended pb signals over 2030–2049 under a high-emission scenario relative to 1980–1999 for three selected models and find statistically significant increases in future pb variance by ∼ 30 %–50 % across deep Arctic basins and the southern South Atlantic. The strengthening appears to be linked to projected changes in high-latitude surface winds and, in the case of the South Atlantic, intensified eddy kinetic energy. The study thus points to possibly new pathways for relating observed pb variability from (future) satellite gravimetry missions to anthropogenic climate change.

Currents and their drivers in the Archipelago Sea: insights from ADCP measurements - Hedi Kanarik; Laura Tuomi; Pekka Alenius; Elina Miettunen; Milla Johansson; Tuomo Roine; Antti Westerlund; Kimmo K. Kahma (Ocean Science, 2025)
The Archipelago Sea (AS) in the Baltic Sea is a complicated fragmented sea area with numerous small islands and islets that are crossed by several deeper straits. The area functions as a major route for both transport and leisure activities on the sea as well as many other forms of the blue economy. Even with the large number of maritime activities in this sea area, knowledge of the currents along the deep channels crossing the AS has been limited due to the lack of quality-assured measurements. To enhance the general understanding of the dynamics in the AS, we have collected and analysed currents in 10 different locations across the AS using acoustic Doppler current profiler (ADCP) current measurements made during numerous short measurement campaigns over the last two decades. Currents in the AS are restricted by the geometry of the area and typically have two main flow directions, with a slightly wider directional distribution at the southern parts of the AS. In open areas of the AS, surface currents have a mean magnitude of around 8 cm s−1 and can momentarily grow to significant magnitudes, as even the shortest measurement time series (of around 4 months) have measured currents exceeding at least 40 cm s−1. The current magnitudes in the long and narrow straits are almost double (14 cm s−1) the general current magnitudes of the area, and current magnitudes up to 115 cm s−1 have been measured at the northern end of the long northeastern strait. The AS currents are mostly driven by local winds, but strong oscillations in the surrounding basins can cause significant currents (up to 80 cm s−1 in narrow straits) if the local winds are weak enough (below 10 m s−1) or from non-optimal directions with respect to the straits. Our analysis shows that especially strong seiche in the Gulf of Finland combined with low sea levels in the Gulf of Bothnia is a significant factor in forcing currents in the AS.

Controls on dense-water formation along the path of the North Atlantic subpolar gyre - Oliver J. Tooth; Helen L. Johnson; Chris Wilson (Ocean Science, 2025)
The North Atlantic subpolar gyre (SPG) plays a fundamental role in the global climate system through the formation of dense North Atlantic Deep Water (NADW) as part of the Atlantic Meridional Overturning Circulation. Observations show pronounced decadal variability in SPG water mass properties; however, it remains unclear to what extent such thermohaline changes impact the formation of dense water. Here, we explore the mechanisms governing dense-water formation along the path of the SPG using Lagrangian water parcel trajectories in an eddy-rich ocean sea ice hindcast. We show that neither the rate of transformation of water parcels across density surfaces nor their thermohaline properties on arrival into the eastern SPG are rate-limiting factors governing dense-water formation. Instead, the total amount of dense water formed during transit around the SPG can be skilfully predicted based solely on the volume transport of light, upper-limb waters arriving into the eastern SPG via the branches of the North Atlantic Current. This relationship between upper-limb volume transport and dense-water formation emerges since the SPG boundary current is long enough for all upper-limb thermal anomalies to be damped during transit. Our findings emphasise the close relationship between the strength of the SPG and subpolar dense-water formation on multi-decadal timescales, such that a stronger SPG circulation following persistent positive phases of the North Atlantic Oscillation results in greater NADW formation along-stream. This underscores the importance of monitoring the state of the SPG for both decadal and longer-term climate predictions.

Application of quality-controlled sea level height observation at the central East China Sea: Assessment of sea level rise - Taek-Bum Jeong; Yong Sun Kim; Hyeonsoo Cha; Kwang-Young Jeong; Jin-Yong Jeong; Jae-Ho Lee (Ocean Science, 2025)
This study presents a state-of-the-art quality control (QC) process for the sea level height (SLH) time series observed at the Ieodo Ocean Research Station (I-ORS) in the central East China Sea, a unique in situ measurement taken in the open sea for over 2 decades at a 10 min interval. The newly developed QC procedure, named Temporally And Locally Optimized Detection (TALOD), has two notable differences in characteristics from the typical ones: (1) spatiotemporally optimized local range check based on the high-resolution tidal prediction model TPXO9 and (2) consideration of the occurrence rate of a stuck value over a specific period. In addition, the TALOD adopts an extreme event flag (EEF) system to provide SLH characteristics during extreme weather. A comparison with the typical QC process, satellite altimetry, and reanalysis products demonstrated that the TALOD method could provide reliable SLH time series with few misclassifications. A budget analysis suggested that the sea level rise at the I-ORS was primarily caused by the barystatic effect, and the trend differences between observations, satellite, and physical processes were related to vertical land motion. It was confirmed that ground subsidence of −0.89 ± 0.47 mm yr−1 is occurring at I-ORS. As a representative of the East China Sea, this qualified SLH time series makes dynamics research possible spanning from a few hours of nonlinear waves to a decadal trend, along with simultaneously observed environmental variables from the air–sea monitoring system at the research station. This TALOD QC method is designed to process SLH observations in the open ocean, but it can be generally applied to SLH data from tidal gauge stations in the coastal regions.

Salinity trends and mass balances in the Mediterranean Sea: revisit the role of air-sea freshwater fluxes and oceanic exchange - Chao Liu; Xinfeng Liang; Lisan Yu (Ocean Science, 2025)
Understanding the drivers of salinity and mass variability in the Mediterranean Sea is critical for assessing regional climate impacts and interpreting long-term water cycle changes. Although previous studies have closed the Mediterranean's mass and salinity budgets within uncertainty ranges, the relative contributions of key boundary fluxes – surface freshwater fluxes (evaporation minus precipitation minus runoff) and Gibraltar exchange – remain unclear. Here, we analyze the Mediterranean budgets from 2003 to 2017 using the ECCO v4r4 ocean state estimate. Our results show that surface freshwater loss, averaging 1.80 ± 0.10 Sv, dominates salinity variability, while the net Gibraltar exchange contributes a smaller, yet essential, salt input of 0.30 ± 0.20 Sv. Despite the saltwater inflow, the Mediterranean exports salt through the strait at −1.48 ± 0.20 Sv due to density differences between Atlantic Water (AW) and Mediterranean Outflow Water (MOW), leading to an overall salinity increase of 0.29 ± 0.09 Sv over the 15-year period. These results are consistent with observed Mediterranean salinification trends and underscore the dominant role of surface evaporation in shaping the basin's long-term evolution. This study provides a physically consistent, basin-integrated benchmark for interpreting Mediterranean water cycle changes under continued climate forcing.

Physical drivers and parameter sensitivities of pearl river-derived sediment dispersal on the Northern South China Sea Shelf: a modeling study - Guang Zhang; Suan Hu; Xiaolong Yu; Heng Zhang; Wenping Gong (Ocean Science, 2025)
This study employs the Coupled-Ocean-Atmosphere-Wave-Sediment-Transport (COAWST) modeling system to quantitatively assess the seasonal suspension, transport, and annual fate of Pearl River-derived sediment (riverine slow-settling single fine grains and high-settling flocs) on the northern continental shelf of the South China Sea (SCS). Following careful model validation, a series of sensitivity experiments were conducted to investigate the effects of tides, waves, background circulation, sediment settling velocity, critical shear stress, and sediment spin-up durations. The results reveal strong seasonal variations in sediment dynamics driven by the East Asian monsoon. During the wet summer, weaker hydrodynamic conditions promote the initial deposition of riverine sediment via the surface buoyant plume. In contrast, stronger winds and waves during winter enhance sediment resuspension and southwestward transport, particularly toward the Beibu Gulf. Spatially, approximately two-thirds of the annual Pearl River-derived sediment load is retained near the Pearl River Estuary. About 9 % reaches the continental shelf east of the estuary, while similar proportions accumulate in the Beibu Gulf and south of Hainan Island. Sensitivity experiments highlight the distinct and significant roles of different physical forcings in sediment dispersal. Tidal dynamics strongly enhance sediment mobilization and transport within the estuary by increasing bottom shear stress, which promotes offshore sediment export and limits local deposition. Wave forcing plays a dominant role in sediment resuspension near the river mouth and along the coast, especially during winter, facilitating sediment redistribution across the shelf. The remotely forced (large-scale, non-local forcing) ambient shelf circulation in summer drives eastward sediment transport, enabling sediment to spread widely across the shelf. Model outcomes are also sensitive to sediment parameterization. The natural seasonal increase in critical shear stress for erosion during winter counteracts part of the wave-enhanced resuspension capacity, thereby reducing resuspension and erosion on the continental shelf east of the Leizhou Peninsula. Higher settling velocities decrease suspended sediment concentrations and promote near-source retention, limiting long-distance transport. Spin-up duration experiments indicate that Pearl River–derived sediment, which enters and accumulates in various regions of the model domain during the first year, continues to migrate southwestward in the second year under the influence of the mean annual flow field. In contrast, the spin-up duration of seabed sediment has little impact on the retentions of Pearl River–derived sediment on the shelf. Overall, this study reveals the transport pathway and fate of the Pearl River-derived sediment and provides a model-based assessment of its seasonal behavior and the sensitivity of suspended sediment dispersal to physical drivers and sediment parameters or conditions on the northern SCS shelf. It identifies key physical drivers regulating sediment transport and deposition patterns, offering new insight into sediment fate in a monsoon-dominated shelf system.

The global ocean mixed layer depth derived from an energy approach based on buoyancy work - Efraín Moreles; Emmanuel Romero; Karina Ramos-Musalem; Leonardo Tenorio-Fernandez (Ocean Science, 2025)
The mixed layer depth (MLD) is critical for understanding ocean-atmosphere interactions and internal ocean dynamics. Traditional methods for determining the MLD, commonly relying on constant temperature and density thresholds, may not adequately address spatial and temporal variations in local oceanographic conditions, thereby limiting their global consistency and applicability. An energy-based definition of the mixed layer could be a more physically consistent alternative to address this issue. We propose a physically derived and energy-based methodology that defines the mixed layer as the energetically homogeneous upper ocean layer in which water parcels can move with little or no buoyancy work. The threshold in buoyancy work, which determines the mixed layer globally throughout the year, was carefully investigated. An energy-based global monthly MLD climatology demonstrated the reliability of the methodology across diverse ocean conditions and its usefulness for studies spanning seasonal to climate time scales, from regional to large spatial scales. Our easy-to-implement MLD methodology provides a robust criterion that is globally and temporally consistent, maintaining quasi-homogeneity in energy, density, and temperature year-round for most of the global ocean. This study promotes the development of MLD energy-based methodologies that could offer significant potential for advancing the study of dynamic and thermodynamic processes, including heat content and vertical exchanges. Our methodology could also serve as a robust tool for validating ocean circulation models and supporting intercomparison studies in initiatives such as the Ocean Model Intercomparison Project (OMIP) and the International Coupled Model Intercomparison Project (CMIP). Future research will explore its applicability to high-frequency processes and regional variability, further enhancing its utility for understanding and modeling oceanic phenomena.

The historical representation and near-future (2050) projections of the Coral Sea current system in CMIP6 HighResMIP - Jodie A. Schlaefer; Clothilde Langlais; Severine Choukroun; Mathieu Mongin; Mark E. Baird (Ocean Science, 2025)
The Coral Sea houses expansive coral reefs. Reef health is inextricably linked to water temperatures, which are regulated by the hydrodynamic environment. The ocean current system in the Coral Sea is dominated by jets of the South Equatorial Current (SEC): the North Vanuatu Jet (NVJ), the North Caledonian Jet (NCJ) and the South Caledonian Jet (SCJ). We investigated the projected near-future (2050) changes in the temperature and transport structure of the Coral Sea using the three highest-resolution climate models from the high-resolution coupled modelling experiment (HighResMIP) conducted within the latest iteration of the Coupled Model Intercomparison Project Phase 6 (CMIP6). We found that the HighResMIP models successfully represented the historical temperature and transport structure of the SEC jets and their El Niño–Southern Oscillation-related variability. Surface ocean warming of 0.78 and 1.12 °C was projected in the Coral Sea under 1.5 and 2 °C global air temperature warming, respectively. The maximum depth of the warming signal deepened by 30 m per decade, penetrating to 400 m by 2050. This indicated the additional thermal stress that could be experienced by Coral Sea ecosystems. Interestingly, the surface warming was associated with a sub-surface cooling between 400 and 600 m. Decreases in the transports of the NVJ and NCJ and an intensification of the SCJ were also projected in the HighResMIP models. The magnitudes of the changes were relatively small (2 % to 7 % of historical means) and of a similar order to the variability in transport associated with the El Niño–Southern Oscillation. Our analysis further showed that the transport projections of the NVJ and NCJ varied with depth, where surface intensifications coincided with the areas of greatest warming. These changes could modify western boundary currents and upwelling dynamics on the Great Barrier Reef shelf.

Tracking marine heatwaves in the Balearic Sea: temperature trends and the role of detection methods - Blanca Fernández-Álvarez; Bàrbara Barceló-Llull; Ananda Pascual (Ocean Science, 2025)
Marine heatwaves (MHWs) are defined as discrete periods of anomalous ocean warming. In the most commonly used MHW determination method, the threshold over which a certain temperature is considered to be an MHW is calculated using a fixed baseline constructed from a common climatology (1982–2011). By this definition, these phenomena have been increasing in frequency and intensity due to global warming, and this is expected to ultimately lead to a saturation point. Significant efforts have been directed towards developing new ways of defining marine heatwaves, motivated by the need to differentiate between long-term temperature trends and extreme events. The Mediterranean Sea serves as an ideal backdrop for comparing different MHW detection methods due to its rapid response to climate change, with higher warming trends than the global ocean. In this work, we evaluate sea surface temperature trends in the Balearic Sea, a subregion of the western Mediterranean, and compare the fixed-baseline MHW detection method with two recently developed alternative methodologies. The first alternative employs a moving climatology to adjust the baseline, while the second method involves detrending the temperature data before detecting MHWs with a fixed baseline. For the period between 1982 and 2023, our analysis reveals a statistically significant warming trend of 0.036 ± 0.001 °C per year, which represents an increase of ∼ 10 % compared to previous studies in the same region due to the inclusion of two particularly warm recent years, 2022 and 2023. Regarding MHWs, all three methods identify major events in 2003 and 2022. However, the fixed-baseline method indicates an increase in MHW frequency and duration over time, a tendency not detected by the other methodologies, since we isolate the extreme events from the long-term warming trend. This study underscores the importance of selecting an appropriate MHW detection method that aligns with the intended impact assessments. Studies performed with a moving baseline or detrended data could be more appropriate to analyse species with higher adaptability, while a fixed baseline could be a better option to study species that are less adaptable and more sensitive to exceeding a critical temperature threshold.

Mapping benthic marine habitats featuring coralligenous bioconstructions: a new approach to support geobiological research - Giuseppe Maruca; Mara Cipriani; Rocco Dominici; Gianpietro Imbrogno; Giovanni Vespasiano; Carmine Apollaro; Francesco Perri; Fabio Bruno; Antonio Lagudi; Umberto Severino; Valentina A. Bracchi; Daniela Basso; et al. (Ocean Science, 2025)
Seabed mapping represents a very useful tool for seascape characterization and benthic habitat study and requires advanced technologies for acquiring, processing, and interpreting remote data. Particularly, acoustic instruments, such as high-resolution swath bathymetry sounders (i.e., multibeam echosounder: MBES), allow us to recognize, identify, and map the extension of benthic habitats without applying invasive mechanical procedures. Bathymetry and backscatter (BS) data are crucial to perform modern habitat mapping. Although the acquisition and processing of bathymetric data follow standardized procedure (e.g., Hydrographic Organization guidelines), and recent studies have proposed recommendations for backscatter acquisition and processing, a broadly validated methodological approach, integrating geomorphometric analysis for benthic habitat mapping, is still lacking. In this work, a new approach for benthic habitat mapping, with a focus on coralligenous bioconstructions, was developed using the open-source software QGIS. This methodology, tested within the Isola Capo Rizzuto Marine Protected Area (Calabria, Italy), is designed to be freely reproducible by researchers working in the field of marine ecosystem monitoring and conservation. Through the proposed mapping procedure, it is possible to (i) identify benthic habitats in selected study areas by combining bathymetry and BS data with geomorphological indices performed in QGIS and (ii) quantitatively define the 2D and 3D distribution of coralligenous bioconstructions in terms of surface covered, thickness, and volume. Moreover, the statistical analysis of quantitative morphometric data allowed for comparison of geometric characteristics of different coralligenous morphotypes. The obtained results, combined with improvement of minimally invasive sampling and geobiological–geochemical characterization, can contribute to the development of protocols aimed at monitoring marine bioconstructed ecosystems, many of which are protected by national and international regulations due to their importance for Mediterranean biodiversity preservation, and plan actions for their protection and persistence.

Regional modeling of internal-tide dynamics around New Caledonia – Part 2: Tidal incoherence and implications for sea surface height observability - Arne Bendinger; Sophie Cravatte; Lionel Gourdeau; Clément Vic; Florent Lyard (Ocean Science, 2025)
New Caledonia, in the southwestern tropical Pacific, has recently been identified as a hot spot for energetic semidiurnal internal tides. In a companion paper, the life cycle of coherent internal tides, characterized by fixed amplitude and phase, was investigated in the region through harmonic analysis of a year-long, hourly time series from numerical simulation output. In this study, we investigate the temporal variability of the internal tide by decomposing the semidiurnal signals into coherent and incoherent components. Semidiurnal barotropic-to-baroclinic energy conversion is largely governed by the coherent component (> 90 %), amplified by a factor of 3 to 7 from neap to spring tides through the interaction of M2 and S2 barotropic tidal currents. Incoherent conversion – negligible in the annual mean – can explain on monthly to intraseasonal scales a notable fraction of variability, modifying semidiurnal conversion by up to ±20 %. The latter is largely explained by local effects, particularly the work of the coherent barotropic tide on incoherent baroclinic bottom pressure amplitude variations, linked to mesoscale-eddy-induced stratification changes. Away from the generation sites, tidal incoherence increases, evident through altered orientation of tidal beams and increasing phase variability, caused mainly by interactions with mesoscale currents. Variations in conversion are not consistently proportional to those in energy flux divergence, suggesting that variations in energy dissipation are linked to additional mechanisms that deserve further investigation. The incoherent sea surface height signature, with a root mean square amplitude of 1–2 cm, is widespread across the domain and introduces limitations in disentangling balanced (near-geostrophic) and unbalanced (wave-like) motions in spectral space. Transition scales – the length scale at which unbalanced motions become dominant over balanced motions – are proven meaningful when inferred from altimetry tracks that align with the main propagation direction of internal tides. However, when not aligned the method is flawed as it does not take into account the anisotropy of internal-tide dynamics.

Synoptic observation of a full mesoscale eddy lifetime and its secondary instabilities in the Gulf of Mexico - Charly de Marez (Ocean Science, 2025)
Mesoscale eddies are crucial to ocean circulation, climate, and tracer transport. Yet their life cycle has never been observed synoptically at high resolution. In this study, I use novel SWOT satellite altimetry data to present the first synoptic characterization of a loop current eddy's life cycle in the Gulf of Mexico, during a 6-month period. SWOT allows for the direct observation of key dynamical processes – such as eddy shielding, high-mode instabilities, and dipolar interactions – that were previously only partially resolved in interpolated altimetry products and mainly described in theoretical or numerical studies. These observations challenge the traditional view of eddies as simple, elliptical structures, emphasizing the role of mesoscale interactions in their evolution. Furthermore, SWOT captures intense submesoscale turbulence at the eddy's rim, revealing secondary instabilities likely driving its decay. These findings not only validate decades of vortex theory but also offer new insights into oceanic turbulence dynamics.

Far-future climate projection of the Adriatic marine heatwaves: a kilometre-scale experiment under extreme warming - Cléa Denamiel (Ocean Science, 2025)
The impact of a far-future extreme warming scenario on Adriatic marine heatwave (MHW) characteristics – including intensity, duration, spatial extent, and associated environmental drivers – is assessed using the Adriatic Sea and Coast (AdriSC) kilometre-scale atmosphere–ocean model. The main aim of this study is to evaluate the added value and limitations of the pseudo-global-warming (PGW) approach, used to force the far-future AdriSC simulation, in projecting Adriatic MHWs. In line with existing knowledge, the results indicate a significant increase in MHW intensity along with a notable expansion in spatial coverage, particularly in the central and eastern Adriatic. Seasonal patterns show that the most intense MHWs occur between May and September, with events extending into late autumn and early winter under extreme warming. This study also reveals several novel insights. First, the Po River plume is identified as a key factor for the onset and decline of MHWs. Lower river discharges are associated with intense MHW onset, while higher discharges aid in heat dissipation during decline phases. As air–sea heat fluxes are demonstrated to play a critical role in MHW onset along the plume, these findings suggest that MHWs are more likely to develop and persist under low Po River discharge conditions, when water clarity increases and solar radiation absorption is enhanced due to reduced suspended sediments and organic matter. Second, the study identifies a gap in MHW activity, potentially linked to the Eastern Mediterranean Transient, highlighting the influence of natural variability on MHW dynamics. However, no correlation is found with the Ionian–Adriatic Bimodal Oscillating System, suggesting the need for further research on oceanographic influences. Consequently, the PGW approach is found to effectively capture the thermodynamic changes influencing the MHWs in the Adriatic Sea despite potentially oversimplifying future MHW dynamics as it assumes stationarity in climate signals. Finally, these findings underscore the urgent need for adaptive strategies to mitigate the impacts of intensified MHWs on marine ecosystems and coastal communities, particularly in vulnerable nearshore areas. Future research should incorporate ensembles of high-resolution projections and assess additional climate stressors to provide a more comprehensive understanding of Adriatic MHWs under future warming.

Effect of nonlinear tide–surge interaction in the Pearl River Estuary during Typhoon Nida (2016) - Linxu Huang; Tianyu Zhang; Shouwen Zhang; Hui Wang (Ocean Science, 2025)
Storm surge is one of the most significant marine dynamic disasters affecting coastal areas worldwide. A comprehensive study of its mechanisms is vital for improving forecasting capabilities and developing more prevention strategies. In this study, a two-dimensional (2D) numerical model based on the Advanced Circulation Model (ADCIRC) was employed to examine the characteristics of storm surges and the mechanisms of tide–surge interaction in the Pearl River Estuary (PRE) during Typhoon Nida (2016). Three distinct model runs were conducted to differentiate between variations in water levels attributable to astronomical tides, storm surges, and their combined effect. The results indicated that storm tides are primarily modulated by tides through tide–surge interactions. The nonlinear effect of tide–surge interactions is primarily generated by the nonlinear local acceleration term and convection term from the tide–surge interactions in the study area, as derived from mathematical terms. However, in regions of shallow water, such as the northern part of the island of Qi'ao and Shenzhen Bay, they are predominantly governed by the nonlinear wind stress term and the bottom friction term. Furthermore, the variations in the y components of the nonlinear momentum terms are more significant than those in the x components. To investigate the impact of the tidal phase on the storm surge response to Typhoon Nida, the timing of the landfall was altered in order to introduce variations in PRE characteristics. The results demonstrate that the contribution ratio of each nonlinear term remains relatively constant, while the magnitudes exhibit fluctuations contingent on the timing of the landfall. However, further studies on additional typhoon events, especially with onshore winds, are needed, together with a comprehensive consideration of the meteorological processes and mechanisms of tidal-wave propagation inside and outside the estuary. The model system could still be improved in the future.

Decoding pelagic ciliate (Ciliophora) community divergences in size spectrum, biodiversity and driving factors globally spanning five temperature zones - Chaofeng Wang; Zhiqiang Xu; Guangfu Luo; Xiaoyu Wang; Yan He; Musheng Lan; Tiancheng Zhang; Wuchang Zhang (Ocean Science, 2025)
The community structure of microzooplanktonic ciliates – encompassing size spectrum, biodiversity and biotic–abiotic interplay – is critical for unravelling their ecological role in marine ecosystems, yet it remains challenging to elucidate on a global scale. To address this knowledge gap, we conducted field observational studies across five temperature zones (North Frigid Zone, NFZ; Sub-Arctic Zone, SAZ; North Temperate Zone, NTZ; Torrid Zone, TZ; South Frigid Zone, SFZ). Our analysis demonstrates a sharp decline in ciliate abundance and biomass below the 100 m layer, with distinct vertical distribution patterns observed in each climate region. Moreover, although abundance of ciliate size spectra exhibited a decrease trend from small to large size spectra globally, there were steeper slope lines observed in both polar zones (NFZ and SFZ) compared to the other temperature zones. Latitudinally, ciliate abundance and tintinnid biodiversity exhibited an anti-phase relationship, where the TZ hosted peak biodiversity, while the polar seas showed the highest abundance. Furthermore, a multivariate biota–environment analysis indicated that temperature has a primary influence on ciliate community constitution in the global marine ecosystem, and the bottom-up control plays a key role in shaping assemblages. In conclusion, these results underscore the unprecedented divergences in ciliate trait structure among five temperature zones and can be taken as a guideline for assessing the potential effects of climate change on pelagic ciliates in future marine realms.

Mesoscale dynamics and transport in the North Brazil Current as revealed by the EUREC4A-OA experiment - Yan Barabinot; Sabrina Speich; Xavier Carton; Pierre L'Hégaret; Corentin Subirade; Rémi Laxenaire; Johannes Karstensen (Ocean Science, 2025)
The North Brazil Current (NBC) rings are key features for interhemispheric water transport, facilitating exchange between the South Atlantic and North Atlantic Ocean. However, significant uncertainties persist regarding the total volume transported by these structures and the water masses they advect. In this study, we integrate high-resolution in situ observations from the EUREC4A-OA field experiment with satellite altimetry to address these knowledge gaps. Using a novel methodology, we estimate that surface NBC rings transport approximately 1.5 Sv, while subsurface eddies contribute between 0.4 and 9.7 Sv, underscoring their critical role in the regional total transport. Combined, these transports may significantly contribute to closing the Atlantic Meridional Overturning Circulation transport at low latitudes. Surface NBC rings predominantly advect Salinity Maximum Waters and fresh waters from the Amazon River, whereas subsurface NBC rings play a critical role in transporting Eastern South Atlantic Central Waters, Western South Atlantic Central Waters, and Antarctic Intermediate Waters northward. We estimate that the heat transport by surface and subsurface NBC rings is 5.8 and 0.3 TW, respectively, which is significantly lower than previous findings. Overall, these findings underscore the pivotal role of subsurface NBC rings as conduits for South Atlantic Waters across the Equator to the tropical North Atlantic. This study confirms the intricate dynamics of NBC rings and their essential role in interhemispheric water transport.