Cross-canyon variability in zooplankton backscattering strength in a river-influenced upwelling area - Macarena Díaz-Astudillo; Manuel I. Castillo; Pedro A. Figueroa; Leonardo R. Castro; Ramiro Riquelme-Bugueño; Iván Pérez-Santos; Oscar Pizarro; Gonzalo S. Saldías (Ocean Science, 2025)
Zooplankton are a key component of food webs in upwelling systems. Their distribution is influenced not only by mesoscale and climate dynamics but also by topography and local currents. Submarine canyons that incise the continental shelf can act as conduits, transporting deep, nutrient-rich waters to shallower regions and promoting coastal biological productivity. Consequently, these canyons facilitate the advection and accumulation of zooplankton. We aimed to describe the spatio-temporal variability in zooplankton distribution (using net samples and acoustic data) and their association with local currents in a long, narrow submarine canyon located in the highly productive continental shelf off central Chile. The backscattering strength (Sv), a proxy for zooplankton biomass, was highly variable on both diurnal and spatial scales. Higher Sv and abundances were found at night, following the classic diel vertical migration pattern. Zooplankton were not uniformly distributed within the canyon. In the surface and mid-depth layers, the canyon walls accumulated more zooplankton than the center, particularly during the nighttime. Within the canyon, the currents were asymmetrical and frequently reversed direction. When the positive along-canyon current was stronger on the northern slope, Sv was also higher on that wall. This pattern was especially evident in the section closer to the canyon head. We show that the Biobio Canyon is a highly dynamic environment where oceanographic conditions can rapidly shift. Our findings suggest a feasible mechanism for zooplankton retention driven mainly by along-canyon flow asymmetry and vertical migrations.

Drivers of high-frequency extreme sea levels around northern Europe – synergies between recurrent neural networks and random forest - Céline Heuzé; Linn Carlstedt; Lea Poropat; Heather Reese (Ocean Science, 2025)
Northern Europe is particularly vulnerable to extreme sea level events as most of its large population and financial and logistical centres are located by the coastline. Policy-makers need information to plan for near- and longer-term events. There is a consensus that, for Europe, in response to climate change, changes to extreme sea level will be caused by mean sea level rise rather than changes in its drivers, meaning that determining current drivers will aid such planning. Here we determine from explainable AI the meteorological and hydrological drivers of high-frequency extreme sea level at nine locations on the wider North Sea–Baltic Sea coast using long short-term memory (LSTM, a type of deep recurrent neural network) and the simpler random forest regression on hourly tide gauge data. LSTM is optimised for targeting the excess values or periods of prolonged high sea level, random forest, the block maxima, or most extreme peaks in sea level. Through the permutation feature of LSTM, we show that the most important drivers of the periods of high sea level over the region are the westerly winds, whereas random forest reveals that the driver of the most extreme peaks depends on the geometry of the local coastline. LSTM is the most accurate overall, although predicting the highest values without overfitting the model remains challenging. Despite being less accurate, random forest agrees well with the LSTM findings, making it suitable for predictions of extreme sea level events at locations with short and/or patchy tide gauge observations.

Multiscale phytoplankton dynamics in a coastal system of the eastern English Channel: the Boulogne-sur-Mer coastal area - Kévin Robache; Zéline Hubert; Clémentine Gallot; Alexandre Epinoux; Arnaud P. Louchart; Jean-Valéry Facq; Alain Lefebvre; Michel Répécaud; Vincent Cornille; Florine Verhaeghe; Yann Audinet; Laurent Brutier; et al. (Ocean Science, 2025)
To study changes in phytoplankton community composition on different timescales, an automated flow cytometer (CytoSub, CytoBuoy b.v.) was deployed at the MAREL Carnot automated monitoring station in Boulogne-sur-Mer (eastern English Channel, France) during spring (2021 and 2022) and summer (2022), following an Eulerian approach. Phytoplankton dynamics were recorded every 2 h, distinguishing 11 phytoplankton functional groups (PFGs) based on optical and fluorescence properties. This enabled detailed characterization of PFG successions, including MicroRED (mostly diatoms) and NanoRED (mostly haptophytes of the species Phaeocystis globosa) transitions in spring, as well as a summer dominance by PicoORG (picocyanobacteria, mostly of the genus Synechococcus) and PicoRED. Four rare events, including a salinity drop (April 2021), strong winds (May 2021 and April 2022), and a marine heat wave (July 2022), caused rapid shifts in phytoplankton community assemblage. Empirical mode decomposition (EMD) and Lomb–Scargle periodogram (LSP) analyses revealed that 85±10 % of variability in total phytoplankton abundance, red fluorescence (a proxy of chlorophyll a), and Shannon diversity occurred on relatively short timescales (9 h to 11 d) for time series of several months, highlighting the value of high-frequency monitoring in capturing ecological dynamics under macrotidal conditions in the eastern English Channel.

Assessing subseasonal forecast skill for use in predicting US coastal inundation risk - John R. Albers; Matthew Newman; Magdalena A. Balmaseda; William Sweet; Yan Wang; Tongtong Xu (Ocean Science, 2025)
Developing predictions of coastal flooding risk on subseasonal timescales (2–6 weeks in advance) is an emerging priority for the National Oceanic and Atmospheric Administration (NOAA). In this study, we assess the ability of two current operational forecast systems, the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (IFS) and the Centre National de Recherches Météorologiques climate model (CNRM), to make subseasonal ensemble predictions of the non-tidal residual component of coastal water levels at United States coastal gauge stations for the period 2000–2019. These models were chosen because they assimilate satellite altimetry at forecast initialization and attempt to predict the mean sea level, including a global mean component whose absence in other forecast systems complicates assessment of tide gauge reforecast skill. Both forecast systems have skill that exceeds damped persistence for forecast leads through 2–3 weeks, with IFS skill exceeding damped persistence for leads up to 6 weeks. Post-processing forecasts to include the inverse barometer effect, derived from mean sea level pressure forecasts, improves skill for relatively short forecast leads (1–3 weeks). Accounting for vertical land motion of each gauge primarily improves skill for longer leads (3–6 weeks), especially for the Alaskan and Gulf coasts; sea-level trends contribute to reforecast skill for both model and persistence forecasts, primarily for the East and Gulf coasts. Overall, we find that current forecast systems have sufficiently high levels of deterministic and probabilistic skill to be used in support of operational coastal flood guidance on subseasonal timescales.

Seasonality of meridional overturning in the subpolar North Atlantic: density flux as a metric for understanding the Atlantic meridional overturning circulation - Alan D. Fox; Neil J. Fraser; Stuart A. Cunningham (Ocean Science, 2025)
The Atlantic meridional overturning circulation (AMOC) has a notable seasonal component. This influences the jet stream and the location, frequency, and intensity of extreme weather events. Understanding this seasonality is important for mitigating the impacts of AMOC changes on European weather and climate. Here, we place meridional overturning and fluxes in a coherent framework. This framework highlights the integral relationship between meridional overturning circulation and property transports, both being functions purely of the overturning streamfunction Ψ. Using this framework, we examine the seasonality observed in overturning and density, temperature, and freshwater fluxes at the Overturning in the Subpolar North Atlantic Program (OSNAP) line in the subpolar North Atlantic. We find the seasonal cycle of the MOC metric (the standard measure of overturning, defined as the maximum of the overturning streamfunction) to be dominated by Ekman transports and the large-scale seasonal cycle of surface density; heat flux to be dominated by barotropic velocity variability; the seasonal cycle of freshwater flux to be dominated by a combination of barotropic velocities and the salinity in the western boundary current; and density flux to reflect a broad range of characteristics and processes. We show that the MOC metric is a poor predictor, on seasonal timescales, of either density fluxes or the more societally relevant ocean heat and freshwater transports. This is due to each of these metrics responding to different physical processes. The MOC metric, on seasonal timescales at least, has very high sensitivity to near-surface physical characteristics in a limited geographical area. These characteristics are not necessarily reflective of the fundamental processes driving overturning. Therefore, we suggest caution in the use of the standard MOC metric when studying overturning and the routine use of the density flux as a valuable additional overturning metric.

Marine data assimilation in the UK: the past, the present, and the vision for the future - Jozef Skákala; David Ford; Keith Haines; Amos Lawless; Matthew J. Martin; Philip Browne; Marcin Chrust; Stefano Ciavatta; Alison Fowler; Daniel Lea; Matthew Palmer; Andrea Rochner; et al. (Ocean Science, 2025)
In the last 2 decades, UK research institutes have led a wide range of developments in marine data assimilation (MDA), covering areas from operational applications in physics and biogeochemistry to fundamental theory. We highlight the emergence of strong collaboration in the UK MDA community over this period and the increasing unification of its tools. We focus on identifying the MDA stakeholder community and current/future areas of impact, as well as current trends and future opportunities. This includes the rapid growth of machine learning (ML)/artificial intelligence (AI) and digital-twin applications. We articulate a vision for the future, including the need for future types of observational data (whether planned missions or hypothetical) and how the community should respond to increases in computational power and new computer architectures (e.g. exascale computing). We contrast the requirements of different MDA areas, including physics, biogeochemistry, and coupled data assimilation (DA). Although the specifics of the vision depend on each area, common themes emerge. We advocate for balanced redistribution of new computational capability among increased model resolution, model complexity, more sophisticated DA algorithms, and uncertainty representation (e.g. ensembles). We also advocate for integrated approaches, such as strongly coupled DA (ocean–atmosphere, physics–biogeochemistry, and ocean–sea ice) and the use of ML/AI components (e.g. for multivariate increment balancing, bias correction, model emulation, observation re-gridding, or fusion).

Subsurface manifestation of marine heat waves in the southwestern Indian Ocean - Clea B. Welch; Neil Malan; Daneeja Mawren; Tamaryn Morris; Janet Sprintall; Juliet C. Hermes (Ocean Science, 2025)
Marine heat waves (MHWs) are extreme events of prolonged, anomalously warm ocean temperatures. Globally, marine heat waves are increasing in frequency and intensity and are responsible for long-term impacts on marine ecosystems, which have devastating socio-economic consequences. A key gap in our understanding of MHWs is how they manifest in the subsurface. This paper uses satellite sea surface temperature (SST) data and in situ subsurface temperature observations from expendable bathythermographs (XBTs) to investigate the anomalous water temperature characteristics associated with surface-identified MHWs in the southwestern Indian Ocean (SWIO) and how they progress through the water column. We find that (1) regions of high eddy activity in the Mozambique Channel and southeast of Madagascar, where eddy kinetic energy is high and SST variability is low, are primarily characterised by the occurrence of abrupt and intense MHWs and that (2) the frequency, duration and intensity of these events are largely associated with mesoscale activity. In these eddy corridors, surface-detected MHW case studies demonstrate a strong, deep-reaching subsurface temperature anomaly signal with a maximum intensity below the surface. The majority of these MHWs are associated with anticyclonic eddies, which provide a possible mechanism for the deep extent of the surface MHWs. Improving our understanding of the interaction between mesoscale features and subsurface MHW characteristics will benefit the prediction of MHWs and management of the regions' biodiversity.

Combining BioGeoChemical-Argo (BGC-Argo) floats and satellite observations for water column estimations of the particulate backscattering coefficient - Jorge García-Jiménez; Ana B. Ruescas; Julia Amorós-López; Raphaëlle Sauzède (Ocean Science, 2025)
As the second-largest carbon reservoir on Earth, the ocean regulates the carbon balance through dissolved and particulate organic carbon (POC) forms. Monitoring carbon cycle processes is key to understanding the climate system. Although most organic carbon in the ocean exists in dissolved form, POC, despite its smaller share, plays a vital role by connecting surface biomass production with the deep ocean and sedimentation processes. POC estimation is achieved by measuring proxies like the particulate backscattering coefficient (bbp) estimated from satellite observations and in situ sensors, such as the BioGeoChemical-Argo (BGC-Argo) floats. Previous studies have integrated data from BGC-Argo floats and satellite sensors, demonstrating the potential of machine learning models to estimate vertical bio-optical properties within the water column. The approach presented here enhances the estimation within the top 250 m of the water column compared with previous works. The estimations are performed in two distinct regions, the North Atlantic and the Subtropical Gyres, and across several layers within two maximum depth limits of 50 and 250 m. Data from BGC-Argo profiles and the Ocean and Land Colour Instrument (OLCI) sensor are used together to build a training dataset for a random forest model, which is applied with different sets of variables. Additional considerations regarding our datasets include short time criteria for matchups (±24 h) and high spatial resolution. The random forest model shows promising results, especially within the first 50 m in the Subtropical Gyres.

Indications of improved seasonal sea level forecasts for the United States Gulf Coast and East Coast using ocean dynamic persistence - Xue Feng; Matthew J. Widlansky; Tong Lee; Ou Wang; Magdalena A. Balmaseda; Hao Zuo; Gregory Dusek; William Sweet; Malte F. Stuecker (Ocean Science, 2025)
Forecasting seasonal sea levels along many coasts remains challenging, with generally lower skills than forecasts for the open oceans. We investigate the influence of ocean dynamics on forecasting monthly sea level anomalies for the United States Gulf Coast and East Coast using the Estimating Circulation and Climate of the Ocean (ECCO) system, which is initialized monthly from 1992 through 2017 and runs forward for 12 months under climatological atmospheric forcing. This approach, which we refer to as an ocean dynamic persistence forecast, demonstrates improved skill compared to both observed damped persistence and the ECMWF SEAS5 climate forecast system when evaluated against observations. At a lead of 4 months, dynamic persistence has the highest anomaly correlation coefficients at 22 out of 39 coastal locations (mostly south of Cape Hatteras). However, improvement in root mean square error is minimal, possibly due to reduced variability in ECCO associated with its climatology forcing and coarse resolution. This study suggests that dynamic persistence offers the potential to improve sea level forecasts beyond the capabilities of damped persistence and a state-of-the-art climate model.

Statistical analysis of ocean currents in the eastern Mediterranean - Yosef Ashkenazy; Hezi Gildor; Aviv Solodoch (Ocean Science, 2025)
We examine the probability density function (pdf) of current speeds at the DeepLev station located in the eastern Mediterranean Sea near the central coast of Israel. The currents cover depths from the surface to over 1.3 km and span a period from November 2016 to March 2024. We estimate the parameters of three typical distributions that are usually used to model the pdfs of currents and wind speed: the Weibull, the general extreme value, and the generalized gamma. We find that the three-parameter generalized gamma distribution best describes the pdfs of the observed current-speed series. Still, the studied current-speed time series may not be long enough to assess the exact values of the underlying pdf as some years exhibit stronger currents that affect the distribution. We also study the time series of the difference between consecutive current speeds and find that the stretched exponential pdf describes better (than the normal distribution) their statistics. The comparison of the measured current-speed pdfs to current-speed pdfs of a high-resolution (1 km) regional circulation model (ROMS) and Copernicus Mediterranean reanalysis daily mean currents (∼ 4.6 km resolution) indicates discrepancies from the data. Our results may help to improve statistical models for ocean currents and the estimation of extreme current events.

Biogeochemical layering and transformation of particulate organic carbon in the Tropical Northwestern Pacific Ocean inferred from δ 13 C - Detong Tian; Xuegang Li; Jinming Song; Jun Ma; Huamao Yuan; Liqin Duan (Ocean Science, 2025)
Particulate organic carbon (POC) serves as the main carrier of the biological pump and determines its transmission efficiency, yet the transformation processes of POC remain incompletely understood. This study reports the vertical distribution of POC, dissolved inorganic carbon (DIC), δ13C-POC, and δ13C-DIC in the tropical Northwestern Pacific Ocean (TNPO). The research identified three distinct biogeochemical layers governing POC transformation: the POC rapid synthesis-degradation layer (RSDL, 0–300 m), the net degradation layer (NDL, 300–1000 m), and the stable layer (SL, 1000–2000 m). From the top to the bottom of the RSDL, δ13C-POC values decreased by an average of 2.23 ‰, while the carbon-to-nitrogen (C:N) ratios increased by an average of 2.3:1, indicating the selective degradation of POC. In the NDL, δ13C-POC and δ13C-DIC exhibited a significant negative correlation (r=0.43, p

Satellite-derived steric height in the Southern Ocean: trends, variability, and climate drivers - Jennifer Cocks; Alessandro Silvano; Alberto C. Naveira Garabato; Oana Dragomir; Noémie Schifano; Anna E. Hogg; Alice Marzocchi (Ocean Science, 2025)
The Southern Ocean circulation plays a central role in regulating the global ocean overturning, ventilating the deep ocean, and driving sea level rise by delivering heat to Antarctic ice shelves. Understanding heat and freshwater content in this region is key to monitoring these global processes and identifying multi-year changes; however, in situ observations are limited and often do not offer the spatial or temporal consistency needed to study long-term variability. Perturbations in steric height can reveal changes in oceanic heat and freshwater content inasmuch as they impact the density of the water column. Here, we show for the first time that the monthly steric height anomaly of the Southern Ocean south of 50° S can be assessed using satellite altimetry and GRACE gravimetry data from 2002 to 2018. Steric height anomalies are validated against in situ Argo float and conductivity–temperature–depth (CTD) data from tagged elephant seals. We find good agreement north of 65° S, but there is increasing uncertainty towards the Antarctic continental shelf due to insufficient validation data, the leakage error, and anti-aliasing in GRACE. The Southern Ocean steric height anomalies capture the expected seasonal cycle of low (high) steric height in winter (summer) and show regionally variable trends during 2002–2018. We find that the variability in steric height is driven predominantly by anomalies in surface heat and freshwater content associated with positive and negative phases of the two major modes of Southern Hemisphere climate variability (the El Niño–Southern Oscillation and Southern Annular Mode). This steric height dataset provides a uniquely comprehensive insight into density anomalies and presents opportunities for further analysis of heat and freshwater fluxes, changes in stratification, or convective regimes across the Southern Ocean.

Turbulent dissipation along contrasting internal tide paths off the Amazon shelf from AMAZOMIX - Fabius Kouogang; Ariane Koch-Larrouy; Jorge Magalhaes; Alex Costa da Silva; Daphne Kerhervé; Arnaud Bertrand; Evan Cervelli; Fernand Assene; Jean-François Ternon; Pierre Rousselot; James Lee; Marcelo Rollnic; et al. (Ocean Science, 2025)
The Amazon shelf break is a key oceanic region where strong internal tides (ITs) are generated, playing a substantial role in climate processes and ecosystems through vertical dissipation and mixing. During the AMAZOMIX survey (2021), currents, hydrography, and turbulence were measured over the M2 tidal period (12.42 h) at multiple stations along both high (HTE) and low (LTE) tidal energy paths, covering IT generation and propagation regions off the Amazon shelf. This dataset provides a unique opportunity to assess IT-driven vertical dissipation and quantify its spatial extent and influence in the region. Microstructure analyses, integrated with hydrographic data, highlighted contrasting dissipation rates. The highest rates occurred at IT generation sites along the HTE paths, while the lowest rates were observed on the slope along the LTE path. Near generation sites, the dissipation rates were elevated, [10−6] W kg−1, with IT shear contributing ∼60 % compared to the mean baroclinic current (MBC) shear. Along IT paths, rates decreased to [10−8] W kg−1 but remained substantial, driven by nearly equal contributions from IT and MBC shear. A key finding was the relative increase in turbulent dissipation ([10−7] W kg−1) ∼230 km from two distinct IT generation sites at the shelf break. This zone of high mixing was located in an area where the general circulation vanished, coinciding with a region of potential constructive interference of IT rays originating from different generation sites. It also aligned with the occurrence of large-amplitude internal solitary waves (ISWs), suggesting that constructive IT ray interference may generate nonlinear ISWs that lead to enhanced dissipation.

Ventilation of the Bay of Bengal oxygen minimum zone by the Southwest Monsoon Current - Peter M. F. Sheehan; Benjamin G. M. Webber; Alejandra Sanchez-Franks; Bastien Y. Queste (Ocean Science, 2025)
Oxygen minimum zones (OMZs) occupy large areas of the tropical subsurface oceans and substantially alter regional biogeochemical cycles. In particular, the removal rate of bio-available nitrogen (denitrification) from the water column in OMZs is disproportionate to their size. The Bay of Bengal is one of the strongest OMZs in the global oceans; however, variable sources of oxygen prevent the onset of large-scale denitrification. The various oxygen-supply mechanisms that maintain oxygen concentrations in the OMZ above the denitrification threshold are currently unknown. Here, using a combination of multi-platform observations and model simulations, we identify an annual supply of oxygen to the Bay of Bengal in the high-salinity core of the Southwest Monsoon Current, a seasonal circulation feature that flows northward into the bay during the South Asian southwest monsoon (i.e. June–September). Oxygen concentrations within the Southwest Monsoon Current (80–100 µmol kg−1) are higher than those of waters native to the bay (i.e.

The Southern Ocean Time Series: a climatological view of hydrography, biogeochemistry, phytoplankton community composition, and carbon export in the Subantarctic Zone - Elizabeth H. Shadwick; Cathryn A. Wynn-Edwards; Ruth S. Eriksen; Peter Jansen; Xiang Yang; Gemma Woodward; Diana Davies (Ocean Science, 2025)
The Southern Ocean Time Series (SOTS) provides highly temporally resolved observations of the physical, chemical, and biological variability in the upper ocean, as well as the export of particulate organic and inorganic carbon to the ocean interior, in the subantarctic region south of Australia. The SOTS observatory focuses on the subantarctic region because of its importance in the formation of mode water and the associated uptake and storage of anthropogenic heat and carbon. The region is also critical for the supply of oxygen to the ocean interior and the export of nutrients to fuel primary production in broad areas of the low-latitude global ocean. The SOTS observatory is the longest running multidisciplinary initiative in the open Southern Ocean and has delivered high-quality observations from the surface to the seafloor for more than a decade and, for some parameters, for over two decades, using two deep-water moorings. The moorings are serviced annually, providing additional opportunities for shipboard sampling and sensor validation and calibration. Using observations collected at the SOTS site between 1997 and 2022, we present the seasonal variability in upper-ocean hydrography, biogeochemistry, phytoplankton and microplankton community composition, and diversity, along with particulate organic and particulate inorganic carbon export to the deep ocean. This climatological view of the region is complemented by a review of recent findings underpinned by observations collected by the SOTS observatory and highlighting the ongoing need for long time series to better understand the Subantarctic Ocean and its response to a changing climate.

Relating Atlantic meridional deep-water transport to ocean bottom pressure variations as a target for satellite gravimetry missions - Linus Shihora; Torge Martin; Anna Christina Hans; Rebecca Hummels; Michael Schindelegger; Henryk Dobslaw (Ocean Science, 2025)
The Atlantic Meridional Overturning Circulation (AMOC) is a salient feature of the climate system that is observed with respect to its strength and variability using a wide range of offshore installations and expensive sea-going expeditions. Satellite-based measurements of mass changes in the Earth system, such as from the Gravity Recovery and Climate Experiment (GRACE) mission, may help monitor these transport variations at the large scale, by measuring associated changes in ocean bottom pressure (OBP) at the boundaries of the Atlantic remotely from space. However, as these signals are mainly confined to the continental slope and are small in magnitude, their detection using gravimetry will likely require specialised approaches. Here, we use the output of a fine-resolution (1/20°) regional ocean model to assess the connection between OBP signals at the western boundary of the North and South Atlantic to changes in the zonally integrated meridional deep-water transport. We find that transport anomalies in the ∼ 1–3 km depth range can be reconstructed using OBP variations spatially averaged over the continental slope, with correlations of 0.75 (0.72) for the North (South) Atlantic and root-mean-square errors of ∼ 1 Sv (sverdrup; 106 m3 s−1), on monthly to inter-annual timescales. We further create a synthetic data set containing OBP signals connected to meridional deep-water-transport anomalies; these data can be included in dedicated satellite gravimetry simulations to assess the AMOC detection capabilities of future mission scenarios and to develop specialised recovery strategies that are needed to track those weak signatures in the time-variable gravity field.

Drivers of the spatiotemporal distribution of dissolved nitrous oxide and air–sea exchange in a coastal Mediterranean area - Susana Flecha; Mercedes de la Paz; Fiz Fernández Pérez; Núria Marbà; Carlos Morell; Eva Alou-Font; Joaquín Tintoré; Iris E. Hendriks (Ocean Science, 2025)
Among the well-known greenhouse gases (GHGs), nitrous oxide (N2O) is the third most impactful, possessing a global warming potential approximately 300 times greater than that of carbon dioxide (CO2) over a century. The distribution of N2O in aquatic environments exhibits notable spatial and temporal variations, and emissions remain inadequately constrained and underrepresented in global N2O emission inventories, particularly for coastal zones. This study focuses on N2O levels and air–sea fluxes in the coastal waters of the Balearic Islands Archipelago in the Western Mediterranean basin. Data were gathered between 2018 and 2023 at three coastal monitoring stations: two on the densely populated island of Mallorca and the third in the well-preserved National Park of the Cabrera Archipelago. Seawater N2O concentrations varied from 6.5 to 9.9 nmol L−1, with no significant differences being detected across the sites. When these sink–source strengths are integrated on an annual basis, the Balearic Sea is close to equilibrium with atmospheric N2O, resulting in a neutral atmosphere–ocean exchange (0.1 ± 0.2 µmolm-2d-1). A consistent seasonal pattern was noted during the study period. Machine learning analysis indicated that seawater temperature was the primary factor influencing N2O concentrations, with lesser contributions from chlorophyll levels and salinity.

Sargassum spp. accumulation and transport by mesoscale eddies - Rosmery Sosa-Gutierrez; Julien Jouanno; Leo Berline (Ocean Science, 2025)
The proliferation of pelagic Sargassum spp. (Sargassum) in the tropical Atlantic has significant ecological and socioeconomic impacts. While large-scale ocean circulation patterns influence the basin-scale distribution of Sargassum, the role of mesoscale eddies in their local accumulation and transport has not been quantitatively assessed so far. This study investigates the relationship between mesoscale eddies and Sargassum dynamics using satellite observations. By analyzing 13 years of remote sensing observations, we demonstrate that both cyclonic and anticyclonic long-lived mesoscale eddies can trap and transport Sargassum. However, results show that, in cyclonic eddies, Sargassum cover is higher and tends to accumulate during its lifetime, while within anticyclonic eddies the Sargassum cover is usually weaker and tends to decrease. These findings align with recent studies highlighting the role of eddies in shaping the distribution of floating debris and provide an important observational basis for the development of Sargassum drift models.

Extreme sensitivity of the northeastern Gulf of Lion (western Mediterranean) to subsurface heatwaves: physical processes and insights into effects on gorgonian populations in the summer of 2022 - Claude Estournel; Tristan Estaque; Caroline Ulses; Quentin-Boris Barral; Patrick Marsaleix (Ocean Science, 2025)
In the summer of 2022, atmospheric conditions characterized by persistent anticyclonic anomalies caused an extreme marine heatwave in the western Mediterranean Sea. Time series of temperature profiles at various points along the northeastern coast of the Gulf of Lion (NW Mediterranean Sea) showed exceptional temperatures down to depths of 30 m, which led to massive mortality of benthic species. A hydrodynamic numerical simulation was used to analyze the physical processes responsible for this subsurface heatwave in a region where the climatology in summer is characterized by northerly winds inducing upwelling alternating with low winds. Firstly, the recurrence of heatwaves limited to the surface was demonstrated, triggered when upwelling stopped and warm water from the Northern Current intruded onto the shelf. More importantly, in August and early September 2022, two episodes of southerly and easterly winds of 8 to 10 m s−1 occurred. The oceanic response to these winds was an alongshore cyclonic current advecting warm water onto the shelf and a downwelling of this warm water to depths of the order of 30 to 40 m. A large part of the Gulf of Lion coast was warmed by these events. However, the northeastern part of the shelf, on either side of the city of Marseille, was by far the area most affected at depth due to the combination of the proximity of the warm surface waters of the Ligurian coast advected by wind-induced currents and the local acceleration of the wind by the continental topography, which intensifies the downwelling of these surface waters. These events are rare in summer, but their impact on the rich benthic ecosystems that characterize the region is dramatic and will only increase with the warming trend in surface waters, which is already close to 1 °C, as seen over the last decade.

M2 monthly and annual mode-1 and mode-2 internal tide atlases from altimetry data and MIOST: focus on the Indo-Philippine archipelago and the region off the Amazon shelf - Michel Tchilibou; Simon Barbot; Loren Carrere; Ariane Koch-Larrouy; Gérald Dibarboure; Clément Ubelmann (Ocean Science, 2025)
The M2 MIOST24 (Multivariate Inversion of Ocean Surface Topography 2024) internal tide atlases described in this study are available in annual (MIOST24a) and monthly (MIOST24m) regional versions for modes 1 and 2 in the Indo-Philippine archipelago and the region off the Amazon shelf. They are derived from 25 years (1993–2017, period 1) of sea level anomalies (SLAs) from altimetry observations. Compared to the MIOST22 (Multivariate Inversion of Ocean Surface Topography 2022) internal tide atlases, MIOST24 incorporates M2 mode-1 and mode-2 wavelengths based on monthly stratification profiles from GLORYS12v1 (1993–2020). The differences between MIOST24a and MIOST22a lead to a RMSD (root mean square difference) of up to 3 cm, reflecting amplitude changes, while the RMSD between MIOST24a and the internal tide atlas HRET (High-Resolution Empirical Tide) indicates both amplitude and phase differences. MIOST24m highlights significant monthly variability of M2 internal tides in the Indo-Philippine archipelago and off the Amazon shelf. In the Amazon region, the internal tide propagates far offshore from March to June but is blocked closer to the coast from August to December. In both regions, the RMSDs between the MIOST24a annual and MIOST24m monthly mode-1 atlases are mainly governed by the phase contribution term. This suggests that the monthly variability of the mode-1 internal tide is more affected by changes in its propagation direction than by fluctuations in amplitude. In contrast, for mode 2, the monthly variability is predominantly associated with amplitude variations, particularly near internal tide generation sites. Variance reductions of SLAs show that MIOST24m outperforms MIOST24a, MIOST22a, and HRET in period 1 in the two regions of interest and in the Amazon region in period 2 (2018–2023). Monthly atlases are therefore recommended to correct the internal tides of the SLAs used to derive them. All these results support the development of MIOST24 global atlases.

Determining the depth and upwelling speed of the equatorial Ekman layer from surface drifter trajectories - Nathan Paldor; Yair De-Leon (Ocean Science, 2025)
In this work, trajectories of more than 500 drogued surface drifters launched in the equatorial ocean since 1979 are analyzed by employing the results of a new Lagrangian theory of poleward transport from the Equator forced by the prevailing trade winds. The Lagrangian theory provides an explicit expression for the depth of the Ekman layer that circumvents the application of the 3D continuity equation that requires calculation of the divergence of horizontal transport, which has been the basis of all previous studies on the subject. The analysis is carried out for drifters launched within 1° of the Equator that reached a final latitude of 3, 4, or 5° north or south of the Equator while also remaining in one hemisphere throughout their entire travel time. The analysis yields robust estimates of 45 m for the Ekman layer's depth and 1.0 m d−1 for the upwelling speed of thermocline water into the layer.

Surface saline lakes in the Mediterranean Sea - Elena Terzić; Clara Gardiol; Ivica Vilibić (Ocean Science, 2025)
In the Levantine basin, it has long been known that salinity can reach a maximum in a thin layer near the surface, particularly during the warm season when summer heating, evaporation, and low mixing prevail. This water mass has been linked to the generation of Levantine intermediate and deep waters, depending on winter heat loss and wind-induced mixing. However, a recent study demonstrated that similar conditions, referred to as surface saline lakes (SSLs), can occur as far north as the Adriatic Sea. To investigate this, we analysed data from Argo profiling floats across all of the Mediterranean basins, focusing on the upper layers (up to 200 m in depth), where such lakes are known to form. We developed an objective algorithm to detect SSLs within profiles, defining a SSL according to a threshold-exceeding salinity gradient at its base. This definition allowed us to estimate SSL depth, SSL temperature and potential density (PDA) gradients at the base, as well as the Schmidt stability index which quantifies the energy needed to mix a SSL. We also ensured the quasi-continuity of Argo profiles throughout the year in our analyses, as SSLs are highly seasonal phenomena. SSLs exhibit minimum or vanishing occurrences between February and April and peak between August and October. SSLs were detected in all of the Mediterranean basins, with the highest prevalence – 65 %–70 % of the profiles between July and December – occurring in the Levantine basin. During the August–October peak, SSLs exceeded 35 % of the monthly profiles in each basin, even in the western Mediterranean, albeit with lower overall salinity levels and variable SSL ranges. These findings underscore the role of the atmospheric heat and water exchange in all of the Mediterranean basins, influencing intermediate and deeper thermohaline properties through wintertime mixing. Despite pronounced interannual and seasonal variability, our analysis of the data showed a significant positive trend in SSL depth, accompanied by decreasing thermohaline gradients (temperature, salinity, and PDA) at SSL bases through the investigated period. The observed changes raise questions about their drivers – whether they indicate ongoing climate-change-induced salinization and shifts in Mediterranean water mass dynamics or are merely manifestations of a multi-decadal variability.

Regional sea level trend budget over 2004–2022 - Marie Bouih; Anne Barnoud; Chunxue Yang; Andrea Storto; Alejandro Blazquez; William Llovel; Robin Fraudeau; Anny Cazenave (Ocean Science, 2025)
Closure of the regional sea level trend budget is investigated over the 2004–2022 time span by comparing trend patterns from the satellite altimetry-based sea level with the sum of contributions, i.e. the thermosteric, halosteric, manometric and GRD (gravitational, rotational, and deformational fingerprints due to past and ongoing land ice melt) components. The thermosteric and halosteric components are based on Argo data (down to 2000 m). For the manometric component, two approaches are considered: one using GRACE/GRACE Follow-On satellite gravimetry data and the other using ocean reanalyses-based sterodynamic sea level data corrected for local steric effects. For the latter, six different ocean reanalyses are considered, including two reanalyses that do not assimilate satellite altimetry data. The results show significantly high residuals in the North Atlantic for both approaches. In a few other regions, small-scale residuals of smaller amplitude are observed and attributed to the finer resolution of altimetry data compared to the coarser resolution of data sets used for the components. The observed strong residual signal in the North Atlantic points to Argo-based salinity errors in this region. However, it is not excluded that other factors also contribute to the reported non-closure of the budget in this area.

Coupling of numerical groundwater–ocean models to improve understanding of the coastal zone - Jiangyue Jin; Manuel Espino; Daniel Fernàndez-Garcia; Albert Folch (Ocean Science, 2025)
Coastal zones are increasingly acknowledged as dynamic yet fragile components of global ecosystems amidst escalating anthropogenic activities and complex land–ocean interactions. Understanding the interactions between groundwater and the ocean is crucial for managing submarine groundwater discharge (SGD) and seawater intrusion (SWI), vital for coastal ecosystem preservation and water resource management. This research proposes an integrated modeling approach that couples groundwater flow and physical oceanographic models to accurately simulate coastal-ocean–groundwater interactions. In this work, a TELEMAC-3D-based three-dimensional hydrodynamic model was initially developed to capture marine conditions with variable salinity and temperature. A MODFLOW 6 groundwater model was subsequently constructed. The models were efficiently coupled using FloPy and TelApy, enabling precise co-simulation of hydrodynamic and groundwater systems. Validation of the coupled model against empirical data confirmed its high fidelity, with errors within acceptable ranges. This coupled model employs dynamic boundary conditions, overcoming the limitations of traditional coastal groundwater models that assume constant salinity. This enhancement significantly improves the accuracy and practicality of simulating SGD processes in the coastal ocean. The bidirectional feedback mechanism within the coupled model strengthens the analysis of interactions between the ocean and groundwater systems. It accounts for variations in the seawater boundary under tidal influence and the reciprocal impact of groundwater dynamics on the hydrodynamic conditions of nearshore waters. This holistic enhancement bolsters the model's hydrological simulation capabilities, providing a more comprehensive depiction of the intricate water–salt exchange mechanisms in coastal systems.

Combining benzalkonium chloride addition with filtration to inhibit dissolved inorganic carbon alteration during the preservation of water sample in radiocarbon analysis - Hiroshi A. Takahashi; Masayo Minami (Ocean Science, 2025)
Benzalkonium chloride (BAC) addition has shown great promise as a disinfectant for measuring δ13C and 14C of dissolved inorganic carbon (DIC) in freshwater samples. However, it was reported that the effectiveness of BAC to prevent DIC change was reduced for the use of seawater samples. The present study aims to evaluate the effectiveness of adding BAC as a disinfectant in carbon isotopic analyses of DIC in water samples. We compared the efficacy of BAC addition, filtration (0.22 µm polytetrafluoroethylene (PTFE) or 0.2–0.45 µm polyether sulfone (PES) filters), and a combination of BAC addition and filtration in preventing DIC alterations caused by biological activity using the freshwater (salinity

Monsoonal influence on floating marine litter pathways in the Bay of Bengal - Lianne C. Harrison; Jennifer A. Graham; Piyali Chowdhury; Tiago A. M. Silva; Danja P. Hoehn; Alakes Samanta; Kunal Chakraborty; Sudheer Joseph; T. M. Balakrishnan Nair; T. Srinivasa Kumar (Ocean Science, 2025)
Marine litter in the Bay of Bengal has been under-studied despite large quantities of mismanaged waste reportedly entering the ocean from its surrounding countries. The seasonal reversal of monsoon currents in this region provides a unique environment for the transport of floating macro-litter. A particle tracking model is used here to investigate source-to-sink connectivity of marine debris between countries via oceanic pathways in the Bay of Bengal. We use an approach considering uniform release of particles along the entire coastline, avoiding the considerable uncertainties associated with assumed riverine sources. Two different simulations are considered, forced with either a high-resolution ocean hindcast developed specifically for the Bay of Bengal or a lower-resolution dataset which includes data assimilation. The vast majority of particles released during our simulations were found to beach within 16 months; most particles beached in their country of origin (57 %–90 %), with connectivity towards Myanmar accounting for the second highest connectivity rates (2 %–29 %) from many countries within the Bay of Bengal. This is likely due to the relatively large size of Myanmar's coastline and that it lies in the path of the East India Coastal Current for much of the year (February–September). Patterns of connectivity were found to change along with the monsoon and the post-monsoon period (October–January) showed a notably greater dispersal of particles than the rest of the year. Both simulations were evaluated using the pathways of undrogued surface drifters, which moved primarily within the open ocean, with better agreement found here for particles advected by data-assimilated ocean velocities. This study will therefore crucially inform future research and policy in this region, providing advice on the benefits and suitability of selecting different modelling approaches independent of assumptions of the source locations or volumes.

On mode water formation and erosion in the Arabian Sea: forcing mechanisms, regionality, and seasonality - Estel Font; Sebastiaan Swart; Puthenveettil Narayana Vinayachandran; Bastien Y. Queste (Ocean Science, 2025)
Mode water acts as a barrier layer controlling surface-to-interior fluxes of key climatic properties. In the Arabian Sea, mode water stores heat and provides an oxygen-rich layer for rapid remineralization, and its subduction is a direct pathway for oxygen into the upper oxygen minimum zone. We use float observations to characterize the properties of the Arabian Sea mode water layer (MWL). The MWL forms when springtime warming stratifies the surface layer and caps the deep surface mixed layer formed during the winter monsoon. During the summer monsoon, a second MWL is formed south of 20° N following the cessation of wind-driven mixing. We use 1D and 3D models to disentangle the contributions of atmospheric and oceanic forcing to this water mass. The 1D model accurately represents the mode water's formation and erosion, showing that atmospheric forcing is the first-order driver, in agreement with observations. However, there are regions where advective processes, eddy mixing, or biological heating are essential for the formation and/or erosion of the MWL. For instance, in the eastern Arabian Sea, freshwater-driven stratification advected via the West Indian Coastal Current reduces the potential for deep mixed layers via convective mixing, resulting in a thinner MWL. The 3D model shows that the MW contributes 5 ± 1 % to the oxygen content of the upper ocean, with its maximum during spring in the northern Arabian Sea (40 ± 17 %), thus highlighting the key role of the water mass in storing and transporting heat and oxygen to the interior.

Satellite altimetry and operational oceanography: from Jason-1 to SWOT - Pierre-Yves Le Traon; Gérald Dibarboure; Jean-Michel Lellouche; Marie-Isabelle Pujol; Mounir Benkiran; Marie Drevillon; Yann Drillet; Yannice Faugère; Elisabeth Remy (Ocean Science, 2025)
The development and evolution of satellite altimetry and operational oceanography are very closely linked. By providing all weather, global, and real-time observations of sea level, a key variable to constrain ocean analysis and forecasting systems, satellite altimetry has had a profound influence on the advancement of operational oceanography. Over the past 20 years, satellite altimetry has been providing a continuous observation of the ocean in near-real time. From the launch of Jason-1 in 2001 to the launch of SWOT in 2022, satellite altimetry capabilities have been regularly improved through refinements of geophysical corrections and processing algorithms, including real-time processing, as well as through the evolution of altimeter radar technology (synthetic aperture radar (SAR) mode, swath altimetry). Resolution has also improved through the use of multiple altimeters and, now, swath altimetry. In parallel, major improvements in ocean prediction systems have occurred, from the Global Data Assimilation Experiment (GODAE) demonstration in the 2000s to the fully operational systems now serving a large range of applications in the 2020s. This paper provides an overview of the development and evolution of satellite altimetry and operational oceanography over the past 20 years in the context of the Mercator Ocean prediction centre, DUACS (data unification and altimeter combination system), and the EU Copernicus Marine Service. The impact of altimetry on the performances of ocean prediction systems (based on observing-system evaluations (OSEs) and observing-system simulation experiments (OSSEs)) is reviewed. The future contribution of swath altimetry to ocean prediction is also discussed. Prospects for the next decade are addressed as part of our conclusions.

Application of the HIDRA2 deep-learning model for sea level forecasting along the Estonian coast of the Baltic Sea - Amirhossein Barzandeh; Matjaž Ličer; Marko Rus; Matej Kristan; Ilja Maljutenko; Jüri Elken; Priidik Lagemaa; Rivo Uiboupin (Ocean Science, 2025)
Sea level predictions, typically derived from 3D hydrodynamic models, are computationally intensive and subject to uncertainties stemming from physical representation and inaccuracies in initial or boundary conditions. As a complementary alternative, data-driven machine learning models provide a computationally efficient solution with comparable accuracy. This study employs the deep-learning model HIDRA2 to forecast hourly sea levels at five coastal stations along the Estonian coastline of the Baltic Sea, evaluating its performance across various forecast lead times. Compared to the regional NEMOBAL and subregional NEMOEST hydrodynamic models, HIDRA2 frequently outperforms both, particularly in terms of overall forecast skill. While HIDRA2 shows limitations in resolving high-frequency sea level variability above (6 h)−1, it effectively reproduces energy in lower-frequency bands below (18 h)−1. Errors tend to average out over longer time windows encompassing multiple seiche periods, enabling HIDRA2 to surpass the overall performance of the NEMO models. These findings underscore HIDRA2's potential as a robust, efficient, and reliable tool for operational sea level forecasting and coastal management in the eastern Baltic Sea region.

Constraining local ocean dynamic sea-level projections using observations - Dewi Le Bars; Iris Keizer; Sybren Drijfhout (Ocean Science, 2025)
The redistribution of ocean water volume under ocean–atmosphere dynamical processes results in sea-level changes. This process, called ocean dynamic sea level (ODSL) change, is expected to be one of the main contributors to sea-level rise along the western European coast in the coming decades. State-of-the-art climate model ensembles are used to make 21st century projections of this process, but there is a large model spread. Here, we use the Netherlands as a case study and show that the ODSL rate of change for the period 1993–2021 correlates significantly with the ODSL anomaly at the end of the century and can therefore be used to constrain projections. Given the difficulty of estimating ODSL changes from observations on the continental shelf, we use three different methods providing seven observational estimates. Despite the broad range of observational estimates, we find that 4 to 16 CMIP6 models have rates above the observational range and 0 to 1 below. We compare four model selection methods which differ in the way the uncertainty in the rate estimation is considered. We find that for stricter selection methods the rate of ODSL is closer to the observational range, and the uncertainty in future change is reduced. The influence of model selection is largest for the low emission scenario SSP1-2.6. We discuss the advantages and disadvantages of those selection methods and their suitability for different users.

Topographic modulation on the layered circulation in the South China Sea - Qibang Tang; Zhongya Cai; Zhiqiang Liu (Ocean Science, 2025)
The South China Sea (SCS) is the largest semi-enclosed marginal sea in the western Pacific. It exhibits a unique vertically rotating cyclonic, anticyclonic, and cyclonic circulation in its upper, middle, and deep layers. Over slope topography, these layered currents interact and significantly shape the structure and intensity of the basin circulation. In this study, we employ process-oriented numerical simulations to investigate how upper-layer processes, characterized by greater magnitude and variability, influence the layered circulation over the irregular topographic slope. The simulations reveal that stronger upper intrusion from the open ocean directly enhances upper-layer circulation, which subsequently strengthens the middle and the deep slope currents. Vorticity dynamics illustrate that changes in the middle and deep slope current are largely related to the vertical stretching (ζ_DIV) induced by bottom geostrophic cross-isobath transport (CGTb). As the upper-layer cyclonic slope current intensifies, it modulates the bottom pressure distribution, resulting in stronger negative ζ_DIV predominantly over the northwestern slope to intensify the middle anticyclone slope current. Similarly, for the deep cyclonic slope current, the CGTb maintains downwelling in the northern part and upwelling over the southern slope. Over the southern slope, the strengthening of the positive CGTb is induced by the increment of the advection of relative vorticity and planetary vorticity in the water column, in which the middle layer has an important contribution, but the upper layer has a minimal impact. Conversely, on the northern slope, the strengthening of the negative CGTb is primarily influenced by the upper layer.

An evaluation of the Arabian Sea Mini Warm Pool's advancement during its mature phase using a coupled atmosphere–ocean numerical model - Sankar Prasad Lahiri; Kumar Ravi Prakash; Vimlesh Pant (Ocean Science, 2025)
A coupled atmosphere–ocean numerical model is used to examine the relative contributions of atmospheric and oceanic processes in developing the Arabian Sea Mini Warm Pool (MWP). The model simulations were performed for three independent years, 2013, 2016, and 2018, through April–June, and the results were compared against observations. The model-simulated sea surface temperature (SST) and sea surface salinity (SSS) bias were less than 1.75 °C and 1 psu, respectively; this bias was minimal in the MWP region. Moreover, the model simulated results effectively represented the presence of the MWP across the three different years. The mixed-layer heat budget analysis indicated that the net surface heat flux raised the mixed-layer temperature tendency of the MWP by a maximum of 0.1 °C d−1 during its development phase. The vertical processes exerted a cooling impact on the temperature tendency throughout May and June with a maximum of −0.08 °C d−1. Nonetheless, the decrease of net surface heat flux emerged as the dominant factor for the dissipation of the MWP. Further, four sensitivity numerical experiments were performed to investigate the comparative consequences of the ocean and atmosphere on the advancement of the MWP. The sensitivity experiments indicated that pre-April ocean conditions in years with a strong MWP resulted in a 136 % increase in MWP intensity in years when MWP SST was close to the climatology, which shows the primary role of oceanic preconditioning in determining MWP strength during strong-MWP years. Once the oceanic preconditions are met, the atmospheric conditions of weak-MWP years lead to an 82 % reduction in MWP intensity relative to normal years, highlighting the detrimental impact of atmospheric forcing under such circumstances. Atmospheric conditions, particularly wind, are critical in influencing the spatial evolution and dissipation of the MWP in the southeastern Arabian Sea (SEAS). A wind shadow zone, characterized by less production of turbulent kinetic energy that does not exist during weak-MWP years, facilitates the spatial expansion of the MWP in SEAS during moderate to strong-MWP years.

Seafloor marine heatwaves outpace surface events in the future on the northwestern European shelf - Robert J. Wilson; Yuri Artioli; Giovanni Galli; James Harle; Jason Holt; Ana M. Queirós; Sarah Wakelin (Ocean Science, 2025)
Marine heatwaves are becoming increasingly frequent across the world's oceans. As a result, there are growing impacts on marine ecosystems due to temperatures exceeding the thermal niche and historical exposure of many species. Anticipating the future frequency and severity of marine heatwaves is necessary. Here, we provide the first projections of future marine heatwaves for the sea surface and seafloor across the northwestern European shelf, which is a critically important marine ecosystem. We use an ensemble of five dynamically downscaled hydrodynamic models under the high-emission scenario Representative Concentration Pathway 8.5 (RCP 8.5). Heatwaves were defined as events lasting at least 5 d where temperatures exceed the 90th percentile of a historical baseline period. The frequency of marine heatwaves at the surface and seafloor is projected to increase significantly during the 21st century under RCP 8.5, with most of the year being projected to be under heatwave conditions by the end of the century. Critically, we find that marine heatwaves are projected to increase in frequency to a greater extent at the seafloor compared to at the sea surface due to their lower levels of natural temperature variation. Similarly, we find that the severity of summer heatwaves at the surface is projected to be lower than that of heatwaves during the rest of the year due to lower climatological variations in temperature outside the summer. The impacts of marine heatwaves in shelf seas are therefore likely to be much more complex than previously thought.

Local versus far-field control on South Pacific Subantarctic mode water variability - Ciara Pimm; Andrew J. S. Meijers; Dani C. Jones; Richard G. Williams (Ocean Science, 2025)
In the South Pacific Subantarctic mode water (SAMW) formation region, central and eastern pools of SAMW have been found to be linked to winter mixed-layer thicknesses that vary strongly interannually and out of phase across the basin. This mixed-layer variability is associated with peaks in sea level pressure variability at a quasi-stationary anomaly situated between the two pools. To investigate how surface forcing, as well as the propagation of upstream anomalies, affects the formation of these SAMW pools, a set of adjoint sensitivity experiments with a density-following feature are conducted. Adjoint sensitivities reveal that local cooling can lead to an increase in the SAMW pool volume through mixed-layer-depth changes and the lateral movement of the northern boundary of the pool. In addition, upstream warming along the Antarctic Circumpolar Current can lead to an increase in the SAMW pool volume through lateral density surface movement shifting the southern boundary polewards. The density properties are advected from upstream to the downstream pool over 1 year. Optimal conditions for SAMW formation involve a combination of local cooling and upstream warming of SAMW formation sites. Hence, South Pacific SAMW variability is particularly sensitive to atmospheric modes which lead to a dipole in heating across the formation sites.

Critical uncoupling between biogeochemical stocks and rates in Ross Sea springtime production–export dynamics - Meredith G. Meyer; Esther Portela; Walker O. Smith Jr.; Karen J. Heywood (Ocean Science, 2025)
Three biogeochemical glider surveys in the Ross Sea between 2010 and 2023 were combined and analysed to assess production–export stock and rate dynamics. As the most productive of any Antarctic continental shelf, the Ross Sea is a site of substantial physical and biogeochemical interest. While this region and its annual bloom have been characterised for decades, logistical constraints, such as ship time and sea ice cover, have prevented a comprehensive understanding of this region over long (> 1–2 months) timescales and at high spatiotemporal resolution. Here, we use high-resolution datasets from autonomous gliders in mass balance equations to calculate short-term (days to weeks) net community production via oxygen concentration, change in particulate organic carbon (POC) concentration over time, and POC export potential during the period of peak primary production in the region (November–February). Our results show an overall decoupling of net community production (NCP), driven by biologic changes in oxygen, from overall biomass concentration as well as changes in POC over time. NCP and carbon change vary between seasons and appear related to changes in ice concentration and stratification. Substantial spatiotemporal variability exists in all datasets, but high-resolution sampling reveals short-term variations that are likely masked in other studies. Our study reinforces the need for high-resolution sampling and supports previous classifications of the Ross Sea as a high-productivity (average NCP range −0.7 to 0.2 g C m−2 d−1), low-export (average changes in POC over time range −0.1 to 0.1 g C m−2 d−1) system during the productive austral spring and sheds additional light on the mechanisms controlling these processes.

Simulated density reorganization on the Weddell Sea continental shelf sensitive to atmospheric forcing - Vanessa Teske; Ralph Timmermann; Cara Nissen; Rolf Zentek; Tido Semmler; Günther Heinemann (Ocean Science, 2025)
The strong Antarctic Slope Front in the southern Weddell Sea limits the present-day transport of modified Warm Deep Water (mWDW) onto the continental shelf and is associated with a characteristic V-shape in the density structure across the continental slope. The mechanisms controlling today's V-shape are well studied, but its future development is not yet well constrained. In this study, we run ocean model simulations for a 21st century Shared Socioeconomic Pathways (SSP) 3-7.0 emission scenario. The forcing is retrieved from atmospheric model output from simulations with a global climate model and from a higher-resolved regional atmospheric model respectively. We find that the resolution of the atmospheric model component influences the simulated future transport of mWDW onto the continental shelf into the Filchner Trough in the southern Weddell Sea through differences in the evolution of the depth and symmetry of the V-shape over the 21st century. In both simulations, reduced sea-ice formation and weakened Ekman downwelling reduce the depth of the V-shape and increase the sensitivity of its position above the slope to seasonal variations in sea-ice production and in the wind field. Using forcing data from an atmosphere model with higher resolution leads to an acceleration of the density redistribution on the continental shelf compared to the simulations forced with coarse-resolution data. This indicates that the SSP3-7.0 climate scenario may have a greater potential for a regime shift from a cold to a warm Filchner Trough through a cross-slope current before the end of the 21st century than suggested by other ocean simulations for the same scenario but with lower atmospheric resolution. As cross-slope currents disturb the continuity of the V-shape, we define a spatial grade of connectivity to quantify the lateral integrity of the V-shape along the continental slope. We find that the integrity of the V-shape reduces with a delay of 3 months after a strong cross-slope current of mWDW enters Filchner Trough. Atmospheric downscaling increases the potential for a regime shift, dominated by warmer summer air temperatures. The Antarctic Slope Front is temporarily disturbed by cross-slope currents but the primary reason for the regime shift is the cross-slope density gradient.

Synoptic patterns associated with high-frequency sea level extremes in the Adriatic Sea - Krešimir Ruić; Jadranka Šepić; Marin Vojković (Ocean Science, 2025)
This study focuses on the classification of synoptic conditions leading to episodes of extreme high-frequency (HF) sea level oscillations in the Adriatic Sea (Mediterranean). Two types of extreme episodes were obtained from sea level time series measured at six tide gauge stations: (i) HF extremes, extracted from HF components (periods shorter than 2 h) of sea level time series and defined as periods in which the HF component was above a threshold value, and (ii) compound extremes, extracted from residual (de-tided) time series and defined as periods in which both HF and residual components were above their respective thresholds. Characteristic synoptic situations preceding both types of extremes were determined using the k-medoids clustering method applied on the ERA5 reanalysis data (mean sea level pressure, temperature at 850 hPa, and geopotential height of 500 hPa level). The structural similarity index measure (SSIM) was used as a distance metric. The data were divided into a training set (from the start of measurements to the beginning of 2018) and a testing set (from the beginning of 2018 to the end of 2020). For each station, the k-medoids method was used to obtain first two and then three clusters with characteristic synoptic patterns called “medoids”. Two distinct patterns related to HF and compound extremes were identified at all stations: (i) a “summer-type” pattern, characterised by a non-gradient mean sea level pressure, warm air advection from the south-southwest at 850 hPa, and the presence of a jet stream at the 500 hPa height, with all three conditions previously found to favour the development of meteorological tsunamis (i.e. the strongest of atmospherically triggered HF sea level oscillations); (ii) a “winter-type” pattern, characterised by pronounced mean sea level pressure gradients favouring winds that induce storm surges, a colder low troposphere, and the presence of a jet stream at the 500 hPa level. Including the third cluster in the analysis led to the extraction of either a novel “bora-type” pattern, involving strong northeast winds at the Bakar and Rovinj stations, or an additional cluster with a medoid that represents the refinement of summer- or winter-type patterns. The extracted medoids of clusters were used to label all days of the testing period. It was shown that HF or compound episodes recorded in the testing period mostly appeared during synoptic situations that highly resembled extracted medoids. The potential of using the k-medoids method for forecasting HF sea level oscillations is discussed.

Dual-tracer constraints on the inverse Gaussian transit time distribution improve the estimation of water mass ages and their temporal trends in the tropical thermocline - Haichao Guo; Wolfgang Koeve; Andreas Oschlies; Yan-Chun He; Tronje Peer Kemena; Lennart Gerke; Iris Kriest (Ocean Science, 2025)
Quantifying the mean state and temporal change of seawater age is crucial for understanding the role of ocean circulation and its change in the climate system. One commonly used technique to estimate the water age is the inverse Gaussian transit time distribution method (IG-TTD), which applies measurements of transient abiotic tracers like chlorofluorocarbon 12 (CFC-12). Here, we use an Earth system model to evaluate how accurately the IG-TTD method infers the mean state and temporal change of true water age from 1981 to 2015 in the tropical thermocline (on isopycnal layer σ0=25.5 kg m−3). To this end, we compared the mean age of IG-TTD (Γ) derived from simulated CFC-12 with the model “truth”, the simulated ideal age. Results show that Γ underestimates the ideal age of 46.0 years by up to 50 %. We suggest that this discrepancy can be attributed to imperfect assumptions about the shapes of the transit time distribution of water parcels in the tropics and the short atmospheric history of CFC-12. Moreover, when only one transient tracer (CFC-12) is available, temporal trends in Γ might be an unreliable indicator and, due to uncertainties in the mixing ratio, may even be of opposite sign to temporal trends in the ideal age. The disparity between Γ and ideal age temporal trends can be significantly reduced by incorporating an additional abiotic tracer with a different temporal evolution, which we show by constraining Γ with sulfur hexafluoride (SF6) in addition to CFC-12.

The satellite chlorophyll signature of Lagrangian eddy trapping varies regionally and seasonally within a subtropical gyre - Alexandra E. Jones-Kellett; Michael J. Follows (Ocean Science, 2025)
Vertical motions of mesoscale ocean eddies modulate the resource environment, productivity, and phytoplankton biomass in the ocean's subtropical gyres. The horizontal circulations can trap or disperse the eddy-driven chlorophyll anomalies, which can be observed from space. From 2 decades of satellite remote sensing observations in the North Pacific Subtropical Gyre (NPSG), we compared the chlorophyll anomalies within “leaky” eddy boundaries identified using an Eulerian sea level anomaly (SLA) method and within strictly coherent “trapping” bounds derived from Lagrangian particle simulations. On average, NPSG Lagrangian coherent vortices maintain stronger chlorophyll anomalies than Eulerian SLA eddies due to the limitation of lateral dilution. This is observed in both cyclones and anticyclones. However, there is variability in the biological signature of trapping by sub-region and season. Eddy trapping of positive chlorophyll anomalies is most significant in the southern regions of the NPSG, counter to expectations from a commonly used Eulerian metric of eddy trapping. We found weak relationships between eddy age and the magnitude of surface chlorophyll anomalies in most long-lived Lagrangian coherent vortices; the strongest exception was in wintertime anticyclones in the lee of the Hawaiian Islands, where chlorophyll increases throughout eddy lifetimes. Overall, our results challenge the assumption that Eulerian-identified mesoscale eddy boundaries are coherent and suggest that Lagrangian trapping, combined with regional and seasonal factors, shapes the chlorophyll concentrations of subtropical mesoscale eddies.

A global summary of seafloor topography influenced by internal-wave-induced turbulent water mixing - Hans van Haren; Henk de Haas (Ocean Science, 2025)
Turbulent water motions are important for the exchange of momentum, heat, nutrients, and suspended matter, including sediments in the deep sea. The motions occur in a deep sea that is generally stably stratified in density. To maintain ocean–density stratification, an irreversible diapycnal turbulent transport is needed. The geological shape and texture of marine topography are important for water mixing, as most deep-sea turbulence is generated via internal waves breaking at sloping seafloors. For example, slopes of semidiurnal internal tidal characteristics can “critically” match the mean seafloor slope. In this paper, the concept of critical slopes is revisited from a global internal-wave turbulence viewpoint using seafloor topography and moored high-resolution temperature sensor data. Observations suggest that turbulence generation via internal-wave breaking at 5 % ± 1.5 % of all seafloors is sufficient to maintain ocean–density stratification. However, most, >90 %, turbulence contributions are found at supercritical, rather than the more limited critical, slopes measured at 1′ scales that cover about 50 % of seafloors at water depths

Coupling ocean currents and waves for seamless cross-scale modeling during Medicane Ianos - Salvatore Causio; Seimur Shirinov; Ivan Federico; Giovanni De Cillis; Emanuela Clementi; Lorenzo Mentaschi; Giovanni Coppini (Ocean Science, 2025)
This study investigates the effects of a two-way wave–circulation coupled modeling framework during extreme weather events, with a particular focus on Medicane Ianos, one of the most intense cyclones to have occurred in the Mediterranean Sea. By utilizing a high-resolution unstructured numerical grid, the study explores wave–current interactions in both open-ocean and coastal environments. To this scope, we developed the first external coupler dealing with the SHYFEM-MPI circulation model and the WAVEWATCH III wave model. The interactions considered in this framework include sea-state-dependent momentum flux, radiation stress, Doppler shift, dynamic water depth for waves, and effective wind speed. The study adopts a rigorous validation of the formulations using idealized benchmarks tailored for these specific processes. Afterwards, the modeling framework was employed in real-case simulations of Medicane Ianos. The model is calibrated, and the ocean variables are rigorously validated against in situ and Earth observation (EO) data, including satellite-based measurements. The study found that wave-induced surge components contribute from 10 % to 30 % of the total water level during the storm and that sea-state-dependent momentum flux during a medicane can influence the vertical structure of the ocean up to 100 m. The accuracy of the wave model improves by around 3 % in terms of RMSE when coupled with a circulation model. This study underscores the importance of such coupled models in accurately forecasting medicanes, storm surges, and their impacts, particularly as climate change intensifies extreme events in the Mediterranean Sea.

Stratification and overturning circulation are intertwined controls on ocean heat uptake efficiency in climate models - Linus Vogt; Jean-Baptiste Sallée; Casimir de Lavergne (Ocean Science, 2025)
The global ocean takes up over 90 % of the excess heat added to the climate system due to anthropogenic emissions, thereby buffering climate change at Earth's surface. A key metric for quantifying the role of the oceanic processes removing this heat from the atmosphere and storing it in the ocean is the ocean heat uptake efficiency (OHUE), defined as the amount of ocean heat uptake per degree of global surface warming. Despite the importance of OHUE, there remain substantial uncertainties concerning the physical mechanisms controlling its magnitude in global climate model simulations: ocean mixed-layer depth, Atlantic Meridional Overturning Circulation (AMOC) strength, and upper-ocean stratification strength have all been previously proposed as controlling factors. In this study, we analyze model output from an ensemble of 28 climate models from the Coupled Model Intercomparison Project, Phase 6 (CMIP6), in order to resolve these apparently divergent explanations. We find that stratification in the mid-latitude Southern Ocean is a key model property setting the value of OHUE due to its influence on Southern Ocean overturning. The previously proposed role of the AMOC in OHUE is explained by a link between stratification model biases in the subpolar North Atlantic and the Southern Ocean. Our analysis thus reconciles previous attempts at explaining controls on OHUE and highlights the importance of interlinked model biases across variables and geographical regions.

Long-term prediction of the Gulf Stream meander using OceanNet: a principled neural-operator-based digital twin - Michael Gray; Ashesh Chattopadhyay; Tianning Wu; Anna Lowe; Ruoying He (Ocean Science, 2025)
Many meteorological and oceanographic processes throughout the eastern US and western Atlantic Ocean, such as storm tracks and shelf water transport, are influenced by the position and warm sea surface temperature of the Gulf Stream (GS) – the region's western boundary current. Due to highly nonlinear processes associated with the GS, predicting its meanders and frontal position has been a long-standing challenge within the numerical modeling community. Although the weather and climate modeling communities have begun to turn to data-driven machine learning frameworks to overcome analogous challenges, there has been less exploration of such models in oceanography. Using a new dataset from a high-resolution data-assimilative ocean reanalysis (1993–2022) for the northwestern Atlantic Ocean, OceanNet (a neural-operator-based digital twin for regional oceans) was trained to predict the GS's frontal position over subseasonal to seasonal timescales. Here, we present the architecture of OceanNet and the advantages it holds over other machine learning frameworks explored during development. We also demonstrate that predictions of the GS meander are physically reasonable over at least a 60 d period and remain stable for longer. OceanNet can generate a 120 d forecast of the GS meander within seconds, offering significant computational efficiency.

Sensitivity study of energy transfer between mesoscale eddies and wind-induced near-inertial oscillations - Yu Zhang; Jintao Gu; Shengli Chen; Jianyu Hu; Jinyu Sheng; Jiuxing Xing (Ocean Science, 2025)
Analyses of current observations and numerical simulations at two moorings in the northern South China Sea reveal the transfer of near-inertial energy between the background currents associated with mesoscale eddies and near-inertial currents (NICs). A series of numerical experiments are conducted to determine important parameters affecting the energy transfer between idealized mesoscale eddies and NICs generated by rotating winds. Speeds of NICs transferred by both cyclonic and anticyclonic mesoscale eddies increase linearly with the wind stress and eddy strength. The transferred NICs in anticyclonic eddies have current amplitudes about 6 times larger than in cyclonic eddies. The translation speed of the mesoscale eddy and the wind rotation frequency also affect the conversion of NICs. The energy transfer rate is elevated with the increase in the positive Okubo–Weiss parameter. A simple theoretical analysis is conducted to verify our findings based on numerical results. Analytical solutions confirm the evident asymmetry of the energy transfer between anticyclonic and cyclonic eddies and quantitatively demonstrate the relationship between the wind stress and the near-inertial energy transferred by mesoscale eddies.

Advances in surface water and ocean topography for fine-scale eddy identification from altimeter sea surface height merging maps in the South China Sea - Xiaoya Zhang; Lei Liu; Jianfang Fei; Zhijin Li; Zexun Wei; Zhiwei Zhang; Xingliang Jiang; Zexin Dong; Feng Xu (Ocean Science, 2025)
The recently launched Surface Water and Ocean Topography (SWOT) satellite mission has reduced the noise levels and increased resolution, thereby improving the ability to detect previously unobserved fine-scale signals. We employed a method to utilize the unique and advanced abilities of SWOT to validate the accuracy of identified eddies in merged maps of a widely used Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) data product and a newly implemented two-dimensional variational method (2DVAR), which uses a 1/12° grid and reduces decorrelation of spatial length scales. SWOT data are more likely to provide detailed comparisons of eddy boundaries for fine-scale to mesoscale structures compared with conventional in situ data (e.g., drifting buoys). The validation results demonstrate that, compared with AVISO, the 2DVAR method exhibited greater consistency with the SWOT observations, especially at small scales, confirming the accuracy and ability of the 2DVAR method in the reconstruction and resolution of fine-scale oceanic dynamical structures.

AdriE: a high-resolution ocean model ensemble for the Adriatic Sea under severe climate change conditions - Davide Bonaldo; Sandro Carniel; Renato R. Colucci; Cléa Denamiel; Petra Pranić; Fabio Raicich; Antonio Ricchi; Lorenzo Sangelantoni; Ivica Vilibić; Maria Letizia Vitelletti (Ocean Science, 2025)
The Adriatic Sea (eastern Mediterranean Basin) is traditionally considered a natural laboratory for studying a number of oceanographic processes of global interest, including coastal dynamics, dense water formation, and thermohaline circulation. More recently, the intensification of the effects of climate change and the increasing awareness of its possible consequences on the natural and socio-economic assets of the Adriatic Basin have opened new research questions and reframed most of the existing ones into a multi-decadal timescale. In this perspective, a description of the possible evolution of the physical oceanographic processes is one of the key requirements for addressing the multi-disciplinary challenges set by climate change, but up to now it has not been possible to combine, for this basin, a sufficiently high resolution in the process description with an estimate of the uncertainty associated with the predictions. This work presents an ensemble modelling approach (AdriE – Adriatic Sea Ensemble) for the kilometre-scale description of hydrodynamics in the Adriatic Sea in an end-of-century time frame. Addressing 3D circulation and thermohaline dynamics within the Regional Ocean Modelling System (ROMS), the ensemble consists of six climate runs encompassing the period from 1987 to 2100 in a severe RCP8.5 scenario forced by the SMHI-RCA4 regional climate model, driven by as many different CMIP5 general climate models made available within the EURO-CORDEX initiative. The climate ensemble is flanked by a dedicated evaluation run for the 1987–2010 period, in which SMHI-RCA4 has been driven by reanalysis fields approximating the best available boundary conditions, thus isolating the intrinsic sources of uncertainty in the RCA4–ROMS modelling chain. In order to allow a direct comparison, the assessment of the model skills in the evaluation run borrows, as far as possible, data and approaches used for the evaluation of a recent kilometre-scale, multi-decadal modelling effort for this region. The model performances are mostly aligned with the state-of-the-art reference. In particular, good results in describing the main features of marine heatwaves and cold spells, such as timing, intensity, and interannual variability, indicate that the AdriE ensemble can effectively be used for studies on the occurrence and effects of thermal extremes in the basin. Future projections suggest an increase in temperature and salinity at upper and intermediate depths, resulting in an overall decrease in water density and possibly in deep ventilation rates. Projected variations are stronger in summer and autumn, and in these seasons the ensemble range is larger than the spatial variability of the quantities and occasionally comparable with the intensity of the climate signal, highlighting the importance of an ensemble approach to treat the climate variability at this timescale. The dataset presented in this study, which can be used for the analysis of coastal and continental margin processes of general interest, is fully available upon request to the corresponding author, and monthly averages of the main quantities are available for each run on a dedicated Zenodo repository.

Turbulent erosion of a subducting intrusion in the Western Mediterranean Sea - Giovanni Testa; Mathieu Dever; Mara Freilich; Amala Mahadevan; T. M. Shaun Johnston; Lorenzo Pasculli; Francesco M. Falcieri (Ocean Science, 2025)
Frontal zones within the Western Alboran Gyre (WAG) are characterized by a density gradient resulting from the convergence of Atlantic and Mediterranean waters. Subduction along isopycnals at the WAG periphery can play a crucial role in upper-ocean ventilation and influences its stratification and biogeochemical cycles. In 2019, physical parameters (comprising temperature, salinity, turbulent kinetic energy dissipation rates) and biogeochemical data (oxygen and chlorophyll a) profiles were collected in transects along the northern edge of the WAG. Several intrusions of subducted water with elevated oxygen, chlorophyll a, and spice anomaly were identified towards the center of the anticyclone. These features had elevated kinetic energy dissipation rates on both their upper and lower boundaries. Analysis of the turbulent fluxes involving heat, salt, oxygen, and chlorophyll a demonstrated a net flux of physical and biogeochemical properties from the intrusions to the surrounding ocean. Either the turbulent or diffusive convection mixing contributed to the observed dilution of the intrusion. Other factors (e.g., water column density stability, variability of the photic layer depth, and organic matter degradation) likely played a role in these dynamics. Enhanced comprehension of the persistence and extent of these features might lead to an improved quantitative parameterization of relevant physical and biogeochemical properties involved in subduction within the study zone.

Overlapping turbulent boundary layers in an energetic coastal sea - Arnaud F. Valcarcel; Craig L. Stevens; Joanne M. O'Callaghan; Sutara H. Suanda (Ocean Science, 2025)
Turbulent mixing properties were directly observed to understand the interactions and overlapping events of wind-forced and tidally forced boundary layers in a deep, weakly stratified coastal sea. Te-Moana-o-Raukawa / Cook Strait of Aotearoa / New Zealand is an 𝒪(200 m) deep, energetic strait, known to experience both strong tidal currents and high wind speeds. More than 𝒪(40 000) quality-controlled turbulence observations were obtained from an ocean glider equipped with a microstructure profiler and a current speed through water sensor. Tidal flows of 𝒪(1 m s−1) and wind speeds of 𝒪(10 m s−1) independently enhanced turbulent dissipation to ϵ=O(10-5Wkg-1) in bottom and surface mixed layers. Over a 4 d period, boundary-generated turbulence was evident in the interior water column on 10 occasions, enhancing interior diapycnal diffusivity levels by 5–35-fold, reaching Kz=𝒪(0.1–1 m2 s−1). In three instances, the surface and bottom mixed layers overlapped. These overlapping boundary layers were present in water depths 5-fold deeper than previously observed, which has implications for the vertical extent of material fluxes from the surface or seafloor. Interior stratification was transient, emerging from far-field advection of low-density surface water, and supported by vertical buoyancy fluxes that were episodically eroded by boundary-generated turbulence. Combining observations with one-dimensional General Ocean Turbulence Model (GOTM) outputs, turbulence interactions in the interior were found to be modulated by wind, tides, and transient stratification fields, in turn influencing the vertical structure of sinks and sources of turbulent kinetic energy. Enhanced vertical transport toward the interior of the near-boundary shear-produced turbulence was found to erode interior stratification. The interplay between antecedent stratification, turbulence generation, and vertical transport allows boundary layers to interact and modulate the vertical structure of seawater properties in deep coastal passages.

Flow patterns, hotspots, and connectivity of land-derived substances at the sea surface of Curaçao in the southern Caribbean - Vesna Bertoncelj; Furu Mienis; Paolo Stocchi; Erik van Sebille (Ocean Science, 2025)
The southern Caribbean Island of Curaçao is abundant in coral reef communities, but they are declining. Land-derived nutrients and pollutants are a potential contributing factor to this decline, since these substances, after entering the ocean, can be transported towards reef sites by ocean currents. To study the movement of the substances and their potential impact on coral reefs, we developed the model SCARIBOS (South CARIBbean Ocean System), with a 1/100° resolution, covering the period from April 2020 to March 2024 (excluding spin-up time) to analyse flow patterns within that period in close proximity to Curaçao. SCARIBOS is used as hydrodynamic input for Lagrangian particle tracking analysis with the Parcels framework, where we assess the distribution of positively buoyant substances and explore connectivity within Curaçao's coastlines as well as with the nearby regions of Aruba, Bonaire, the Venezuelan islands, and a portion of the Venezuelan mainland. Results reveal two dominant processes: the northwest-directed Caribbean Current and weaker cyclonic eddies moving in the opposite direction. These flow patterns influence hotspot locations, with higher accumulation of positively buoyant substances occurring during eddy events. Our analysis also highlights increased particle accumulation of land-derived substances in the northwest of Curaçao, corresponding to the prevailing currents. While the focus is on land-derived nutrients and pollutants, this methodology can be extended to study other particle types such as plastic debris and coral larvae, providing valuable insights for marine conservation efforts and environmental management.

Enhanced resolution capability of SWOT sea surface height measurements and their application in monitoring ocean dynamics variability - Yong Wang; Shengjun Zhang; Yongjun Jia (Ocean Science, 2025)
The wavenumber spectrum of sea surface height along ground profiles is commonly determined to quantify the magnitude of detectable ocean dynamics features by altimetry missions. In this paper, wavenumber spectra were calculated and compared for HY-2B, SARAL/AltiKa, Sentinel-3A, and SWOT (Surface Water and Ocean Topography). The wavenumber power spectral density (PSD) of sea surface height (SSH) was averaged using weighted methods across multiple along-track profiles within defined boxes. The deduced resolution capabilities were also compared and analysed, evaluated using the relevant definition of one-dimensional mesoscale resolution capability. We verified that the latest wide-swath SWOT mission offers significantly improved measurements. For example, in the vicinity of the Kuroshio, the one-dimensional mesoscale resolution of SWOT is about 25 km, twice the resolution capability of conventional satellites. In addition, the quality of measurements declined obviously over regions where the eddy kinetic energy becomes larger. Finally, the scale of change in global ocean dynamics between 60° N and 60° S was analysed using cross-power-spectrum analysis based on SWOT data from two 21 d cycles. The results showed significant geographic and temporal variations in the ocean dynamics variability scales, which are mainly relative to sea state variability. The regions with large scales of ocean dynamics variability are concentrated in oceans with strong currents and unstable sea states, such as the Kuroshio Current, the Gulf Stream, and the Antarctic Circumpolar Current. In addition, the scale of ocean dynamics variability is not necessarily large where eddy kinetic energy is large, such as the Equator and the Northwest Indian Ocean Current area. Ocean dynamics variability also varies across seasons. Meanwhile, SWOT 1 d repeat cycle data were also utilized to analyse the sub-mesoscale variability of the ocean.

Quantification of Baltic sea water budget components using dynamic topography - Vahidreza Jahanmard; Artu Ellmann; Nicole Delpeche-Ellmann (Ocean Science, 2025)
Accurate quantification of the Baltic Sea's dynamics and water budget components is essential for understanding both seasonal and long-term variations influenced by climate change. In this study, we utilize dynamic topography (DT), referenced to a geoid-based chart datum, to derive dynamic water volume and to improve estimates of the main water balance components, such as river runoff and water exchange through the Danish Straits. We utilize DT for the period from 2017 to mid-2021, which was corrected for vertical sea level biases and whose vertical datum thus coincides with the geoid. Our findings reveal seasonal dynamic volume variations, with a minimum in spring (78.9 ± 60 km3) and a maximum in autumn and winter (121 ± 57 and 124 ± 80 km3, respectively). Anomalies in DT highlight a specific region (northern Baltic Proper) as representing equilibrium mean DT for the entire Baltic Sea, while areas in the eastern and southern Baltic are prone to extremes. Barotropic exchange analysis shows that no major Baltic inflows occurred during the study period, with small to medium inflows averaging 1.6 km3 d−1 in autumn and winter, while outflows averaged 2.36 km3 d−1. River discharge, indirectly calculated from the water budget, peaked in summer (2.08 km3 d−1) and was lowest in autumn (1.26 km3 d−1), with hydrological models underestimating flows in these seasons. As a result, the method and results show great potential for quantification, validation, and a better understanding of the dynamics of the Baltic Sea, especially with a changing climate.

Marine heatwaves in the Mediterranean Sea: a convolutional neural network study for extreme event prediction - Antonios Parasyris; Vassiliki Metheniti; Nikolaos Kampanis; Sofia Darmaraki (Ocean Science, 2025)
In recent decades, the Mediterranean Sea has experienced a notable rise in the occurrence and intensity of extreme warm temperature events, referred to as marine heatwaves (MHWs). Hence, the ability to forecast Mediterranean MHWs in the short term is an area of ongoing research. Here, we introduce a novel machine learning (ML) approach specifically tailored for short-term predictions of MHWs in the basin using an attention U-Net convolutional neural network. Trained on daily sea surface temperature anomalies (SSTAs) and gridded fields of MHW presence and absence between 1982–2017, our model generates a spatiotemporal forecast of MHW occurrence up to 7 d in advance. To ensure robust performance, we explore various configurations, including different forecast horizons and U-Net architectures, number of input days, features, and different subset splits of train–test datasets. Comparative analysis against a persistence benchmark reveals an improvement of 15 % in forecasting accuracy of MHW presence for a 7 d forecast horizon. We also demonstrate an improvement of MHW prediction accuracy as the forecast horizon decreases, albeit with a smaller discrepancy between the persistence benchmark, which also results in high accuracy for the 3 d forecasts. Our proposed ML methodology offers a data-driven prediction of MHWs with reduced computational requirements, which can be applied across different regions of the global ocean, providing relevant stakeholders and management authorities with essential lead time for implementing effective mitigation strategies.

Wave-resolving Voronoi model of the Rouse number for sediment entrainment - Johannes Lawen (Ocean Science, 2025)
To integrate wave and sediment transport modeling, a computationally extensive wave-resolving Voronoi-mesh-based simulation has been developed to improve upon heretofore separate sediment and spectral wave modeling. Sediment entrainment by wave motion and fine scales of the dynamic Rouse number distribution across the seabed were brought into focus. The entirely parallelized wave-resolving hydrodynamic model is demonstrated for nearshore beach waters adjacent to artificial islands in Doha Bay. The nested model was validated with tidal time series for three locations and two seasons.

Chlorophyll shading reduces zooplankton diel migration depth in a high-resolution physical–biogeochemical model - Mathieu A. Poupon; Laure Resplandy; Jessica Garwood; Charles Stock; Niki Zadeh; Jessica Y. Luo (Ocean Science, 2025)
Zooplankton diel vertical migration (DVM) is critical to ocean ecosystem dynamics and biogeochemical cycles, by supplying food and injecting carbon into the mesopelagic ocean (200–800 m). The deeper the zooplankton migrate, the longer the carbon is sequestered away from the atmosphere and the deeper the ecosystems they feed. Sparse observations show variations in migration depths over a wide range of temporal and spatial scales. A major challenge, however, is to understand the biological and physical mechanisms controlling this variability, which is critical for assessing impacts on ecosystem and carbon dynamics. Here, we introduce a migrating zooplankton model for medium and large zooplankton that explicitly resolves diel migration trajectories and biogeochemical fluxes. This model is integrated into the MOM6-COBALTv2 ocean physical–biogeochemical model and is applied in an idealized high-resolution (9.4 km) configuration of the North Atlantic. The model skillfully reproduces observed North Atlantic migrating zooplankton biomass and DVM patterns. Evaluation of the mechanisms controlling zooplankton migration depth reveals that chlorophyll shading decreases zooplankton migration depths by 60 m in the subpolar gyre compared with the subtropical gyre, with pronounced seasonal variations linked to the spring bloom. Fine-scale spatial effects (

Dissipation ratio and eddy diffusivity of turbulent and salt finger mixing derived from microstructure measurements - Jianing Li; Qingxuan Yang; Hui Sun (Ocean Science, 2025)
Eddy diffusivity is usually estimated using the Osborn relation assuming a constant dissipation ratio of 0.2. In this study, we examine dissipation ratios and eddy diffusivities of turbulent mixing and salt finger mixing based on microstructure datasets. We find that the dissipation ratio of turbulence, ΓT, is highly variable with a median value clearly greater than 0.2, which shows strong seasonal variation and decreases slightly with depth in the western equatorial Pacific but obviously increases with depth in the midlatitude Atlantic. ΓT is jointly modulated by the Ozmidov scale to the Thorpe scale ratio ROT and the buoyancy Reynolds number Reb, namely ΓT∝ROT-4/3 ⋅ Reb1/2. The eddy diffusivity based on observed ΓT is larger than that estimated with 0.2 and presents a much stronger bottom enhancement. The eddy diffusivities of heat and salt for a salt finger are calculated using two “analogical” Osborn equations, and their corresponding “effective” dissipation ratios ΓθF and ΓSF are examined. ΓθF scatters over 2 orders of magnitude with a median value of 0.47 and is mostly linearly correlated with ΓSF as ΓSF≈ 5 ΓθF. The density flux ratio for a salt finger decreases sharply with a density ratio Rρ smaller than 2.4 but regrows to a larger value with Rρ exceeding 2.4. The salt-finger-induced eddy diffusivities also increase with depth, with some being comparable to even stronger ones than the mean turbulent ones. This study highlights the influences of variable dissipation ratios and different mixing types on eddy diffusivity estimates and should help the improvement of mixing estimate and parameterization.

Coupled estimation of internal tides and turbulent motions via statistical modal decomposition - Igor Maingonnat; Gilles Tissot; Noé Lahaye (Ocean Science, 2025)
We present a data-driven modal-decomposition method that extracts the part of an incoherent internal tidal wave that correlates with the proper orthogonal decomposition (POD) of a turbulent mesoscale flow. This method exploits the a priori knowledge that the incoherent internal tide arises from interactions between an incident wave and the turbulent flow and also exploits the corresponding statistical correlation between the two types of motions. The method is presented and tested in an idealised framework based on the rotating-shallow-water model, where we provide a physical interpretation for the decomposition method based on theoretical considerations. Using idealised simulations with a plane wave propagating through a zonal turbulent jet, we first propose the use of the modal-decomposition method as a data analysis technique to understand how the wave is scattered by the flow. In a second step, we construct an estimation algorithm capable of separating the entangled contributions of the wave and mesoscale motions from a single sea surface height snapshot. This algorithm, which consists of estimating the POD coefficients of the turbulent flow shared by the wave and jet modes, is particularly suitable for configurations where the jet contribution to the sea surface height (SSH) is larger than that of the wave.

Generation of super-resolution gap-free ocean colour satellite products using data-interpolating empirical orthogonal functions (DINEOF) - Aida Alvera-Azcárate; Dimitry Van der Zande; Alexander Barth; Antoine Dille; Joppe Massant; Jean-Marie Beckers (Ocean Science, 2025)
In this work we present a super-resolution approach for deriving high-spatial-resolution and high-temporal-resolution ocean colour satellite datasets. The technique is based on DINEOF (data-interpolating empirical orthogonal functions), a data-driven method that uses the spatio-temporal coherence of analysed datasets to infer missing information. DINEOF is used here to effectively increase the spatial resolution of satellite data and is applied to a combination of Sentinel-2 and Sentinel-3 datasets. The results show that DINEOF is able to infer the spatial variability observed in the Sentinel-2 data to the Sentinel-3 data while reconstructing missing information due to clouds and reducing the amount of noise in the initial dataset. In order to achieve this, the Sentinel-2 and Sentinel-3 datasets have undergone the same pre-processing, including a comprehensive, region-independent, and pixel-based automatic switching scheme for choosing the most appropriate atmospheric correction and ocean colour algorithm to derive in-water products. The super-resolution DINEOF has been applied to two different variables (turbidity and chlorophyll) and two different domains (Belgian coastal zone and the whole of the North Sea), and the sub-mesoscale variability of the turbidity along the Belgian coastal zone has been studied.

Ocean wave spectrum bias correction through energy conservation for climate change impacts - Andrea Lira Loarca; Giovanni Besio (Ocean Science, 2025)
A novel bias adjustment technique for 2D directional wave spectra is presented, which accounts for the intra-annual temporal variability of waves and the conservation of the wave energy integrated parameter and its extreme distribution, allowing for shifts in frequency and direction given by the global climate model and regional climate model (GCM–RCM) climate signal for the complete multimodal energy distribution. This work is the first attempt to address the biases inherent in GCM–RCM wave spectrum simulations for an assessment of the magnitudes of the projected changes in a climate change scenario. The bias-correction method is applied to a multimodel ensemble of 17 EURO-CORDEX regional simulations of wave spectra in 11 locations of the Mediterranean Sea. Climate change impacts are assessed by means of the changes between the bias-adjusted ensemble and hindcast wave spectra for mid-century conditions from 2034 until 2060 and end-of-century conditions from 2074 until 2100. The results highlight the need for novel bias-correction techniques that address the complexity of the possible directional and frequency shifts due to climate change, in order to provide an accurate assessment of projected future changes in the wave climate.

Operational hydrodynamic service as a tool for coastal flood assessment - Xavier Sánchez-Artús; Vicente Gracia; Manuel Espino; Manel Grifoll; Gonzalo Simarro; Jorge Guillén; Marta González; Agustín Sanchez-Arcilla (Ocean Science, 2025)
Coastal communities are increasingly vulnerable to storm impacts, highlighting the urgent need for predictive tools and enhanced preparedness. In this work, a comprehensive, high-resolution hydrodynamic operational service using the XBeach model is presented and tested for three urban beaches in Barcelona, NW Mediterranean Sea. The operational system is designed to provide early warnings of coastal flooding at a 3 d horizon. The operational architecture is based on Python scripts combined with task automation tools, ensuring a user-friendly system implemented on a standard desktop computer. Hydrodynamic validation of the model is carried out using data gathered during a field campaign in 2022, when a high-intensity storm occurred, resulting in a root mean square error of around 0.4 m and a skill score assessment index of 0.82. Flooding predictions were validated using videometry systems, yielding satisfactory Euclidean distances of less than 5 m for storms close to the topobathymetry collection. For storms occurring years earlier, the distances ranged between 7–15 m, underscoring the need for regular topobathymetry updates to maintain forecasting accuracy. The service provides a warning system with a specific categorisation of the event, enabling the end-users to prepare for a possible flooding. The strategy is currently running in operational mode, issuing alert warnings at the correct severity level. The outcome assists in decision-making relating to such events by utilising the operational system. The presented methodology is easily adaptable and replicable to meet user requirements or to be applied in other areas of interest.

An estimate of the eddy diffusivity tensor from observed and simulated Lagrangian trajectories in the Benguela Upwelling System - Ria Oelerich; Birte Gülk; Julia Dräger-Dietel; Alexa Griesel (Ocean Science, 2025)
Lateral mixing of unresolved processes in ocean models is usually parameterized with a scalar diffusivity, although the observed mixing can be highly anisotropic. Estimating the full diffusivity tensor from Lagrangian dispersion observations is challenging because shear dispersion from background currents can prevent the diffusive limit from being reached. This study investigates the diffusivity tensor with Lagrangian single- and pair-particle statistics in the Benguela upwelling region, using (1) a set of Lagrangian trajectories derived from a recent drifter dataset with an hourly resolution and background currents from the Ocean Surface Current Analysis Real-time (OSCAR) and (2) another trajectory set from simulations using the 1/10° Parallel Ocean Program (POP) simulation. Theory predicts that pair-particle diffusivities, which are expected to be independent of background mean flows, are twice the single-particle diffusivities if the pair velocities are uncorrelated. In this study, it is found that, although pair-particle diffusivities are much less influenced by mean flow, they are generally significantly smaller than twice the single-particle diffusivities. Subtracting the mean flow reduces this discrepancy and improves convergence in both methods, although single-particle diffusivities remain higher. Velocity autocorrelations decay faster than pair correlations, with mean flow subtraction accelerating decorrelation, especially in the zonal direction. The pair correlation term in the diffusivity equation contributes significantly to the differences between single-particle and pair-particle diffusivities, explaining why pair-particle diffusivities are generally smaller, making them a less accurate estimate in diffusive parameterizations. In both the POP simulation and the observations, convergence properties improve significantly after mean flow subtraction. Mean flow removal plays a critical role in achieving convergence in the components of the diffusivity tensor and in the major-axis component after diagonalization. The significant anisotropy in the diffusivity tensor is mainly explained by the anisotropy in the Lagrangian integral timescales, while the major-axis component of the velocity variance tensor is only about 1.2 times the minor-axis component. The motions that are not resolved by the OSCAR surface current product but are captured by the surface drifters contribute significantly to the diffusivities, accounting for 8 % and 42 % of the contributions to the zonal and meridional components, respectively, after mean flow subtraction. This study highlights the importance of including the full diffusivity tensor in the Benguela upwelling region in lateral mixing parameterizations.

Internal-wave-induced dissipation rates in the Weddell Sea Bottom Water gravity current - Ole Pinner; Friederike Pollmann; Markus Janout; Gunnar Voet; Torsten Kanzow (Ocean Science, 2025)
This study investigates the role of wave-induced turbulence in the dynamics of the Weddell Sea Bottom Water gravity current. The current transports dense water from its formation sites on the shelf to the deep sea and is a crucial component of the Southern Ocean overturning circulation. The analysis is based on velocity records from a mooring array deployed across the continental slope between January 2017 and January 2019 as well as vertical profiles of temperature and salinity measured on various ship expeditions on a transect along the array. Previous studies suggest that internal waves may play a crucial role in driving turbulence within gravity currents; however, this influence has not been quantitatively assessed. To quantify the contribution of internal waves to turbulence in this particular gravity current along the continental slope, we employ three independent methods for estimating dissipation rates. First, we use a Thorpe scale approach to compute total, process-independent dissipation rates from density inversions in density profiles. Second, we apply the fine-structure parameterization to estimate wave-induced dissipation rates from vertical profiles of strain, calculated from temperature and salinity profiles. Third, we estimate wave energy levels from moored velocity time series and deduce wave-induced dissipation rates by applying a formulation that is at the heart of the fine-structure parameterization. Turbulence is highest at the shelf break and decreases towards the deep sea, in line with decreasing strength of wave-induced turbulence. We observe a two-layer structure of the gravity current, a strongly turbulent bottom layer about 60–80 m thick, and an upper, more quiescent interfacial layer. In the interfacial layer, internal waves induce an important part of the dissipation rate and therefore drive entrainment of warmer upper water into the gravity current. A precise quantification of the contribution is complicated by large method uncertainties. A comparison with turbulence measurements up- and downstream of our study site indicates that the processes dominating turbulence generation may depend on the location along the Weddell Sea Bottom Water gravity current: on the shelf trapped waves are most important, on the continental slope breaking internal waves dominate, and in the basin symmetric instability is likely the main driver of turbulence.

Decadal changes in phytoplankton functional composition in the Eastern English Channel: possible upcoming major effects of climate change - Zéline Hubert; Arnaud P. Louchart; Kévin Robache; Alexandre Epinoux; Clémentine Gallot; Vincent Cornille; Muriel Crouvoisier; Sébastien Monchy; Luis Felipe Artigas (Ocean Science, 2025)
Global change is known to exert a considerable impact on marine and coastal ecosystems, affecting various parameters such as sea surface temperature (SST), runoff, circulation patterns and the availability of limiting nutrients (like nitrogen, phosphorus and silicon), with each influencing phytoplankton communities differently. This study is based on weekly to fortnightly in vivo fine-spatial-resolution (∼ 1 km) phytoplankton observations along an nearshore–offshore gradient in the French waters of the Eastern English Channel in the Strait of Dover. The phytoplankton functional composition was addressed by automated “pulse-shape recording” flow cytometry, coupled with the analysis of environmental variables over the last decade (2012–2022). This method allows for the characterization of almost the entire phytoplankton size range (from 0.1 to 800 µm width) and the determination of the abundance of functional groups based on optical single-cell signals (fluorescence and scatter). We explored seasonal, spatial and decadal dynamics in an environment strongly influenced by tides and currents. Over the past 11 years, the SST has shown an increasing trend at all stations, with nearshore waters warming faster than offshore waters (+1.05 °C vs. +0.93 °C). Changes in nutrient concentrations have led to imbalances in nutrient ratios (N:P:Si) relative to reference nutrient ratios. However, a return to balanced ratios has been observed since 2019. The phytoplankton total abundance has also increased over the aforementioned decade, with a higher contribution of small-sized cells (picoeukaryotes and picocyanobacteria) and a decrease in microphytoplankton, particularly near the coast. Based on an analysis of environmental parameters and phytoplankton abundance, the winters of 2013–2014 and 2019–2020 were identified as shifting periods in this time series. These changes in the phytoplankton community, favoring the smallest groups, could lead to a reduction in the productivity of coastal marine ecosystems, which could, in turn, affect higher trophic levels and the entire food web.

River discharge impacts coastal southeastern tropical Atlantic sea surface temperature and circulation: a model-based analysis - Léo C. Aroucha; Joke F. Lübbecke; Peter Brandt; Franziska U. Schwarzkopf; Arne Biastoch (Ocean Science, 2025)
The southeastern tropical Atlantic (SETA) coastal region sustains highly productive fisheries and marine ecosystems, thus having immeasurable socio-economic importance for southwestern African coastal countries. It is characterized by high sea surface temperature (SST) variability and significant freshwater input from land mainly due to Congo River discharge. In this study, using high-resolution ocean model sensitivity experiments, we show that the presence of low-salinity waters from the river discharge increases the mean state SST in the SETA coastal fringe by about 0.26 °C on average and by up to 0.9 °C from south of the Congo River to the Angola–Benguela Front (ABF). North of the Congo River up to about 4° S, this input significantly reduces the mean state SST by more than 1 °C. We demonstrate that the impact of river discharge on SST is associated with a halosteric effect, which modifies the sea surface height gradient and alters geostrophic currents, producing a southward coastal geostrophic flow, with an onshore geostrophic component to the south of the Congo River. Hence, advective warming and downwelling are generated south of the river mouth. Furthermore, the southward advection generated by the low-salinity waters pushes the ABF further south. Concomitantly north of the Congo's mouth, the sea surface height gradient generates a northward geostrophic coastal current with an offshore geostrophic component, which is associated with advective cooling and upwelling north of 6° S. These results draw attention to the freshwater impact on SSTs and ocean surface dynamics, especially in the projected climate change scenario of continuously increasing land-to-ocean discharge.

The influence of a submarine canyon on the wind-driven downwelling circulation over the continental shelf - Pedro A. Figueroa; Gonzalo S. Saldías; Susan E. Allen (Ocean Science, 2025)
The response of a coastal ocean model, simulating a typical eastern boundary system, to downwelling-favorable winds with and without the presence of a submarine canyon is studied. Three contrasting bathymetric configurations, considering shelves with different depths and slopes, are evaluated. Experiments without a submarine canyon represent the well-known downwelling circulation and cross-shore structure with a downwelling front and the development of frontal instabilities generating density anomalies in the bottom layer. The presence of the submarine canyon drives important changes in cross-shore flows, with opposing velocities on either side of the canyon. Onshore (offshore) and downward (upward) velocities develop in the upstream (downstream) side of the canyon in the time-dependent and advective phases. Instabilities develop and are modified principally downstream of the canyon. Overall, the net impact of the canyon is to enhance offshore and downward transport. However, particle tracking experiments reveal that particles can become trapped inside the canyon in an anticyclonic circulation when the particles pass the canyon over the continental slope or when particles inside the canyon are affected by downwelling conditions. Overall, ∼20 %–23 % (∼15 %–18 %) of particles released directly upstream (in the canyon) at depths below the continental shelf become trapped inside the canyon until the end of the simulations (15 d).

Alongshore sediment transport analysis for a semi-enclosed basin: a case study of the Gulf of Riga, the Baltic Sea - Tarmo Soomere; Mikołaj Zbigniew Jankowski; Maris Eelsalu; Kevin Ellis Parnell; Maija Viška (Ocean Science, 2025)
The properties of wave-driven sediment transport and the dimensions of single sedimentary compartments are often radically different in different parts of semi-enclosed waterbodies with an anisotropic wind climate. The western, southern and eastern shores of the Gulf of Riga are a remote part of the more than 700 km long interconnected sedimentary coastal system of the eastern Baltic Sea from Samland in Kaliningrad District, Russia, to Pärnu Bay, Estonia. Even though shores of the gulf are generally straight or gently curved, the presence of small headlands and variations in the orientation of the coastline give rise to numerous fully or partially separated sedimentary compartments. We decompose sedimentary shores of this gulf into single compartments and cells based on the analysis of wave-driven potential sediment transport using high-resolution wave time series and the Coastal Engineering Research Centre (CERC) approach. The western shore of the Gulf of Riga forms a large interconnected sedimentary system with intense sediment transport that is largely fed by sand transported from the Baltic proper. The southern shore has much less intense sediment transport and is mostly an accumulation area. The south-eastern sector of the gulf is an end-station of counter-clockwise sand transport. The eastern shore consists of several almost isolated sedimentary cells and contains a longer segment where clockwise transport predominates. The transport rates along different shore segments show extensive interannual variations but no explicit trends in the period 1990–2022.

Detection and tracking of carbon biomes via integrated machine learning - Sweety Mohanty; Lavinia Patara; Daniyal Kazempour; Peer Kröger (Ocean Science, 2025)
In the framework of a changing climate, it is useful to devise methods capable of effectively assessing and monitoring the changing landscape of air–sea CO2 fluxes. In this study, we developed an integrated machine learning tool to objectively classify and track marine carbon biomes under seasonally and interannually changing environmental conditions. The tool was applied to the monthly output of a global ocean biogeochemistry model at 0.25° resolution run under atmospheric forcing for the period 1958–2018. Carbon biomes are defined as regions having consistent relations between surface CO2 fugacity (fCO2) and its main drivers (temperature, dissolved inorganic carbon, alkalinity). We detected carbon biomes by using an agglomerative hierarchical clustering (HC) methodology applied to spatial target–driver relationships, whereby a novel adaptive approach to cut the HC dendrogram based on the compactness and similarity of the clusters was employed. Based only on the spatial variability of the target–driver relationships and with no prior knowledge of the cluster location, we were able to detect well-defined and geographically meaningful carbon biomes. A deep learning model was constructed to track the seasonal and interannual evolution of the carbon biomes, wherein a feed-forward neural network was trained to assign labels to detected biomes. We find that the area covered by the carbon biomes responds robustly to seasonal variations in environmental conditions. A seasonal alternation between different biomes is observed over the North Atlantic and Southern Ocean. Long-term trends in biome coverage over the 1970–2018 period, namely a 1 % to 2 % per decade expansion of the subtropical biome in the North Atlantic and a 0.5 % to 1 % per decade expansion of the subpolar biome in the Southern Ocean, are suggestive of long-term climate shifts. Our approach thus provides a framework that can facilitate the monitoring of the impacts of climate change on the ocean carbon cycle and the evaluation of carbon cycle projections across Earth system models.

Observed bottom warming in the East Siberian Sea driven by the intensified vertical mixing - Xiaoyu Wang; Longjiang Mu; Xianyao Chen (Ocean Science, 2025)
The East Siberian Sea (ESS) features the broadest continental shelf on Earth and contains nearly 80 % of the world's subsea permafrost. A persistently cold bottom layer, with temperatures at freezing point, inhibits the downward transport of heat, thus preventing the thawing of permafrost and subsequent methane release from sediments. However, in early September 2016, we observed an unprecedented warming of over 3 °C at the bottom of the water column, approximately 46 m deep in the ESS, following a relatively moderate Arctic cyclone. We attribute this notable bottom warming to enhanced wave-induced vertical mixing, which facilitates the well-mixed Arctic marginal seas and allows surface heat to reach the bottom layer. As sea ice continues to retreat on the Arctic continental shelf, wind-driven waves have longer fetch to grow. Consequently, even moderate cyclones can trigger substantial vertical mixing, a phenomenon not previously documented. Given the accelerated warming of the Arctic and the rapid decline of sea ice, we anticipate that more open water will foster the growth of larger wind-driven waves and intensified vertical mixing, leading to greater heat influx to the bottom layers of Arctic shelves in the future.

Technical note: Large offsets between different datasets of seawater isotopic composition – an illustration of the need to reinforce intercalibration efforts - Gilles Reverdin; Claire Waelbroeck; Antje H. L. Voelker; Hanno Meyer (Ocean Science, 2025)
We illustrate offsets in surface seawater isotopic composition between recent public datasets from the Atlantic Ocean and the subtropical southeastern Indian Ocean. The observed offsets between datasets often exceed 0.10 ‰ in δ18O and 0.50 ‰ in δ2H. They might in part originate from different sampling of seasonal, interannual, or spatial variability. However, they likely mostly originate from different instrumentations and protocols used to measure the water samples. Estimation of the systematic offsets is required before merging the different datasets in order to investigate the spatiotemporal variability of isotopic composition in the world ocean surface waters. This highlights the need to actively share seawater isotopic composition samples dedicated to specific intercomparison of data produced in different laboratories and to promote best practices, a task to be addressed by the new Scientific Committee of Oceanic Research (SCOR) working group 171.

Technical note: Spectral slopes in a deep, weakly stratified ocean and coupling between sub-mesoscale motion and small-scale mechanisms - Hans van Haren (Ocean Science, 2025)
Large, basin-wide ocean circulations are complex non-linear dynamical systems. They include small-scale physical processes such as transport by sub-mesoscale eddies and turbulence-generating breaking of internal waves. To date, however, knowledge is lacking on the precise interactions between the different processes. In this note, a potential contributor to the interactions is investigated using spectra from deep-sea-moored observations. In weakly stratified waters, continuous spectral slopes are observed that extend from sub-mesoscales across the internal wave band to the turbulence range. In the latter, the governing slope can be distinctly different from the inertial subrange of shear turbulence and is described as the buoyancy subrange of convection turbulence. At sub-inertial frequencies, the slope's extension describes either quasi-gyroscopic waves or sub-mesoscale eddies. Such cross-spectral correspondence suggests a potential feedback mechanism stabilizing large-scale ocean circulations.

Control of spatio-temporal variability of ocean nutrients in the East Australian Current - Megan Jeffers; Christopher C. Chapman; Bernadette M. Sloyan; Helen Bostock (Ocean Science, 2025)
The East Australian Current (EAC), the South Pacific's southward-flowing western boundary current, dominates the marine environment of the eastern coast of Australia. Upwelling of deep EAC nutrient-rich water into the oligotrophic surface waters is very important for maintaining upper-ocean productivity. However, the role of EAC dynamics in upper-ocean nutrient variability and resulting productivity is poorly understood. In this study, we use physical and biogeochemical data collected from 2012–2022 to improve understanding of the variability of the nutrients in the upper water column at ∼ 27° S, a subtropical region strongly influenced by the EAC. The 10-year data set shows that there is a seasonal increase in nutrient concentrations in the upper water column (0–200 m) in the austral spring (September–November) and autumn (March–May) and a minimum in winter (June–August). We also find that the nutrient concentrations in the upper water column are influenced by the position of the EAC jet. Two main modes of variability in the EAC's position are identified: an inshore mode with the jet flowing along the continental slope and an offshore mode with the current core detached from the continental slope and flowing over the adjacent abyssal plain. The position of the EAC jet influences the location of upwelling of nutrient-rich water at depth (> 200 m). For the EAC inshore mode, cooler nutrient-rich waters are restricted to the area of the continental shelf and slope that is inshore of the EAC. The offshore mode exhibits a wider distribution of nutrient-rich waters over both the inshore shelf and slope and the offshore abyssal Tasman Sea. Our analysis highlights the important interactions between the highly variable EAC and the distribution of high-nutrient waters, which has implications for primary production, fisheries, and the biological carbon pump.

Anthropogenic pressures driving the salinity intrusion in the Guadalquivir estuary: insights from 1D numerical simulations - Sara Sirviente; Juan J. Gomiz-Pascual; Marina Bolado-Penagos; Sabine Sauvage; José M. Sánchez-Pérez; Miguel Bruno (Ocean Science, 2025)
The study presents a dynamic analysis of the present-day behavior of salinity concentration in the Guadalquivir estuary and evaluates the impact of anthropogenic pressures on the dynamics of the horizontal salinity gradient. A one-dimensional (1D) hydrodynamic model with an advection and dispersion module is used to study the effects of human pressure involved in the salinity concentration along the estuary. The observations, which correspond to continuous measurements taken during different oceanographic campaigns from 2021 to 2023, show an excessive salt intrusion in the estuary (with salinities of 5 psu at 60 km from the mouth) compared to the idealized situation when anthropogenic water withdrawals are absent. This highlights the need to include a water withdrawal term in the simulations to accurately reproduce the system's real behavior, thereby reflecting the magnitude of anthropogenic pressures. The model successfully reproduces the observations when this forcing factor is included. Under constant low-flow conditions, experiments show that increasing water withdrawals leads to an increase in the horizontal salinity gradient upstream. Similarly, under constant water withdrawal conditions, a decrease in the horizontal salinity gradient is observed when freshwater flows exceed 40 m3 s−1. Variations in anthropogenic pressures, such as water withdrawals for agriculture or the saline industry and reductions in freshwater flow, play a fundamental role in the evolution of salinity along the estuary. Under the current circumstances, the Guadalquivir estuary requires an urgent regulation of these uses in order to avoid further damage to the aquatic ecosystems.

Impact of extratropical cyclones on coastal circulation in a semi-enclosed bay within the Humboldt Current System - Josse Contreras-Rojas; Piero Mardones; Marcus Sobarzo (Ocean Science, 2025)
This study examines the hydrodynamic response of the Gulf of Arauco, a semi-enclosed bay within the Humboldt Current System, to the passage of extratropical cyclones and their associated changes in wind patterns during the austral winter. Characterization of these cyclones over central Chile and their effect on the gulf's meridional wind was conducted using ERA5 atmospheric pressure fields in conjunction with a cyclone tracking algorithm. The hydrodynamic response was assessed through acoustic Doppler current profiler (ADCP) observations at two strategic sites, providing valuable insights into the currents at the gulf's connections with the open ocean. Measurements were conducted from July to September 2016 and May to July 2018. Additionally, the main modes of subtidal current variability were compared with the local wind through coherence wavelets, revealing a direct influence of cyclones on the modulation of the gulf's currents. Our findings suggest that intense northerly wind events, associated with the passage of extratropical cyclones, can cause surface water transport into the gulf, accumulating at the gulf's head. This would result in a pronounced pressure gradient, driving a water outflow through both connections with the open ocean, thereby altering the coastal circulation patterns. As the northerly wind decreases, the surface inflow in the northwest region attenuates, allowing the water to exit at shallower depths. This mechanism suggests cyclones play a vital role in renewing the waters of semi-enclosed bays such as the Gulf of Arauco, potentially reducing the water residence times. Consequently, these insights provide a broader understanding of wind-driven coastal dynamics, highlighting their significant impacts on marine ecosystems and coastal management in similar semi-enclosed bays globally. By contributing to the broader knowledge of the interaction between atmospheric and oceanic processes in coastal regions, this research offers a comparative perspective applicable to other regions affected by similar atmospheric phenomena.

Application of wave–current coupled sediment transport models with variable grain properties for coastal morphodynamics: a case study of the Changhua River, Hainan - Yuxi Wu; Enjin Zhao; Xiwen Li; Shiyou Zhang (Ocean Science, 2025)
This study introduces an integrated sand transport model that considers wave and current actions alongside variable grain properties to explore sediment dynamics in river deltas. The research delves into a case study of a river delta region, examining sediment transport over a substantial stretch of the river's lower course. The study incorporates topographic data, sediment sampling, and remote sensing to validate the model against observed suspended sediment concentrations at a key monitoring station. The results reveal substantial sediment deposition in both the estuary and lower reaches of the river, influenced by hydrodynamic conditions and geological settings. Deposition patterns in the estuary are primarily driven by coastal currents and wave action, while river channel deposition is linked to river constriction and flow velocity variations. The study demonstrates that the residual current in the region consistently flows towards a nearby bay, suggesting that sediment in the lower reaches of the river will be directed by this residual flow. The study underscores the pivotal roles of current and wave action in sediment transport within a multi-branched estuary characterized by low sediment concentrations, which can inform coastal management and environmental planning.

Ocean carbon sink assessment via temperature and salinity data assimilation into a global ocean biogeochemistry model - Frauke Bunsen; Judith Hauck; Sinhué Torres-Valdés; Lars Nerger (Ocean Science, 2025)
Global ocean biogeochemistry models are frequently used to derive a comprehensive estimate of the global ocean carbon uptake. These models are designed to represent the most important processes of the ocean carbon cycle, but the idealized process representation and uncertainties in the initialization of model variables lead to errors in their predictions. Here, observations of ocean physics (temperature and salinity) are assimilated into the ocean biogeochemistry model FESOM2.1-REcoM3 over the period 2010–2020 to study the effect on the air–sea carbon dioxide (CO2) flux and other biogeochemical (BGC) variables. The assimilation nearly halves the model–observation differences in sea surface temperature and salinity, with modest effects on the modeled ecosystem and CO2 fluxes. The main effects of the assimilation on the air–sea CO2 flux occur on small scales in highly dynamic regions, which pose challenges to ocean models. Its largest imprint is in the Southern Ocean during winter. South of 50° S, winter CO2 outgassing is reduced; thus the regional CO2 uptake increases by 0.18 Pg C yr−1 through the assimilation. Other particularly strong regional effects on the air–sea CO2 flux are located in the area of the North Atlantic Current (NAC). However, the effect on the global ocean carbon uptake is a comparatively small increase by 0.05 Pg C yr−1 induced by the assimilation, yielding a global mean uptake of 2.78 Pg C yr−1 for the period 2010–2020.

Hydrographic section along 55° E in the Indian and Southern oceans - Katsuro Katsumata; Shigeru Aoki; Kay I. Ohshima; Michiyo Yamamoto-Kawai (Ocean Science, 2025)
A hydrographic section along 55° E, south of 30° S, was visited from December 2019 to January 2020 as the first occupation under the Global Ocean Ship-based Hydrographic Investigations Program. The water column was measured from the sea surface to 10 dbar above the bottom with eddy-resolving station spacings and state-of-the-art accuracy. The upper profile was characterised by a conspicuous front between 42.5 and 43° S and a cold-core eddy at 39° S. The front was identified as the confluence of Subtropical and Subantarctic fronts. The Agulhas Return Current front was found at 41.6° S. When combined with the section north of 30° S observed in 2018, another subsurface front was found in dissolved oxygen around 28° S at depths of 1500 to 3000 dbar. In the eastern Weddell–Enderby Abyssal Plain, no obvious large-scale flow was observed at depths greater than 3000 dbar. We used transient tracers to estimate isopycnal diffusivity there to be 72±16 m2 s−1. Antarctic Bottom Water in the basin consisted of water masses originating from the Cape Darnley region (0 %–35 %) and Weddell Sea Deep Water (5 %–75 %) diluted by Lower Circumpolar Deep Water above. These snapshot observations not only confirm hydrographic features reported earlier in the Madagascar and Crozet basins, but also describe the diffusive nature of the deep to bottom circulation in the Weddell–Enderby Abyssal Plain.

Uncertainties in the finite-time Lyapunov exponent in an ocean ensemble prediction model - Mateusz Matuszak; Johannes Röhrs; Pål Erik Isachsen; Martina Idžanović (Ocean Science, 2025)
Lagrangian coherent structures (LCSs) are transient features in the ocean circulation that describe particle transport, revealing information about transport barriers and accumulation or dispersion regions. The method of finite-time Lyapunov exponents (FTLEs) uses Lagrangian data to approximate LCSs under certain conditions. In this study FTLEs are used to characterize flow field features in a high-resolution regional ocean forecast system. Generally, trajectory simulations, such as Lagrangian trajectories, inherit uncertainty from the underlying ocean model, bearing substantial uncertainties as a result of chaotic and turbulent flow fields. As the FTLE characterizes the flow, which may impact particle transport, we aim to investigate the uncertainty of FTLE fields at any given time using an ensemble prediction system (EPS) to propagate velocity field uncertainty into the FTLE analysis. In addition, velocity fields often evolve rapidly in time, and we therefore also evaluate the time variability of FTLE fields. We find that averaging over ensemble members can reveal robust FTLE ridges, i.e., FTLE ridges that exist across ensemble realizations. Likewise, time averaging can reveal persistent FTLE ridges, i.e., ridges that occur over extended periods of time. In addition, large-scale FTLE ridges are more robust and persistent than small-scale ridges. Averaging of FTLE fields is thus effective at removing short-lived and unpredictable structures and may provide the means to employ FTLE analysis in forecasting applications that require the ability to separate uncertain from certain flow features.

Investigation of the impact of complex coastline geometry on the evolution of storm surges along the eastern coast of India: a sensitivity study using a numerical model - Pawan Tiwari; Ambarukhana D. Rao; Smita Pandey; Vimlesh Pant (Ocean Science, 2025)
We conduct a comprehensive investigation of the generation of storm surges along the coast in response to complex coastline geometry using a standalone advanced circulation (ADCIRC) model. The study deals with sensitivity experiments using various idealized concave and/or convex model domains with the same intensity in terms of parallel cyclone tracks. It demonstrates that a sharp curvature, along with the landfall location of each track within the domain, has more influence on the surge evolution. Peak surges (PSs) are generated in the domain with tracks possessing strong onshore winds, while intense alongshore winds are responsible for PSs spreading along the coast. The time evolution of both positive and negative surges along the coast is also demonstrated. The propagation of energy density per unit length associated with surge waves is computed for concave coasts to explain the funneling effect. Using parallel tracks, the development of PSs is also seen, with actual intricate coastal stretch having concave and convex coastlines along the eastern coast of India, which is consistent with the findings of idealized experiments. Further simulations are carried out along real coastlines, with the different approach angles of the track exhibiting that maximum PSs are not always aligned with the east of the track. Depending on the track angle, PSs may also develop on the western side. Experiments on real coastlines also indicate that peak surge in the concave coastline is more influenced by the cyclone's radius of maximum winds. Surge generation with different radii of maximum winds is seen to the western side of the track, though the cyclone does not experience landfall in this region. Simulation with a recent cyclone, Michaung, reveals that the occurrence of storm tides is also seen to the west of the landfall location as the track moves parallel to the coast, which agrees with the observations. This study shows the importance of the local coastline configuration, particularly for the concave coasts, in relation to the intensification of storm surges.

Turbulent heat flux dynamics along the Dotson and Getz ice-shelf fronts (Amundsen Sea, Antarctica) - Blandine Jacob; Bastien Y. Queste; Marcel D. du Plessis (Ocean Science, 2025)
In coastal polynyas, where sea-ice formation and melting occur, it is crucial to have accurate estimates of heat fluxes in order to predict future sea-ice dynamics. The Amundsen Sea Polynya is a coastal polynya in Antarctica that remains poorly observed by in situ observations because of its remoteness. Consequently, we rely on models and reanalysis that are un-validated against observations to study the effect of atmospheric forcing on polynya dynamics. We use austral summer 2022 shipboard data to understand the turbulent heat flux dynamics in the Amundsen Sea Polynya and evaluate our ability to represent these dynamics in ERA5. We show that cold- and dry-air outbreaks from Antarctica enhance air–sea temperature and humidity gradients, triggering episodic heat loss events. The ocean heat loss is larger along the ice-shelf front, and it is also where the ERA5 turbulent heat flux exhibits the largest biases, underestimating the flux by up to 141 W m−2 due to its coarse resolution. By reconstructing a turbulent heat flux product from ERA5 variables using a nearest-neighbor approach to obtain sea surface temperature, we decrease the bias to 107 W m−2. Using a 1D model, we show that the mean co-located ERA5 heat loss underestimation of 28 W m−2 led to an overestimation of the summer evolution of sea surface temperature (heat content) by +0.76 °C (+8.2×107 J) over 35 d. By obtaining the reconstructed flux, the reduced heat loss bias (12 W m−2) reduced the seasonal bias in sea surface temperature (heat content) to −0.17 °C (−3.30 × 107 J) over the 35 d. This study shows that caution should be applied when retrieving ERA5 turbulent flux along the ice shelves and that a reconstructed flux using ERA5 variables shows better accuracy.

Benefits of a second tandem flight phase between two successive satellite altimetry missions for assessing instrumental stability - Michaël Ablain; Noémie Lalau; Benoit Meyssignac; Robin Fraudeau; Anne Barnoud; Gérald Dibarboure; Alejandro Egido; Craig Donlon (Ocean Science, 2025)
Five successive reference missions, TOPEX/Poseidon, Jason-1, Jason-2, Jason-3, and more recently Sentinel-6 Michael Freilich, have ensured the continuity and stability of the satellite altimetry data record. Tandem flight phases have played a key role in verifying and ensuring the consistency of sea level measurements between successive altimetry reference missions and thus the stability of sea level measurements. During a tandem flight phase, two successive reference missions follow each other on an identical ground track at intervals of less than 1 min. Observing the same ocean zone simultaneously, the differences in sea level measurements between the two altimetry missions mainly reflect their relative errors. Relative errors are due to instrumental differences related to altimeter characteristics (e.g., altimeter noise) and processing of altimeter measurements (e.g., retracking algorithm), precise orbit determination, and mean sea surface. Accurate determination of systematic instrumental differences is achievable by averaging these relative errors over periods that exceed 100 d. This enables for the precise calibration of the two altimeters. The global mean sea level offset between successive altimetry missions can be accurately estimated with an uncertainty of about ±0.5 mm ([16 %–84 %] confidence level). Nevertheless, it is only feasible to detect instrumental drifts in the global mean sea level exceeding 1.0 to 1.5 mm yr−1 due to the brief duration of the tandem phase (9 to 12 months). This study aims to propose a novel cross-validation method with a better ability to assess the instrumental stability (i.e., instrumental drifts in the global mean sea level trends). It is based on the implementation of a second tandem flight phase between two successive satellites a few years after the first one. Calculating sea level differences during the second tandem phase provides an accurate evaluation of relative errors between the two successive altimetry missions. With a second tandem phase that is long enough, the systematic instrumental differences in sea level will be accurately reevaluated. The idea is to calculate the trend between the systematic instrumental differences made during the two tandem phases. The uncertainty in the trend is influenced by the length of each tandem phase and the time intervals between the two tandem phases. Our findings show that assessing the instrumental stability with two tandem phases can achieve an uncertainty below ±0.1 mm yr−1 ([16 %–84 %] confidence level) at the global scale for time intervals between the two tandem phases that are higher than 4 years or more and where each tandem phase lasts at least 4 months. On regional scales, the gain is greater, with an uncertainty of ±0.5 mm yr−1 ([16 %–84 %] confidence level) for spatial scales of about 1000 km or more. With regard to the scenario foreseen for the second phase between Jason-3 and Sentinel-6 Michael Freilich planned for early 2025, 2 years and 9 months after the end of the first tandem phase, the instrumental stability could be assessed with an uncertainty of ±0.14 mm yr−1 on the global scale and ±0.65 mm yr−1 for spatial scales of about 1000 km ([16 %–84 %] confidence level). In order to achieve a larger benefit from the use of this novel cross-validation method, this involves regularly implementing double tandem phases between two successive altimetry missions in the future.

Internal tides off the Amazon shelf in the western tropical Atlantic: analysis of SWOT Cal/Val mission data - Michel Tchilibou; Loren Carrere; Florent Lyard; Clément Ubelmann; Gérald Dibarboure; Edward D. Zaron; Brian K. Arbic (Ocean Science, 2025)
This study focuses on the internal tides (ITs) off the Amazon shelf in the tropical Atlantic. It is based on 2 km horizontally gridded observations along the swaths of SWOT (Surface Water and Ocean Topography) track 20 during the calibration/validation phase (Cal/Val, 1 d orbit) from late March to early July 2023. We evaluate the amplitude of M2, N2, and S2 frequencies and use the M2 atlas as an internal tide correction model for SWOT observations. Internal tide amplitudes (models or atlases) are first derived by harmonic analysis of the SWOT sea level anomaly (SLA). The estimation is improved by performing a principal component analysis before the harmonic analysis. The results compare very well with the high-resolution empirical tide (HRET) internal tide model, the reference product for internal tide corrections in altimetry observations. The coherent mode 1 and mode 2 M2 can be distinguished in the internal tide model derived from SWOT, while the higher modes with their strong SLA signature are seen mostly in the incoherent part. In comparison to HRET, the correction of SWOT observations with SWOT-based atlases may be more relevant for this track.

Blending 2D topography images from the Surface Water and Ocean Topography (SWOT) mission into the altimeter constellation with the Level-3 multi-mission Data Unification and Altimeter Combination System (DUACS) - Gerald Dibarboure; Cécile Anadon; Frédéric Briol; Emeline Cadier; Robin Chevrier; Antoine Delepoulle; Yannice Faugère; Alice Laloue; Rosemary Morrow; Nicolas Picot; Pierre Prandi; Marie-Isabelle Pujol; et al. (Ocean Science, 2025)
The Surface Water and Ocean Topography (SWOT) mission delivers unprecedented swath-altimetry products. Despite SWOT's 2D coverage and precision, its Level-2 ocean products suffer from the same limitations as their counterparts from nadir altimetry missions. To achieve the mission's primary science objectives, the space agencies generate Level-2 ocean products with SWOT alone. In contrast, some research domains and applications require consistent multi-mission observations, such as the Level-3 ocean products provided by the Data Unification and Altimeter Combination System (DUACS) for almost 3 decades and with 20 different satellites. In this paper, we describe how we extended the Level-3 algorithms to handle SWOT's unique swath-altimeter data. We also illustrate and discuss the benefits, relevance, and limitations of Level-3 swath-altimeter products for various research domains.

Dynamics of salt intrusion in complex estuarine networks: an idealised model applied to the Rhine–Meuse Delta - Bouke Biemond; Wouter M. Kranenburg; Ymkje Huismans; Huib E. de Swart; Henk A. Dijkstra (Ocean Science, 2025)
Many deltas in the world consist of a network of connected channels. We identify and quantify the characteristics of salt intrusion in such systems using an idealised model. The Rhine–Meuse Delta is selected as a prototype example of a complex network with many channels. The model is able to capture the characteristics of the tide-dominated water level variations due to the main tidal component and the salinity time series for 1 year of observations. Quantification of tidally averaged salt transport components shows that transport related to exchange flow is dominant in the seaward, deep parts of the network, but tidal dispersion is dominant in shallower channels further inland. Close to the network junctions, a tidally averaged downgradient salt transport is generated by the tidal currents, which is explained by the phase differences between the tidal currents in the different channels. Salt overspill is confined to the most seaward part of the Rhine–Meuse Delta. The magnitudes of the response times of different channels to changes in discharge increase with the distance to the estuary mouth and with decreasing net water transport through the channel. In channels without a subtidal discharge, response times are a factor of 2–4 larger than in the other channels. The effect of changes in the depth on the extent of salt intrusion strongly depends on where the change takes place. If the change is within the salt intrusion range, deepening will locally increase salt intrusion due to an increase in salt transport by the exchange flow. If the change is outside the salt intrusion range, changes to the net water transport dominate the response of the salt intrusion.

Circulation of Baffin Bay and Hudson Bay waters on the Labrador shelf and into the subpolar North Atlantic - Elodie Duyck; Nicholas P. Foukal; Eleanor Frajka-Williams (Ocean Science, 2025)
In the coming decades increasing amounts of freshwater are predicted to enter the subpolar North Atlantic from Greenland and the Arctic. If this additional freshwater reaches the regions where deep convection occurs, it could potentially dampen ventilation and the formation of deep waters. In this study, we use a surface drifter dataset spanning the period 1990–2023 to investigate the pathways followed by waters originating from Davis Strait and Hudson Strait on the Labrador shelf and into the interior subpolar North Atlantic. Recent drifter deployments in the region allow for an improved understanding of the circulation on the Labrador shelf, in particular its northern part, where prior data were sparse. We show that waters originating from Davis Strait and Hudson Strait remain on the shelf as they flow downstream until they reach the Newfoundland shelf. This confirms that very little exchange takes place between the Labrador shelf and the interior Labrador Sea. Decomposing the Labrador shelf into five regions, we further describe typical pathways for these waters and show that extensive exchanges take place between the coastal and shelf-break branches of the Labrador Current. Our results suggest that if an increasing amount of freshwater reaches the Labrador shelf, it would not directly affect the Labrador Sea convection region; instead, it would lead to the formation of a salinity anomaly off the Grand Banks, which could then circulate around the subpolar North Atlantic.

Non-negligible impact of Stokes drift and wave-driven Eulerian currents on simulated surface particle dispersal in the Mediterranean Sea - Siren Rühs; Ton van den Bremer; Emanuela Clementi; Michael C. Denes; Aimie Moulin; Erik van Sebille (Ocean Science, 2025)
Numerical simulations of marine surface particle dispersal are a crucial tool for addressing many outstanding issues in physical oceanography of societal relevance, such as marine plastic pollution. However, the quality of these Lagrangian simulations depends on the ability of the underlying numerical model to represent prevailing ocean circulation features. Here, we investigate how simulated marine surface particle dispersal changes if the – often omitted or only approximated – impact of wind-generated surface waves on upper-ocean circulation is considered. We use velocity fields from a high-resolution coupled ocean–wave model simulation and a complementary stand-alone ocean model simulation for the Mediterranean Sea to answer the following questions: (1) how does the explicit representation of waves impact simulated surface particle dispersal, and what is the relative impact of Stokes drift and wave-driven Eulerian currents? (2) How accurately can the wave impact be approximated by the commonly applied approach of advecting particles with non-wave-driven Eulerian currents and Stokes drift from stand-alone ocean and wave models? We find that the representation of surface waves tends to increase the simulated mean Lagrangian surface drift speed in winter through the dominant impact of Stokes drift and tends to decrease the mean Lagrangian surface drift speed in summer through the dominant impact of wave-driven Eulerian currents. Furthermore, simulations that approximate the surface wave impact by including Stokes drift (but ignoring wave-driven Eulerian currents) do not necessarily yield better estimates of surface particle dispersal patterns with explicit wave impact representation than simulations that do not include any surface wave impact. Our results imply that – whenever possible – velocity fields from a coupled ocean–wave model should be used for surface particle dispersal simulations.

Estimating ocean currents from the joint reconstruction of absolute dynamic topography and sea surface temperature through deep learning algorithms - Daniele Ciani; Claudia Fanelli; Bruno Buongiorno Nardelli (Ocean Science, 2025)
Our study focuses on absolute dynamic topography (ADT) and sea surface temperature (SST) mapping from satellite observations, with the primary objective of improving the satellite-derived ADT (and derived geostrophic currents) spatial resolution. Retrieving consistent high-resolution ADT and SST information from space is challenging, due to instrument limitations, sampling constraints, and degradations introduced by the interpolation algorithms used to obtain gap-free (L4) analyses. To address these issues, we developed and tested different deep learning methodologies, specifically convolutional neural network (CNN) models that were originally proposed for single-image super resolution. Building upon recent findings, we conduct an Observing System Simulation Experiment (OSSE) relying on Copernicus numerical model outputs (with respective temporal and spatial resolutions of 1 d and 1/24°), and we present a strategy for further refinements. Previous OSSEs combined low-resolution L4 satellite equivalent ADTs with high-resolution “perfectly known” SSTs to derive high-resolution sea surface dynamical features. Here, we introduce realistic SST L4 processing errors and modify the network to concurrently predict high-resolution SST and ADT from synthetic, satellite equivalent L4 products. This modification allows us to evaluate the potential enhancement in the ADT and SST mapping while integrating dynamical constraints through tailored, physics-informed loss functions. The neural networks are thus trained using OSSE data and subsequently applied to the Copernicus Marine Service satellite-derived ADTs and SSTs, allowing us to reconstruct super-resolved ADTs and geostrophic currents at the same spatiotemporal resolution of the model outputs employed for the OSSE. A 12-year-long time series of super-resolved geostrophic currents (2008–2019) is thus presented and validated against in situ-measured currents from drogued drifting buoys and via spectral analyses. This study suggests that CNNs are beneficial for improving standard altimetry mapping: they generally sharpen the ADT gradients, with consequent correction of the surface currents direction and intensities with respect to the altimeter-derived products. Our investigation is focused on the Mediterranean Sea, quite a challenging region due to its small Rossby deformation radius (around 10 km).

The alongshore tilt of mean dynamic topography and its implications for model validation and ocean monitoring - Christoph Renkl; Eric C. J. Oliver; Keith R. Thompson (Ocean Science, 2025)
Mean dynamic topography (MDT) plays an important role in the dynamics of shelf circulation. Coastal tide gauge observations in combination with the latest generation of geoid models are providing estimates of the alongshore tilt of MDT with unprecedented accuracy. Additionally, high-resolution ocean models are providing better representations of nearshore circulation and the associated tilt of MDT along their coastal boundaries. It has been shown that the newly available geodetic estimates can be used to validate model predictions of coastal MDT variability on global and basin scales. On smaller scales, however, there are significant variations in alongshore MDT that are of the same order of magnitude as the accuracy of the geoid models. In this study, we use a regional ocean model of the Gulf of Maine and Scotian Shelf (GoMSS) to demonstrate that the new observations of geodetically referenced coastal sea level can also provide valuable information for the validation of such high-resolution models. The predicted coastal MDT is in good agreement with coastal tide gauge observations referenced to the Canadian Gravimetric Geoid 2013 – Version A (CGG2013a), including a significant tilt of alongshore MDT along the coast of Nova Scotia. Using the validated GoMSS model and two idealized models, we show that this alongshore tilt of MDT can be interpreted in two complementary, and dynamically consistent, ways: in the coastal view, the tilt of MDT along the coast can provide a direct estimate of the average alongshore current. In the regional view, the tilt provides a measure of area-integrated nearshore circulation. This highlights the value of using geodetic MDT estimates for model validation and ocean monitoring.

Assessing the thermohaline coherence of mesoscale eddies as described from in situ data - Yan Barabinot; Sabrina Speich; Xavier Carton (Ocean Science, 2025)
Mesoscale eddies are a ubiquitous feature of the global ocean. According to Lagrangian theory, these eddies often transport a distinct water mass within their cores, making them materially coherent. This study aims to determine if such a distinct water mass exists in eddy cores, thereby verifying their material coherence using in situ data, despite the lack of temporal continuity. We introduce the term “thermohaline coherence” to describe this approach. Identifying such a water mass would signal Lagrangian transport from the eddy formation region. We analyzed the water masses at the cores of various eddies sampled during eight research cruises using high-resolution data (approximately 20 km horizontally and 10 m vertically). We revisited coherence definitions and checked data accuracy. Comparing the horizontal positions of these core anomalies with eddy surface signatures revealed that surface data alone are insufficient for characterizing the eddy material coherence. To calculate eddy volumes, we compare thermohaline anomalies with other criteria, and we present two methods for extrapolating eddy volumes from a single hydrographic section. The results show that the outermost closed contour of the Brunt–Väisälä frequency anomaly at each depth provides a reliable approximation for the eddy boundary.

Understanding uncertainties in the satellite altimeter measurement of coastal sea level: insights from a round-robin analysis - Florence Birol; François Bignalet-Cazalet; Mathilde Cancet; Jean-Alexis Daguze; Wassim Fkaier; Ergane Fouchet; Fabien Léger; Claire Maraldi; Fernando Niño; Marie-Isabelle Pujol; Ngan Tran (Ocean Science, 2025)
The satellite radar altimetry record of sea level has now surpassed 30 years in length. These observations have greatly improved our knowledge of the open ocean and are now an essential component of many operational marine systems and climate studies. But the use of altimetry close to the coast remains a challenge from both a technical and scientific point of view. Here, we take advantage of the recent availability of many new algorithms developed for altimetry sea level computation to quantify and analyze the uncertainties associated with the choice of algorithms when approaching the coast. To achieve this objective, we did a round-robin analysis of radar altimetry data, testing a total of 21 solutions for waveform retracking, correcting sea surface heights and finally deriving sea level variations. Uncertainties associated with each of the components used to calculate the altimeter sea surface heights are estimated by measuring the spread of sea level values obtained using the various algorithms considered in the round-robin for this component. We intercompare these uncertainty estimates and analyze how they evolve when we go from the open ocean to the coast. At regional scale, complementary analyses are performed through comparisons with independent tide gauge observations. The results show that tidal corrections and uncertainties in the mean sea surface can be significant contributors to uncertainties in sea level estimates in many coastal regions. However, improving the quality and robustness of the retracking algorithm used to derive both the range and the sea state bias correction is today the main factor to bring accurate altimetry sea level data closer to the shore than ever before.

Convolutional neural networks for sea surface data assimilation in operational ocean models: test case in the Gulf of Mexico - Olmo Zavala-Romero; Alexandra Bozec; Eric P. Chassignet; Jose R. Miranda (Ocean Science, 2025)
Deep learning models have demonstrated remarkable success in fields such as language processing and computer vision, routinely employed for tasks like language translation, image classification, and anomaly detection. Recent advancements in ocean sciences, particularly in data assimilation (DA), suggest that machine learning can emulate dynamical models, replace traditional DA steps to expedite processes, or serve as hybrid surrogate models to enhance forecasts. However, these studies often rely on ocean models of intermediate complexity, which involve significant simplifications that present challenges when transitioning to full-scale operational ocean models. This work explores the application of convolutional neural networks (CNNs) in data assimilation within the context of the HYbrid Coordinate Ocean Model (HYCOM) in the Gulf of Mexico. The CNNs are trained to correct model errors from a 2-year, high-resolution (1/25°) HYCOM dataset, assimilated using the Tendral Statistical Interpolation System (T-SIS). The CNNs are trained to replicate the increments generated by the T-SIS data assimilation package, aiming to correct model forecasts of sea surface temperature (SST) and sea surface height (SSH). The inputs to the CNNs include real satellite observations of SST from the Group for High Resolution Sea Surface Temperature (GHRSST), along-track altimeter SSH observations (ADT), the model background state (previous forecast), and the innovations (differences between observations and background). We assess the performance of the CNNs across five controlled experiments, designed to provide insights into their application in environments governed by full primitive equations, real observations, and complex topographies. The experiments focus on evaluating (1) the architecture and complexity of the CNNs, (2) the type and quantity of observations, (3) the type and number of assimilated fields, (4) the impact of training window size, and (5) the influence of coastal boundaries. Our findings reveal significant correlations between the chosen training window size – a factor not commonly examined – and the CNNs' ability to assimilate observations effectively. We also establish a clear link between the CNNs' architecture and complexity and their overall performance. This research uses artificial intelligence to enhance ocean forecasting in the Gulf of Mexico. By using convolutional neural networks, the study improves predictions of sea temperatures and heights by integrating real satellite data with existing models. Through five comprehensive experiments, the team found that the amount of training data and the design of the neural networks significantly affect accuracy. These insights pave the way for faster, more reliable ocean models, benefiting environmental monitoring and maritime operations.

Long-term variability and trends in the Agulhas Leakage and its impacts on the global overturning - Hendrik Großelindemann; Frederic S. Castruccio; Gokhan Danabasoglu; Arne Biastoch (Ocean Science, 2025)
Agulhas Leakage transports relatively warm and salty Indian Ocean waters into the Atlantic Ocean and as such is an important component of the global ocean circulation. These waters are part of the upper limb of the Atlantic meridional overturning circulation (AMOC), and Agulhas Leakage variability has been linked to AMOC variability. Agulhas Leakage is expected to increase under a warming climate due to a southward shift in the Southern Hemisphere westerlies, which could further influence the AMOC dynamics. This study uses a set of high-resolution preindustrial control, historical and transient simulations with the Community Earth System Model (CESM) with a nominal horizontal resolution of 0.1° for the ocean and sea ice and 0.25° for the atmosphere and land. At these resolutions, the model represents the necessary scales to investigate Agulhas Leakage transport variability and its relation to the AMOC. The simulated Agulhas Leakage transport of 19.7 ± 3 Sv lies well within the observed range of 21.3 ± 4.7 Sv. A positive correlation between the Agulhas Current and the Agulhas Leakage is shown, meaning that an increase of the Agulhas Current transport leads to an increase in Agulhas Leakage. The Agulhas Leakage impacts the strength of the AMOC through Rossby wave dynamics that alter the cross-basin geostrophic balance with a time lag of 2–3 years. Furthermore, the salt transport associated with the Agulhas Leakage influences AMOC dynamics through the salt–advection feedback by reducing the AMOC's freshwater transport at 34° S. The Agulhas Leakage transport indeed increases under a warming climate due to strengthened and southward-shifting winds. In contrast, the Agulhas Current transport decreases due to a decrease in the Indonesian Throughflow and the strength of the wind-driven subtropical gyre. The increase in the Agulhas Leakage is accompanied by a higher salt transport into the Atlantic Ocean, which could play a role in the stability of the AMOC via the salt–advection feedback.

Enhanced bed shear stress and mixing in the tidal wake of an offshore wind turbine monopile - Martin J. Austin; Christopher A. Unsworth; Katrien J. J. Van Landeghem; Ben J. Lincoln (Ocean Science, 2025)
Tidal flow past offshore wind farm (OWF) infrastructure generates a turbulent vortex wake. The wake is hypothesised to enhance seabed stress and water column turbulence mixing, thereby affecting seabed mobility, water column stratification and the transport of nutrients and oxygen and resulting in ecological impacts. We collect novel hydrodynamic data 40 m from an OWF monopile over a spring–neap cycle and use high-frequency velocity measurements to quantify turbulence. Outside of the wake, we observe a classical depth-limited boundary layer, with strong turbulence production and dissipation forced by tidal shear at the seabed. Inside the wake, turbulence production, dissipation and stress are enhanced throughout the full water column and are maximised in the upper half of the water column, where they correspond to a strong mean velocity deficit. Our results show that the seabed drag coefficient is doubled from Cd=3.5×10-3 to 7.8×10-3, suggesting greater seabed mobility, and the eddy viscosity is increased by 1 order of magnitude, indicating enhanced water column mixing. This research provides some valuable insight as OWFs expand into deeper seasonally stratified waters using both bottom-fixed and floating structures, where the addition of enhanced wake turbulence may have broad impacts as the additional mixing energy is added to regions with low rates of background mixing.

Integrating wide-swath altimetry data into Level-4 multi-mission maps - Maxime Ballarotta; Clément Ubelmann; Valentin Bellemin-Laponnaz; Florian Le Guillou; Guillaume Meda; Cécile Anadon; Alice Laloue; Antoine Delepoulle; Yannice Faugère; Marie-Isabelle Pujol; Ronan Fablet; Gérald Dibarboure (Ocean Science, 2025)
Real-time observation of ocean surface topography is essential for various oceanographic applications. Historically, these observations have mainly relied on satellite nadir altimetry data, which were limited to observation scales greater than approximately 60 km. However, the recent launch of the wide-swath Surface Water Ocean Topography (SWOT) mission in December 2022 marks a significant advancement, enabling the two-dimensional global observation of finer-scale oceanic scales (∼ 15 km). While the direct analysis of the two-dimensional content of these swaths can provide valuable insights into ocean surface dynamics, integrating such data into mapping systems presents several challenges. This study focuses on integrating the SWOT mission into multi-mission mapping systems. Specifically, it examines the contribution of the SWOT mission to both the current nadir altimetry constellation (six/seven nadirs) and a reduced nadir altimetry constellation (three nadirs). Our study indicates that within the current nadir altimetry constellation, SWOT's impact is moderate, as existing nadir altimeters effectively constrain surface dynamics. However, in a hypothetical scenario where a reduced nadir altimetry constellation is envisioned to be operational by 2030, the significance of wide-swath data in mapping becomes more pronounced. Alternatively, we found that data-driven and dynamical mapping systems can significantly participate in refining the resolution of the multi-mission gridded products. Consequently, integrating high-resolution ocean surface topography observations with advanced mapping techniques can enhance the resolution of satellite-derived products, providing promising solutions for studying and monitoring sea-level variability at finer scales. However, to fully exploit SWOT's capabilities, future research will need to focus on innovations in data gridding and assimilation to extend mapping beyond geostrophically balanced flows.

A new vision of the Adriatic Dense Water future under extreme warming - Cléa Denamiel; Iva Tojčić; Petra Pranić (Ocean Science, 2025)
We use the Adriatic Sea and Coast (AdriSC) kilometer-scale atmosphere–ocean model to assess the impact of a far-future extreme-warming scenario on the formation, spreading, and accumulation of North Adriatic Dense Water (NAddW) over the entire basin, including the Jabuka Pit accumulation site, and Adriatic Deep Water (AdDW) over the Southern Adriatic Pit (SAP). Our key findings differ from previous studies that used coarser Mediterranean climate models and did not update the thresholds for dense-water and deep-water definitions to account for the far-future background density changes caused by warmer sea surface temperatures. We show that surface buoyancy losses at NAddW generation sites, driven by evaporation, are expected to increase by 15 % under extreme warming, despite a 25 % reduction in the intensity and spatial extent of Bora winds. As a result, future NAddW formation will remain similar to present conditions. However, the volume of dense water in the Jabuka Pit will decrease due to the increased far-future stratification. Additionally, dense-water transport between the Jabuka Pit and the deepest part of the SAP will stop, as future NAddW will be lighter than the AdDW. Regarding Ionian–Adriatic exchanges, extreme warming will not affect the impact of the bimodal oscillation system on the Adriatic salinity variability, but future AdDW dynamics will be determined by density changes in the northern Ionian Sea. Our findings highlight the complexity of climate change impacts on Adriatic atmosphere–ocean processes and the importance of high-resolution models for more accurate far-future projections in the Adriatic Sea.

Disentangling spring–neap suspended particulate matter (SPM) dynamics in estuaries - Yoeri M. Dijkstra; Dennis D. Bouwman; Henk M. Schuttelaars (Ocean Science, 2025)
Suspended particulate matter (SPM) concentrations in estuaries have been observed to vary strongly over the spring–neap cycle through complex interactions between trapping and re-suspension. However, a systematic framework for analysing the processes causing this spring–neap SPM variability in general is missing. In this study we set up such a framework, consisting of three tiers. First, by studying the sediment transport capacity, how the locations of sediment trapping change over the spring–neap cycle is identified. Second, how the transport capacity affects the sediment stock and bottom pool of sediment is studied. This bottom pool only adapts gradually to the changing transport conditions, incorporating a lag or memory effect. Using a two-timescale analysis it is shown that this slow movement of the bottom pool is the leading source of such lag effects. Third, the SPM concentration is explained from an almost instantaneously balanced exchange between the bottom pool and the water column through re-suspension and deposition. We demonstrate the use of this framework on two model cases implemented in the idealised width-averaged iFlow model: an idealised test case where the sediment dynamics do not affect the water motion and a case representative of the Loire estuary, with strong feedback between sediment and the water motion through sediment-induced damping of turbulence. The first is illustrative as it allows a full understanding in terms of cause and effect between water motion, transport, and SPM concentration. In the more realistic Loire case, the SPM dynamics cannot be explained in terms of cause and effect but can explain the trapping locations and timing of maximum concentrations in a systematic way in terms of the governing physical mechanisms.

Surface circulation characterization along the middle southern coastal region of Vietnam from high-frequency radar and numerical modeling - Thanh Huyen Tran; Alexei Sentchev; Thai To Duy; Marine Herrmann; Sylvain Ouillon; Kim Cuong Nguyen (Ocean Science, 2025)
Coastal water dynamics along the Vietnamese middle southern coast (VMSC) region, part of the South China Sea, are highly complex with large spatiotemporal variability whose drivers are not yet well understood. For the first time, high-resolution surface current data from high-frequency radar (HFR) measurements were obtained in this region during the early (transition) phase of the Asian summer monsoon. The data were used for comparison with simulation results from a circulation model, SYMPHONIE, and ultimately to optimize the wind forcing in the model. Both modeling and HFR were able to show the spatial and temporal evolution of the surface circulation, but some discrepancies were found between model and HFR data on some days, coinciding with the evolution of the wind. Two methods were used to optimize the wind forcing, namely the ensemble perturbation smoother (EnPS) and the wind correction method using wind-driven surface currents (EkW). Both methods achieved a significant reduction (∼ 36 %–40 %) in the error of the surface current velocity fields compared to the measured data. Optimized winds obtained from the two methods were compared with satellite wind data for validation. The results show that both optimization methods performed better in the far field, where topography no longer affects the coastal surface circulation. The optimization results revealed that the surface circulation is driven not only by winds but also by other factors such as intrinsic ocean variability, which is not entirely controlled by boundary conditions. This indicates the potential usefulness of large velocity datasets and other data fusion methods to effectively improve modeling results.

Monitoring the coastal–offshore water interactions in the Levantine Sea using ocean color and deep supervised learning - Georges Baaklini; Julien Brajard; Leila Issa; Gina Fifani; Laurent Mortier; Roy El Hourany (Ocean Science, 2024)
Understanding and tracking the surface circulation of the Levantine Sea present significant challenges, particularly close to the coast. This difficulty arises due to two main factors: the limited availability of in situ observations and the increasing inaccuracies in altimetry data close to the coastline. Here, we propose a new approach to monitor the interaction between offshore and coastal waters. In this approach, we develop a pattern detection model using deep learning by training the U-Net model on ocean color data to track the interactions between the coastal and offshore water in the Levantine Sea. The results showed the presence of notable variations in the behavior of coastal currents as they progress northward beyond 33.8° E. As these coastal currents become increasingly unstable, they exhibit continuous pinching-off events that are missed by conventional observational tools. These pinching-off events, especially observed along the Lebanese coast, manifest in various patterns that evolve simultaneously. Typically, these patterns have a relatively short lifespan of a few weeks, appearing and disappearing rapidly. However, these structures can evolve into larger eddies that endure for over 4 months in some years, especially in the northern part of the Lebanese coasts. Although these structures could be observed during all the seasons, spring consistently records the lowest activity of these structures. Overall, we showed that the pinching-off events were always observed in the eastern part of the Levantine Sea. On the contrary, in the southern part, along the Egyptian coasts, the coastal flow is more stable in the southern region, where these events are less frequently observed, with more than 63 % of the total observations not exhibiting any pinching-off events. Moreover, when these events occur in the south, their spatial extent is notably limited. This research not only sheds light on previously missed (or underestimated) coastal current dynamics in the Levantine Sea but also highlights the crucial need to increase in situ observations to advance our understanding of this region's complex oceanographic processes.

Contrasting two major Arctic coastal polynyas: the role of sea ice in driving diel vertical migrations of zooplankton in the Laptev and Beaufort seas - Igor A. Dmitrenko; Vladislav Petrusevich; Andreas Preußer; Ksenia Kosobokova; Caroline Bouchard; Maxime Geoffroy; Alexander S. Komarov; David G. Babb; Sergei A. Kirillov; David G. Barber (Ocean Science, 2024)
The diel vertical migration (DVM) of zooplankton is one of the largest species migrations to occur globally and is a key driver of regional ecosystems and the marine carbon pump. Previously thought to be hampered by the extreme light regime prevailing in the Arctic Ocean, observations have revealed that DVM does occur in ice-covered Arctic waters and suggest the decline in Arctic sea ice may thereby impact DVM and its role in the Arctic ecosystem. However, coastal polynyas present a unique environment where open water or thin, nearly translucent, ice prevail when offshore winds advect the ice pack away from the coast, allowing light into the surface waters and potentially disrupting DVM. Here, four yearlong time series of acoustic backscatter collected by moored acoustic Doppler current profilers at two opposite sides of the circumpolar polynya system at the Laptev Sea shelf (2007–2008) and the Beaufort Sea shelf (2005–2006) were used to examine the annual cycle of acoustic scattering and therefore the annual cycle of DVM in these areas. The acoustic time series were used along with atmospheric and oceanic reanalysis and satellite data to interpret the results. Our observations show that DVM started to occur once the ice-free surface or under-ice layer irradiance exceeds a certain threshold (from ∼0.3 to 3.3 lx), which is about 2 to 10 times lower in the Beaufort Sea compared to the Laptev Sea. In the Laptev Sea, based on our data and methodology, DVM could not be detected during polar night. In contrast, civil twilight in the Beaufort Sea is sufficient to trigger DVM through polar night. This difference in DVM between the Laptev and Beaufort seas is not entirely assigned to the 3° difference in latitude between the mooring positions as it is also due to the different light threshold required to trigger DVM, different zooplankton communities' composition, and potentially different depths and predation pressures. We find examples in both the Laptev and Beaufort seas where the formation of polynyas and large leads caused DVM to abruptly cease or be disrupted, which we attribute to predator avoidance by the zooplankton in response to higher polar cod (Boreogadus saida) abundance near the open water. Finally, light attenuation by sea ice in the Beaufort Sea caused DVM to extend onto the polar day until the summer solstice. Overall, our results highlight the role of sea ice in disrupting synchronized DVM, the spatial variability in the relationship between sea ice and DVM, and the potential ecological impact of significant trends toward a more extensive circumpolar Arctic coastal polynya as part of changing ice conditions in the Arctic Ocean.

Assessing the impact of future altimeter constellations in the Met Office global ocean forecasting system - Robert R. King; Matthew J. Martin; Lucile Gaultier; Jennifer Waters; Clément Ubelmann; Craig Donlon (Ocean Science, 2024)
Satellite altimeter measurements of sea surface height (SSH) are a crucial component of current operational ocean forecasting systems. The launch of the Surface Water and Ocean Topography (SWOT) wide-swath altimeter (WiSA) mission is bringing a step change in our observing capacity with 2D mesoscale structures now able to be observed over the global ocean. Proposals are now being considered for the make-up of the future altimeter constellation. In this study we use Observing System Simulation Experiments (OSSEs) to compare the impact of additional altimeter observations from two proposed future satellite constellations. We focus on the expected impact on the Met Office operational ocean analysis and forecasting system of assimilating an observation network including either 12 nadir altimeters or 2 wide-swath altimeters. Here we show that an altimeter constellation of 12 nadir altimeters produces greater reductions in the errors for SSH, surface currents, temperature, and salinity fields compared to a constellation of 2 wide-swath altimeters. The impact is greatest in the dynamic western boundary current (WBC) regions where the nadir altimeters can reduce the SSH RMS (root-mean-square) error by half, while the wide-swath altimeter only reduces this by one-quarter. A comparison of the spatial scales resolved in daily SSH fields also highlights the superiority of the nadir constellation in our forecasting system. We also highlight the detrimental impact spatially correlated errors could have on the immediate use of wide-swath altimeter observations. However, we still achieve promising impacts from the assimilation of wide-swath altimetry, and work is ongoing to develop improved methods to account for spatially correlated observation errors within our data assimilation scheme.

Dynamically downscaled seasonal ocean forecasts for North American east coast ecosystems - Andrew C. Ross; Charles A. Stock; Vimal Koul; Thomas L. Delworth; Feiyu Lu; Andrew Wittenberg; Michael A. Alexander (Ocean Science, 2024)
Using a 1/12° regional model of the Northwest Atlantic Ocean (MOM6-NWA12), we downscale an ensemble of retrospective seasonal forecasts from a 1° global forecast model. To evaluate whether downscaling improved the forecast skill for surface temperature and salinity and bottom temperature, the global and downscaled forecasts are compared with each other and with a reference forecast of persistence using anomaly correlation. Both sets of forecasts are also evaluated on the basis of mean bias and ensemble spread. We find that downscaling significantly improved the forecast skill for monthly sea surface temperature anomalies in the Northeast US Large Marine Ecosystem, a region that global models have historically struggled to skillfully predict. The downscaled sea surface temperature (SST) predictions for this region were also more skillful than the persistence baseline across most initialization months and lead times. Although some of the SST prediction skill in this region stems from the recent rapid warming trend, prediction skill above persistence is generally maintained after removing the contribution of the trend, and patterns of skill suggestive of predictable processes are also preserved. While downscaling mainly improved the SST anomaly prediction skill in the Northeast US region, it improved bottom temperature and sea surface salinity anomaly skill across many of the marine ecosystems along the North American east coast. Although improvements in anomaly prediction via downscaling were ubiquitous, the effects of downscaling on prediction bias were mixed. Downscaling generally reduced the mean surface salinity biases found in the global model, particularly in regions with sharp salinity gradients (the Northern Gulf of Mexico and the Northeast US). In some cases, however, downscaling amplified the surface and bottom temperature biases found in the global predictions. We discuss several processes that are better resolved in the regional model and contribute to the improved skill, including the autumn reemergence of temperature anomalies and advection of water masses by coastal currents. Overall, the results show that a downscaled high-resolution model can produce improved seasonal forecast skill by representing fine-scale processes that drive predictability.

Transient Attracting Profiles in the Great Pacific Garbage Patch - Luca Kunz; Alexa Griesel; Carsten Eden; Rodrigo Duran; Bruno Sainte-Rose (Ocean Science, 2024)
A major challenge for cleanup operations in the Great Pacific Garbage Patch is the daily prediction of plastic concentrations that allows identifying hotspots of marine debris. Lagrangian simulations of large particle ensembles are the method in use and effectively reproduce observed particle distributions at synoptic scales 𝒪(1000 km). However, they lose accuracy at operational scales 𝒪(1–10 km), and operators regularly experience differences between predicted and encountered debris accumulations within the garbage patch. In Lagrangian methods, it would be common to ask this question: where do objects go as they follow the current? Here, we take a different approach and instead ask this question: which locations attract material? The recently developed concept of Transient Attracting Profiles (TRAPs) provides answers because TRAPs uncover the most attracting regions of the flow. TRAPs are the attracting form of hyperbolic Objective Eulerian Coherent Structures and are computable from the instantaneous strain field on the ocean surface. They describe flow features which attract drifting objects and could facilitate offshore cleanups that are currently taking place in the Great Pacific Garbage Patch. However, the concept remains unapplied since little is known about the persistence and attraction of these features, specifically within the Pacific. Therefore, we compute a 20-year dataset of daily TRAP detections from satellite-derived mesoscale velocities within the North Pacific subtropical gyre. We are the first to track these instantaneous flow features as they propagate through space and time. It allows us to study the life cycle of TRAPs, which can range from days to seasons and lasts an average of 6 d. We show how long-living TRAPs with lifetimes beyond 30 d intensify and weaken over their life cycle, and we demonstrate that the evolution stage of TRAPs affects the motion of nearby surface drifters. Our findings indicate that, at the mesoscale, operators in the Great Pacific Garbage Patch should search for long-living TRAPs that are at an advanced stage of their life cycle. These TRAPs are the most likely to induce a large-scale confluence of drifting objects and their streamlining into hyperbolic pathways. Such a streamlined bypass takes, on average, 5 d and creates an opportunity to filter the flow around TRAPs. But we also find TRAPs that retain material over multiple weeks where we suspect material clustering at the submesoscale. Prospective research could investigate this further by applying our algorithms to soon-available high-resolution observations of the flow. Our analysis contributes to a better understanding of TRAPs, which can even benefit other offshore operations besides ocean cleanups, such as optimal drifter deployment, oil spill containment, and humanitarian search and rescue.

Hydrography and circulation below Fimbulisen Ice Shelf, East Antarctica, from 14 years of moored observations - Julius Lauber; Tore Hattermann; Laura de Steur; Elin Darelius; Agneta Fransson (Ocean Science, 2024)
Future mass loss from the East Antarctic Ice Sheet represents a major uncertainty in projections of future sea level rise. Recent studies have highlighted the potential vulnerability of the East Antarctic Ice Sheet to atmospheric and oceanic changes, but long-term observations inside the ice shelf cavities are rare. Here, we present new insights from observations from three oceanic moorings below Fimbulisen Ice Shelf from 2009 to 2023. We examine the characteristics of intrusions of modified Warm Deep Water (mWDW) across a sill connecting the cavity to the open ocean and investigate seasonal variability of the circulation and water masses inside the cavity using an optimum multiparameter analysis. In autumn, the water below the 345 m deep central part of the ice shelf is composed of up to 30 % solar-heated, buoyant Antarctic Surface Water (ASW), separating colder Ice Shelf Water from the ice base and affecting the cavity circulation on seasonal timescales. At depth, the occurrence of mWDW is associated with the advection of cyclonic eddies across the sill into the cavity. These eddies reach up to the ice base. The warm intrusions are observed most often from January to March and from September to November, and traces of mWDW-derived meltwater close to the ice base imply an overturning of these warm intrusions inside the cavity. We suggest that this timing is set by both the offshore thermocline depth and the interactions of the Antarctic Slope Current with the ice shelf topography over the continental slope. Our findings provide a better understanding of the interplay between shallow inflows of ASW contributions and deep inflows of mWDW for basal melting at Fimbulisen Ice Shelf, with implications for the potential vulnerability of the ice shelf to climate change.

Ensemble reconstruction of missing satellite data using a denoising diffusion model: application to chlorophyll a concentration in the Black Sea - Alexander Barth; Julien Brajard; Aida Alvera-Azcárate; Bayoumy Mohamed; Charles Troupin; Jean-Marie Beckers (Ocean Science, 2024)
Satellite observations provide a global or near-global coverage of the World Ocean. They are however affected by clouds (among others), which severely reduce their spatial coverage. Different methods have been proposed in the literature to reconstruct missing data in satellite observations. For many applications of satellite observations, it has been increasingly important to accurately reflect the underlying uncertainty of the reconstructed observations. In this paper, we investigate the use of a denoising diffusion model to reconstruct missing observations. Such methods can naturally provide an ensemble of reconstructions where each member is spatially coherent with the scales of variability and with the available data. Rather than providing a single reconstruction, an ensemble of possible reconstructions can be computed, and the ensemble spread reflects the underlying uncertainty. We show how this method can be trained from a collection of satellite data without requiring a prior interpolation of missing data and without resorting to data from a numerical model. The reconstruction method is tested with chlorophyll a concentration from the Ocean and Land Colour Instrument (OLCI) sensor (aboard the satellites Sentinel-3A and Sentinel-3B) on a small area of the Black Sea and compared with the neural network DINCAE (Data-INterpolating Convolutional Auto-Encoder). The spatial scales of the reconstructed data are assessed via a variogram, and the accuracy and statistical validity of the reconstructed ensemble are quantified using the continuous ranked probability score and its decomposition into reliability, resolution, and uncertainty.

Influence of river runoff and precipitation on the seasonal and interannual variability of sea surface salinity in the eastern North Tropical Atlantic - Clovis Thouvenin-Masson; Jacqueline Boutin; Vincent Échevin; Alban Lazar; Jean-Luc Vergely (Ocean Science, 2024)
In tropical regions, the freshwater flux entering the ocean originates primarily from precipitation and, to a lesser extent when considering basin-scale averages, from continental rivers. Nevertheless, at the regional scale, river flows can have a significant impact on the surface ocean dynamics. Riverine freshwater modifies salinity and, therefore, density, stratification, and circulation. With its particular coastline and high cumulative river discharge, as well as its being in the vicinity of the intertropical convergence zone (ITCZ), the eastern part of the North Tropical Atlantic (e-NTA) region off northwestern Africa is a particularly interesting location to study the linkage between precipitation, river outflow, and sea surface salinity (SSS). Here, we focus on the regional e-NTA SSS seasonal cycle and interannual variability and on the impact of using various river runoff and precipitation forcing data sets to simulate SSS with a regional model. The simulated SSS values are compared with the Climate Change Initiative (CCI) satellite SSS values; in situ SSS values from Argo floats, ships, and a coastal mooring; and the GLORYS reanalysis SSS values. An analysis of the mixed-layer salinity budget is then conducted. Overall, the simulations reproduce the seasonal cycle and interannual variability well despite a positive mean model bias north of 15° N. The seasonal cycle is impacted by the phasing of the different runoff products. The mixed-layer SSS decrease during the rainy season is mainly driven by precipitation followed by runoff by means of horizontal advection and is partly compensated for by vertical mixing. In terms of interannual anomalies, river runoff has a more direct impact on SSS than precipitation. This study highlights the importance of properly constraining river runoff and precipitation to simulate realistic SSS values and the importance of observing SSS in coastal regions to validate such constraints.

Multiple timescale variations in fronts in the Seto Inland Sea, Japan - Menghong Dong; Xinyu Guo (Ocean Science, 2024)
The Seto Inland Sea (SIS) is a critical semi-enclosed coastal sea in Japan, characterized by intricate coastlines and narrow straits that give rise to various fronts. Despite extensive research on tidal fronts, knowledge gaps persist regarding their spatiotemporal dynamics, particularly in certain poorly documented regions. Additionally, the understanding of thermohaline fronts, which emerge during winter, requires further investigation. We aimed to enhance our understanding of tidal and thermohaline fronts in the SIS by analyzing their dynamic processes, including intra-tidal and spring–neap tidal cycles, seasonal variations, and anomalous frontal variability. Using a gradient-based algorithm with an advanced contextual feature-preserving median filter, we processed the high-resolution sea surface temperature dataset to detect and quantify tidal and thermohaline fronts. Our analysis revealed the presence of numerous tidal fronts, predominantly influenced by the M2 tide, across the SIS, with substantial spatial variations in amplitude due to complex coastlines and narrow straits. Intra-tidal movements of tidal fronts corresponded to ebb and flood currents, while spring–neap tidal cycles and seasonal shifts influenced frontal positions and intensities. Additionally, thermohaline fronts were identified in certain regions during winter, characterized by large horizontal temperature and salinity gradients. This study enhances the understanding of tidal and thermohaline fronts in the SIS, emphasizing the importance of intra-tidal and wind-driven influences on frontal dynamics. However, limited observational coverage and resolution emphasize the need for further research to explore long-term temporal changes and better grasp the influences of ambient currents and wind patterns. Such insights are vital for effective coastal management and environmental monitoring in the SIS region.

Assessing storm surge model performance: what error indicators can measure the model's skill? - Rodrigo Campos-Caba; Jacopo Alessandri; Paula Camus; Andrea Mazzino; Francesco Ferrari; Ivan Federico; Michalis Vousdoukas; Massimo Tondello; Lorenzo Mentaschi (Ocean Science, 2024)
A well-validated storm surge numerical model is crucial, offering precise coastal hazard information and serving as a basis for extensive databases and advanced data-driven algorithms. However, selecting the best model setup based solely on common error indicators like the root-mean-square error (RMSE) or Pearson correlation does not always yield optimal results. To illustrate this, we conducted 34-year high-resolution simulations for storm surge under barotropic (BT) and baroclinic (BC) configurations using atmospheric data from ERA5 and a high-resolution downscaling of the Climate Forecast System Reanalysis (CFSR) developed by the University of Genoa (UniGe). We combined forcing and configurations to produce three datasets: (1) BT-ERA5, (2) BC-ERA5, and (3) BC-UniGe. The model performance was assessed against nearshore station data using various statistical metrics. While RMSE and Pearson correlation suggest BT-ERA5, i.e., the coarsest and simplest setup, is the best model (followed by BC-ERA5), we demonstrate that these indicators are not always reliable for performance assessment. The most sophisticated model (BC-UniGe) shows worse values of RMSE or Pearson correlation due to the so-called “double penalty” effect. Here we propose new skill indicators that assess the ability of the model to reproduce the distribution of the observations. This, combined with an analysis of values above the 99th percentile, identifies BC-UniGe as the best model, while ERA5 simulations tend to underestimate the extremes. Although the study focuses on the accurate representation of storm surge by the numerical model, the analysis and proposed metrics can be applied to any problem involving the comparison between time series of simulation and observation.

Tide–surge interaction observed at Singapore and the east coast of Peninsular Malaysia using a semi-empirical model - Zhi Yang Koh; Benjamin S. Grandey; Dhrubajyoti Samanta; Adam D. Switzer; Benjamin P. Horton; Justin Dauwels; Lock Yue Chew (Ocean Science, 2024)
Tide–surge interaction plays a substantial role in determining the characteristics of coastal water levels over shallow regions. We study the tide–surge interaction observed at seven tide gauges along Singapore and the east coast of Peninsular Malaysia, focusing on the timing of extreme non-tidal residuals relative to tidal high water. We propose a modified statistical framework using a no-tide–surge interaction (no-TSI) null distribution that accounts for asymmetry and variation in the duration of tidal cycles. We find that our modified framework can mitigate false-positive signals of tide–surge interaction in this region. We find evidence of tide–surge interaction at all seven locations, with characteristics varying smoothly along the coastline: the highest non-tidal residuals are found to occur most frequently before tidal high water in the south, both before and after tidal high water in the central region, and after tidal high water in the north. We also propose a semi-empirical model to investigate the effects of tidal-phase alteration, which is one mechanism of tide–surge interaction. Results of our semi-empirical model reveal that tidal-phase alteration caused by storm surges is substantial enough to generate significant change in the timing of extreme non-tidal residuals. To mitigate the effect of tidal-phase alteration on return level estimation, skew surge can be used. We conclude that (1) tide–surge interaction influences coastal water levels in this region, (2) our semi-empirical model provides insight into the mechanism of tidal-phase alteration, and (3) our no-TSI distribution should be used for similar studies globally.

Turbulent features of nearshore wave–current flow - Massimiliano Marino; Carla Faraci; Bjarne Jensen; Rosaria Ester Musumeci (Ocean Science, 2024)
Waves and currents influence nearly all nearshore physical processes. Their complex interaction gives birth to complex turbulence features that are far from being completely understood. In this regard, previous studies mainly focused on mean flow or inferred turbulent features from averaged velocities, seldom examining turbulent fluctuations. Moreover, the dynamics of wave–current flow have mostly been replicated in experimental channel setups, i.e., overlooking the natural occurrence of waves and longshore currents intersecting at a near-orthogonal angle. In the present work, the hydrodynamics of near-orthogonal wave–current interaction are investigated through a physical model study. Experiments were carried out in a laboratory basin in the presence of fixed sand and gravel beds, where current-only, wave-only, and combined flow tests were performed. Flow velocities were measured by means of acoustic Doppler velocimeters, through which time-averaged, phase-averaged, and turbulent velocities were obtained. Results revealed two main features of the wave–current flow. First, we observed that the superposition of waves does not necessarily induce an increase in the current bed shear stresses. Indeed, depending on bed roughness, current freestream velocity and wave orbital velocity, enhancements or reductions of the current bed shear were observed. Moreover, application of quadrant analysis revealed a periodic evolution of the current turbulent bursts. Specifically, the number of current turbulent ejections and sweeps is reduced or increased as the wave phase progresses from antinodes to nodes and from nodes to antinodes, respectively.

A three-quantile bias correction with spatial transfer for the correction of simulated European river runoff to force ocean models - Stefan Hagemann; Thao Thi Nguyen; Ha Thi Minh Ho-Hagemann (Ocean Science, 2024)
In ocean or Earth system model applications, the riverine freshwater inflow is an important flux affecting salinity and marine stratification in coastal areas. However, in climate change studies, the river runoff based on climate model output often has large biases on local, regional, or even basin-wide scales. If these biases are too large, the ocean model forced by the runoff will drift into a different climate state compared to the observed state, which is particularly relevant for semi-enclosed seas such as the Baltic Sea. To achieve low biases in riverine freshwater inflow in large-scale climate applications, a bias correction is required that can be applied in periods where runoff observations are not available and that allows spatial transferability of its correction factors. In order to meet these requirements, we have developed a three-quantile bias correction that includes different correction factors for low-, medium-, and high-percentile ranges of river runoff over Europe. Here, we present an experimental setup using the Hydrological Discharge (HD) model and its high-resolution (1/12°) grid. First, bias correction factors are derived at the locations of the downstream stations with available daily discharge observations for many European rivers. These factors are then transferred to the respective river mouths and mapped to neighbouring grid boxes belonging to ungauged catchments. The results show that the bias correction generally leads to an improved representation of river runoff. Especially over northern Europe, where many rivers are regulated, the three-quantile bias correction provides an advantage compared to a bias correction that only corrects the mean bias of the river runoff. Evaluating two NEMO (Nucleus for European Modelling of the Ocean) model simulations in the German Bight indicated that the use of the bias-corrected discharges as forcing leads to an improved simulation of sea surface salinity in coastal areas. Although the bias correction is tailored to the high-resolution HD model grid over Europe in the present study, the methodology is suitable for any high-resolution model region with a sufficiently high coverage of river runoff observations. It is also noted that the methodology is applicable to river runoff based on climate hindcasts, as well as on historical climate simulations where the sequence of weather events does not match the actual observed history. Therefore, it may also be applied in climate change simulations.

Upper-ocean changes with hurricane-strength wind events: a study using Argo profiles and an ocean reanalysis - Jacopo Sala; Donata Giglio; Addison Hu; Mikael Kuusela; Kimberly M. Wood; Ann B. Lee (Ocean Science, 2024)
As the Earth's climate warms, the intensity and rain rate of tropical cyclones (TCs) is projected to increase. TCs intensify by extracting heat energy from the ocean; hence, a better understanding of upper-ocean changes with the TC passage is helpful to improve our understanding of air–sea interactions during and after the event. This work uses Argo float observations and the HYbrid Coordinate Ocean Model (HYCOM) ocean reanalysis to describe characteristics of upper-ocean changes with hurricane-strength wind events. We study the association of these changes with the vertical structure of the salinity profile before the event, i.e., increasing versus decreasing with depth. We also study the contribution of changes in salinity to upper-ocean density changes in each case. Results show that in regions where pre-event salinity increases with depth there is a corresponding statistically significant increase in upper-ocean salinity; vice versa, we observe a significant decrease in upper-ocean salinity in regions where pre-event salinity decreases with depth. Consistent with previous studies, temperature decreases in both regions. As near-surface temperature decreases, upper-ocean density increases, and the increase is larger where pre-event salinity increases with depth. Changes in upper-ocean properties from Argo and HYCOM are overall consistent with wind-driven vertical mixing of near-surface waters with colder and higher-salinity (or lower-salinity) waters below. Resulting changes in ocean stratification have implications for air–sea interactions during and after the event, with potential impacts on weather events that follow.

Alkalinity sources in the Dutch Wadden Sea - Mona Norbisrath; Justus E. E. van Beusekom; Helmuth Thomas (Ocean Science, 2024)
Total alkalinity (TA) is an important chemical property that plays a decisive role in the oceanic buffering capacity with respect to CO2. TA is mainly generated by weathering on land as well as by various anaerobic metabolic processes in the water and sediments. The Wadden Sea, located in the southern North Sea, is hypothesized to be a source of TA for the North Sea, but quantifications are scarce. This study shows observations of TA, dissolved inorganic carbon (DIC), and nutrients in the Dutch Wadden Sea in May 2019. Surface samples were taken along several transects in order to investigate spatial distribution patterns and compare them with data from the late 1980s. A tidal cycle was sampled to further shed light on TA generation and potential TA sources. We identified the Dutch Wadden Sea as a source of TA and estimated an export of 6.6 Mmol TA per tide to the North Sea. TA was generated in the sediments, with deep pore water flow during low tide enriching the surface water. A combination of anaerobic processes and CaCO3 dissolution were potential TA sources in the sediments. We deduce that seasonality and the associated nitrate availability specifically influence TA generation by denitrification, which is low in spring and summer.

Expanding seawater carbon dioxide and methane measuring capabilities with a Seaglider - Claudine Hauri; Brita Irving; Dan Hayes; Ehsan Abdi; Jöran Kemme; Nadja Kinski; Andrew M. P. McDonnell (Ocean Science, 2024)
Warming, ocean acidification, and deoxygenation are increasingly putting pressure on marine ecosystems. At the same time, thawing permafrost and decomposing hydrates in Arctic shelf seas may release large amounts of methane (CH4) into the water column, which could accelerate local ocean acidification and contribute to climate change. The key parameters to observing and understanding these complex processes and feedback mechanisms are vastly undersampled throughout the oceans. We developed carbon dioxide (CO2) and CH4 gliders, including standard operational procedures, with the goal that CO2 and CH4 measurements will become more common for glider operations. The Seagliders with integrated Contros HydroC CO2 or CH4 sensors also include conductivity, temperature, depth, oxygen, chlorophyll a, backscatter, and fluorescent dissolved organic matter sensors. Communication via satellite allows for near-real-time data transmission, sensor adjustments, and adaptive sampling. Several sea trials with the CO2 Seaglider in the Gulf of Alaska and data evaluation with discrete water and underway samples suggest nearly “weather-quality” CO2 data as defined by the Global Ocean Acidification Network. A winter mission in Resurrection Bay, Alaska, provided the first insights into the water column inorganic carbon dynamics during this otherwise undersampled season. The CH4 Seaglider passed its flight trials in Resurrection Bay but needs to be tested during a field mission in an area with CH4 concentrations beyond background noise. Both sensing systems are available to the science community through the industry partners (Advanced Offshore Operations and -4H-JENA engineering GmbH) of this project.

TEOS-10 and the climatic relevance of ocean–atmosphere interaction - Rainer Feistel (Ocean Science, 2024)
Unpredicted observations in the climate system, such as recent excessive ocean warming, are often lacking immediate causal explanations and are challenging numerical models. As a highly advanced mathematical tool, the Thermodynamic Equation of Seawater – 2010 (TEOS-10) was established by international bodies as an interdisciplinary standard and is recommended for use in geophysics, such as, and in particular, in climate research. From its very beginning, the development of TEOS-10 was supported by Ocean Science through publishing successive stages and results. Here, the history and properties of TEOS-10 are briefly reviewed. With focus on the air–sea interface, selected current problems of climate research are discussed, and tutorial examples for the possible use of TEOS-10 in the associated context are presented, such as topics related to ocean heat content, latent heat, and the rate of marine evaporation; properties of sea spray aerosol; or climatic effects of low-level clouds. Appended to this article, a list of publications and their metrics is provided for illustrating the uptake of TEOS-10 by the scientific community, along with some continued activities, addressing still pending, connected issues such as uniform standard definitions of uncertainties of relative humidity, seawater salinity, or pH. This article is dedicated to the jubilee celebrating 20 years of Ocean Science. This article is also dedicated to the memory of Wolfgang Wagner, who sadly and unexpectedly passed away on 12 August 2024. His contributions to TEOS-10 are truly indispensable constituents; Wolfgang was an essential co-author of various related documents and articles. He will be deeply missed. All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither they return again. The King James Bible: Ecclesiastes, 450–150 BCE He wraps up the waters in his clouds, yet the clouds do not burst under their weight. Holy Bible: New International Version, Job 26:8 Of the air, the part receiving heat is rising higher. So, evaporated water is lifted above the lower air. Leonardo da Vinci: Primo libro delle acque, Codex Arundel, ca. 1508 Two-thirds of the Sun's energy falling on the Earth's surface is needed to vaporize … water … as a heat source for a gigantic steam engine. Heinrich Hertz: Energiehaushalt der Erde, 1885 The sea-surface interaction is obviously a highly significant quantity in simulating climate. Andrew Gilchrist and Klaus Hasselmann: Climate Modelling, 1986 The climate of the Earth is ultimately determined by the temperatures of the oceans. Donald Rapp: Assessing Climate Change, 2014

North Atlantic Subtropical Mode Water properties: intrinsic and atmospherically forced interannual variability - Olivier Narinc; Thierry Penduff; Guillaume Maze; Stéphanie Leroux; Jean-Marc Molines (Ocean Science, 2024)
This study investigates the contributions of the ocean's chaotic intrinsic variability (CIV) and atmospherically forced variability to the interannual fluctuations in the North Atlantic Subtropical Mode Water (STMW) properties. Utilizing a 0.25° regional 50-member ocean–sea-ice ensemble simulation driven by an original surface forcing method and perturbed initially, the forced variability in STMW properties is estimated from ensemble mean fluctuations, while CIV is determined from deviations around the ensemble mean within each member. The model successfully captures the main features of STMW, showing correct agreement with observation-based ARMOR3D data in terms of location, seasonality, mean temperature and volume, as well as interannual variance of STMW's main properties. CIV significantly impacts STMW, explaining 10 %–13 % and 28 %–44 % of the interannual variance of its geometric and thermohaline mean properties, respectively, with a maximum imprint on STMW temperature. Observation-based and simulated intrinsic-to-total variance ratios are mostly consistent, dispelling concerns about a signal-to-noise paradox. This study also illustrates the advantages of ensemble simulations over single simulations in understanding oceanic fluctuations and attributing them to external drivers, while also cautioning against overreliance on individual simulation assessments.

Temperature effect on seawater f CO 2 revisited: theoretical basis, uncertainty analysis and implications for parameterising carbonic acid equilibrium constants - Matthew P. Humphreys (Ocean Science, 2024)
The sensitivity of the fugacity of carbon dioxide in seawater (fCO2) to temperature (denoted υ, reported in % °C−1) is critical for the accurate fCO2 measurements needed to build global carbon budgets and for understanding the drivers of air–sea CO2 flux variability across the ocean. However, understanding and computing υ have been restricted to either using empirical functions fitted to experimental data or determining it as an emergent property of a fully resolved marine carbonate system, and these two approaches are not consistent with each other. The lack of a theoretical basis and an uncertainty estimate for υ has hindered resolving this discrepancy. Here, we develop a new approach for calculating the temperature sensitivity of fCO2 based on the equations governing the marine carbonate system and the van 't Hoff equation. This shows that, to first order, ln (fCO2) should be proportional to 1/tK (where tK is temperature in kelvin), rather than to temperature, as has previously been assumed. This new approach is, to first order, consistent with calculations from a fully resolved marine carbonate system, which we have incorporated into the PyCO2SYS software. Agreement with experimental data is less convincing but remains inconclusive due to the scarcity of direct measurements of υ, particularly above 25 °C. However, the new approach is consistent with field data, performing better than any other approach for adjusting fCO2 by up to 10 °C if spatiotemporal variability in its single fitted coefficient is accounted for. The uncertainty in υ arising from only measurement uncertainty in the main experimental dataset where υ has been directly measured is in the order of 0.04 % °C−1, which corresponds to a 0.04 % uncertainty in fCO2 adjusted by +1 °C. However, spatiotemporal variability in υ is several times greater than this, so the true uncertainty due to the temperature adjustment in fCO2 adjusted by +1 °C using the most widely used constant υ value is around 0.24 %. This can be reduced to around 0.06 % using the new approach proposed here, and this could be further reduced by more measurements. The spatiotemporal variability in υ arises mostly from the equilibrium constants for CO2 solubility and carbonic acid dissociation (K1∗ and K2∗), and its magnitude varies significantly depending on which parameterisation is used for K1∗ and K2∗. Seawater fCO2 can be measured accurately enough that additional experiments should be able to detect spatiotemporal variability in υ and distinguish between different parameterisations for K1∗ and K2∗. Because the most widely used constant υ was coincidentally measured from seawater with roughly global average υ, our results are unlikely to significantly affect global air–sea CO2 flux budgets, but they may have more important implications for regional budgets and studies that adjust by larger temperature differences.

MAESSTRO: Masked Autoencoders for Sea Surface Temperature Reconstruction under Occlusion - Edwin Goh; Alice Yepremyan; Jinbo Wang; Brian Wilson (Ocean Science, 2024)
This study investigates the use of a masked autoencoder (MAE) to address the challenge of filling gaps in high-resolution (1 km) sea surface temperature (SST) fields caused by cloud cover, which often result in gaps in the SST data and/or blurry imagery in blended SST products. Our study demonstrates that MAE, a deep learning model, can efficiently learn the anisotropic nature of small-scale ocean fronts from numerical simulations and reconstruct the artificially masked SST images. The MAE model is trained and evaluated on synthetic SST fields and tested on real satellite SST data from the Visible Infrared Imaging Radiometer Suite (VIIRS) sensor on the Suomi NPP satellite. We demonstrate that the MAE model trained on numerical simulations can provide a computationally efficient alternative for filling gaps in satellite SST. MAE can reconstruct randomly occluded images with a root mean square error (RMSE) of under 0.2 °C for masking ratios of up to 80 %. A trained MAE model in inference mode is exceptionally efficient, requiring 3 orders of magnitude (approximately 5000×) less time compared to the conventional approaches of cubic radial basis interpolation and Kriging tested on a single CPU. The ability to reconstruct high-resolution SST fields under cloud cover has important implications for understanding and predicting global and regional climates and detecting small-scale SST fronts that play a crucial role in the exchange of heat, carbon, and nutrients between the ocean surface and deeper layers. Our findings highlight the potential of deep learning models such as MAE to improve the accuracy and resolution of SST data at kilometer scales. This presents a promising avenue for future research in the field of small-scale ocean remote sensing analyses.

Intensified upwelling: normalized sea surface temperature trends expose climate change in coastal areas - Miguel Ángel Gutiérrez-Guerra; María Dolores Pérez-Hernández; Pedro Vélez-Belchí (Ocean Science, 2024)
Eastern boundary upwelling systems (EBUSs) provide valuable natural resources due to their high primary production. However, there is significant uncertainty in how climate change may affect the mechanisms that sustain these ecosystems in the future. Therefore, assessing the effects of climate change on EBUSs under the current global warming scenario is crucial for efficient ecosystem management. In 1990, Andrew Bakun suggested an increase in the upwelling intensity due to the rise of the ocean–land pressure gradient. Since there is a significant link between thermal gradients and offshore Ekman transport, we use sea level pressure (SLP) and deseasonalized sea surface temperature (SST) data from remote sensing to elucidate this hypothesis and validate it using in situ observations. SST is an indicator of coastal upwelling, and our long-term analysis of monthly and deseasonalized SST records shows that the seasonal and synoptic processes have minimal influence on the SST–upwelling intensity relationship. Upwelling within the same EBUS is not usually evenly distributed along coastlines, leading to upwelling in specific areas, referred to as upwelling centers. We compare the SST trends in the main upwelling centers of the four EBUSs with those in open ocean waters through a new index, αUI, designed to characterize upwelling changes in the EBUS. An a-dimensional number allows us to normalize the trends independently of the upwelling system and compare all of them. Furthermore, we have complemented the SST index with sea level pressure gradient data. This new index (supported by SLP gradient trends) indicates intensification in all the EBUSs, revealing a coherent pattern within EBUSs in the same ocean (i.e., Canarian and Benguela or Californian and Humboldt upwelling systems).

The formation and ventilation of an oxygen minimum zone in a simple model for latitudinally alternating zonal jets - Eike E. Köhn; Richard J. Greatbatch; Peter Brandt; Martin Claus (Ocean Science, 2024)
An advection–diffusion model coupled to a simple dynamical ocean model is used to illustrate the formation and ventilation of an oxygen minimum zone. The advection–diffusion model carries a passive tracer with a source at the western boundary and a Newtonian damping term to mimic oxygen consumption. The dynamical model is a non-linear one-and-a-half-layer reduced-gravity model. The latter is forced by an annually oscillating mass flux confined to the near-equatorial band that, in turn, leads to the generation of mesoscale eddies and latitudinally alternating zonal jets at higher latitudes. The model uses North Atlantic geometry and develops a tracer minimum zone remarkably similar in location to the observed oxygen minimum zone in the eastern tropical North Atlantic (ETNA). This is despite the absence of wind forcing for a subtropical gyre and the shadow zone predicted by the ventilated thermocline theory. Although the model is forced only at the annual period, the model nevertheless exhibits decadal and multidecadal variability in its spun-up state. The associated trends are comparable to observed trends in oxygen within the ETNA oxygen minimum zone. Notable exceptions are the multi-decadal decrease in oxygen in the lower-oxygen minimum zone and the sharp decrease in oxygen in the upper-oxygen minimum zone between 2006 and 2013.

Subsurface floats in the Filchner Trough provide the first direct under-ice tracks of the circulation on shelf - Jean-Baptiste Sallée; Lucie Vignes; Audrey Minière; Nadine Steiger; Etienne Pauthenet; Antonio Lourenco; Kevin Speer; Peter Lazarevich; Keith W. Nicholls (Ocean Science, 2024)
Bottom water formation in the Weddell Sea and mass loss from the Filchner–Ronne Ice Shelf are tightly linked by the supply of Warm Deep Water to the continental shelf. Heavy sea ice cover and icebergs restrict ship access and upper-ocean measurements by moorings, compelling us to try new sampling methods. We present results from the first dedicated under-sea-ice float experiment tracking circulation on the continental shelf of the eastern Weddell Sea. Seven Apex profiling floats were deployed in 2017 at three different locations on the eastern Weddell Sea continental shelf, targeting the inflowing modified Warm Deep Water (mWDW), as well as the outflowing Ice Shelf Water (ISW). The floats capture a warm mWDW regime with southward inflow over the eastern continental shelf and a cold ISW regime with a recirculation of ISW in the Filchner Trough throughout the 4 years of observations. We provide the first Lagrangian in situ confirmation that the mWDW flowing onto the continental shelf follows two pathways: the eastern flank of the Filchner Trough and a small trough on the shallow shelf farther east. In the present circulation regime, this warm water is blocked from reaching the ice shelf cavity due to the presence of the thick ISW layer inside the Filchner Trough. The floats' trajectories and hydrography reveal the dynamically active front, flow reversal, and eddying motion between these two water masses along the eastern flank of the Filchner Trough.

Analyses of sea surface chlorophyll a trends and variability from 1998 to 2020 in the German Bight (North Sea) - Felipe de Luca Lopes de Amorim; Areti Balkoni; Vera Sidorenko; Karen Helen Wiltshire (Ocean Science, 2024)
Satellite remote sensing of ocean colour properties allows observation of the ocean with high temporal and spatial coverage, facilitating the better assessment of changes in marine primary production. Ocean productivity is often assessed using satellite-derived chlorophyll a concentrations, a commonly used proxy for phytoplankton concentration. We used the Copernicus GlobColour remote sensing chlorophyll a surface concentration to investigate seasonal and non-seasonal variability, temporal trends, and changes in spring bloom chlorophyll a magnitude. Complementary, we analysed the chlorophyll a relationship with sea surface temperature and mixed-layer depth in the German Bight from 1998 to 2020. Empirical orthogonal functions were employed in order to investigate dominant spatial and temporal patterns (modes) related to the main processes of chlorophyll a variability. Multi covariance analysis was used to extract the dominant structures that maximize the covariance between chlorophyll a and sea surface temperature mixed-layer depth fields. High levels of chlorophyll a were found near the coast, showing a decreasing gradient towards offshore waters. A significant chlorophyll a positive trend was observed close to the Elbe estuary and adjacent area, while 55 % of the German Bight was characterized by a significant chlorophyll a negative trend. The chlorophyll a non-seasonal variability showed that the first four modes explained around 45 %, with the first and second modes related to inter- and intra-annual variability, respectively, observed in the temporal principal components spectral analyses. Monthly chlorophyll a concentration anomalies co-varied by 45 % with sea surface temperature anomalies and 23 % with mixed-layer depth anomalies. The monthly averages of chlorophyll a anomaly fields were suitable to investigate long-term trends and variability. The rising water temperature, combined with its indirect effects on other variables, can partially explain the observed trends in chlorophyll a.

Importance of tides and winds in influencing the nonstationary behaviour of coastal currents in offshore Singapore - Jun Yu Puah; Ivan D. Haigh; David Lallemant; Kyle Morgan; Dongju Peng; Masashi Watanabe; Adam D. Switzer (Ocean Science, 2024)
Coastal currents significantly impact port activities, coastal landform morphodynamics, and ecosystem functioning. It is therefore necessary to understand the physical characteristics and natural variability of these currents within coastal settings. Traditional methods, such as harmonic analysis, assume stationarity of tide-driven currents and may thus not be applicable to systems modulated by variable nontidal inputs and processes. Here we deployed eight tilt current meters in shallow (

Study of the role of abyssal ocean stratification in the rearrangement of potential vorticity through the water column - Beatrice Giambenedetti; Nadia Lo Bue; Vincenzo Artale (Ocean Science, 2024)
Observations of abyssal variability performed in the Ionian Sea (Mediterranean Sea) have revealed the presence of a dense, stable abyssal layer, whose thermohaline and dynamical properties changed drastically over a decade. Building upon these available observations, we aim to investigate the role that stratification can play in the transmission of vorticity throughout the water column to the abyss and, in turn, in the redistribution of energy stored in the deep sea, with a set of stationary states. A quasi-geostrophic level model equipped with four coupled layers, a free surface, and a mathematical artifice for parameterizing decadal time evolution has been considered, proving that the relative-thickness and relative-density differences among the layers are the two critical factors that determine the dynamical characteristics of this rearrangement. The variability in ocean stratification is a relevant aspect that can activate deep and intermediate dynamics, engaging in the propagation and stabilization of signals throughout the water column. This demonstrates the non-negligible active connection between the dynamics of the bottom layers and the surface. The theoretical framework and parameterization used are based on specific observations made in the Ionian Sea over the last decades but retain general applicability to all ocean basins that are characterized by the presence of a stratified, dense water mass in their deep and intermediate layers.

Seasonal variability in the semidiurnal internal tide – a comparison between sea surface height and energetics - Harpreet Kaur; Maarten C. Buijsman; Zhongxiang Zhao; Jay F. Shriver (Ocean Science, 2024)
We investigate the seasonal variability in the semidiurnal internal tide steric sea surface height (SSSH) and energetics using 8 km global Hybrid Coordinate Ocean Model (HYCOM) simulations with realistic forcing and satellite altimeter data. In numerous previous studies, SSSH has been used to explore the seasonal changes in internal tides. For the first time, we compare the seasonal variability in the semidiurnal internal tide SSSH with the seasonal variability in the semidiurnal baroclinic energetics. We explore the seasonal trends in SSSH variance, barotropic to baroclinic conversion rate, kinetic energy, available potential energy, and pressure flux for the semidiurnal internal tides. We find that the seasonal cycle of monthly semidiurnal SSSH variance in the Northern Hemisphere is out of phase with the Southern Hemisphere. This north–south phase difference and its timing are in agreement with altimetry. The amplitudes of the seasonal variability in SSSH variance are about 10 %–15 % of their annual mean values when zonally averaged. The normalized amplitude of the seasonal variability is higher for the SSSH variance than for the energetics. The largest seasonal variability is observed in Georges Bank and the Arabian Sea, where the seasonal trends of monthly SSSH variance and energetics are in phase. However, outside these hotspots, the seasonal variability in semidiurnal energetics is out of phase with semidiurnal SSSH variance, and a clear phase difference between the Northern Hemisphere and Southern Hemisphere is lacking. While the seasonal variability in semidiurnal energy is driven by seasonal changes in barotropic to baroclinic conversion, semidiurnal SSSH variance is also modulated by seasonal changes in surface stratification. Surface-intensified stratification at the end of summer enhances the surface perturbation pressures, which enhance the SSSH amplitudes.

High-resolution numerical modelling of seasonal volume, freshwater, and heat transport along the Indian coast - Kunal Madkaiker; Ambarukhana D. Rao; Sudheer Joseph (Ocean Science, 2024)
Seasonal reversal of winds and equatorial remote forcing influences the circulation of the Arabian Sea (AS) and Bay of Bengal (BoB) basins in the northern Indian Ocean. In this study, we numerically modelled the physical characteristics of the AS and BoB, using the Massachusetts Institute of Technology general circulation model (MITgcm) at a high spatial resolution of 1/20° forced with climatological initial and boundary conditions. The simulated temperature, salinity, and flow fields were validated with satellite and in situ datasets. We then studied the exchange of coastal waters by evaluating transports computed from the model simulations. The alongshore volume transport on the eastern coast is stronger with high seasonal variability due to the poleward-flowing western boundary current and equatorward-flowing East Indian Coastal Current. West coast transport is influenced by large intraseasonal oscillations. The alongshore freshwater transport is an order less than the alongshore volume transport. Out of the net volume transport, freshwater accounts for a maximum of 6.03 % during the southwestern monsoon season, followed by 4.85 % in the post-monsoon season. We observe an inverse relationship between alongshore freshwater and volume transport on the western coast and a direct relationship on the eastern coast. The contribution of eddy-induced heat and freshwater transport was also examined. The relation between net heat transport and net heat flux illustrates the role of coastal currents and equatorial forcing in dissipating heat within the coastal waters. We observed that meridional heat transport over the AS is stronger than over the BoB. Both basins act as a heat source during the summer monsoon and heat sink during the winter. This high-resolution model setup simulates all the important physical climatological patterns, leading to a better understanding of the state of the northern Indian Ocean.

Predicting particle catchment areas of deep-ocean sediment traps using machine learning - Théo Picard; Jonathan Gula; Ronan Fablet; Jeremy Collin; Laurent Mémery (Ocean Science, 2024)
The ocean's biological carbon pump plays a major role in climate and biogeochemical cycles. Photosynthesis at the surface produces particles that are exported to the deep ocean by gravity. Sediment traps, which measure deep-carbon fluxes, help to quantify the carbon stored by this process. However, it is challenging to precisely identify the surface origin of particles trapped thousands of meters deep due to the influence of ocean circulation on the sinking path of carbon. In this study, we conducted a series of numerical Lagrangian experiments in the Porcupine Abyssal Plain region of the North Atlantic and developed a machine learning approach to predict the surface origin of particles trapped in a deep-ocean sediment trap. Our numerical experiments support the predictive performance of the machine learning approach, and surface conditions appear to provide valuable information for accurately predicting the source area, suggesting a potential application with satellite data. We also identify factors that potentially affect prediction efficiency, and we show that the best predictions are associated with low kinetic energy and the presence of mesoscale eddies above the trap. This new tool could provide a better link between satellite-derived sea surface observations and deep-ocean sediment trap measurements, ultimately improving our understanding of the biological-carbon-pump mechanism.

Exploring variability in climate change projections on the Nemunas River and Curonian Lagoon: coupled SWAT and SHYFEM modeling approach - Natalja Čerkasova; Jovita Mėžinė; Rasa Idzelytė; Jūratė Lesutienė; Ali Ertürk; Georg Umgiesser (Ocean Science, 2024)
This study advances the understanding of climate projection variabilities in the Nemunas River, Curonian Lagoon, and southeastern Baltic Sea continuum by analyzing the output of a coupled ocean and drainage basin modeling system forced by a subset of climate models. A dataset from a downscaled high-resolution regional atmospheric climate model driven by four different global climate models was bias-corrected and used to set up the hydrological (Soil and Water Assessment Tool, SWAT) and hydrodynamic (Shallow water HYdrodynamic Finite Element Model, SHYFEM) modeling system. This study investigates the variability and trends in environmental parameters such as water fluxes, timing, nutrient load, water temperature, ice cover, and saltwater intrusions under Representative Concentration Pathway 4.5 and 8.5 scenarios. The analysis highlights the differences among model results underscoring the inherent uncertainties in projecting climatic impacts, hence highlighting the necessity of using multi-model ensembles to improve the accuracy of climate change impact assessments. Modeling results were used to evaluate the possible environmental impact due to climate change through the analysis of the cold-water fish species reproduction season. We analyze the duration of cold periods (

A new airborne system for simultaneous high-resolution ocean vector current and wind mapping: first demonstration of the SeaSTAR mission concept in the macrotidal Iroise Sea - David L. McCann; Adrien C. H. Martin; Karlus A. C. de Macedo; Ruben Carrasco Alvarez; Jochen Horstmann; Louis Marié; José Márquez-Martínez; Marcos Portabella; Adriano Meta; Christine Gommenginger; Petronilo Martin-Iglesias; Tania Casal (Ocean Science, 2024)
Coastal seas, shelf seas and marginal ice zones are dominated by small-scale ocean surface dynamic processes that play a vital role in the transport and exchange of climate-relevant properties such as carbon, heat, water and nutrients between land, ocean, ice and atmosphere. Mounting evidence indicates that ocean scales below 10 km have far-ranging impacts on air–sea interactions, lateral ocean dispersion, vertical stratification, ocean carbon cycling and marine productivity – governing exchanges across key interfaces of the Earth system, the global ocean, and atmosphere circulation and climate. Yet, these processes remain poorly observed at the fine spatial and temporal scales necessary to resolve them. The Ocean Surface Current Airborne Radar (OSCAR) is a new airborne instrument with the capacity to inform these questions by mapping vectorial fields of total ocean surface currents and winds at high resolution over a wide swath. Developed for the European Space Agency (ESA), OSCAR is the airborne demonstrator of the satellite mission concept SeaSTAR, which aims to map total surface current and ocean wind vectors with unprecedented accuracy, spatial resolution and temporal revisit across all coastal seas, shelf seas and marginal ice zones. Like SeaSTAR, OSCAR is an active microwave synthetic aperture radar along-track interferometer (SAR-ATI) with optimal three-azimuth sensing enabled by unique highly squinted beams. In May 2022, OSCAR was flown over the Iroise Sea, France, in its first scientific campaign as part of the ESA-funded SEASTARex project. The campaign successfully demonstrated the capabilities of OSCAR to produce high-resolution 2D images of total surface current vectors and near-surface ocean vector winds, simultaneously, in a highly dynamic, macrotidal coastal environment. OSCAR current and wind vectors show excellent agreement with ground-based X-band-radar-derived surface currents, numerical model outputs and NovaSAR-1 satellite SAR imagery, with root mean square differences from the X-band radar better than 0.2 m s−1 for currents at 200 m resolution. These results are the first demonstration of simultaneous retrieval of total current and wind vectors from a high-squint three-look SAR-ATI instrument and the first geophysical validation of the OSCAR and SeaSTAR observing principle. OSCAR presents a remarkable new ocean observing capability to support the study of small-scale ocean dynamics and air–sea interactions across the Earth's coastal, shelf and polar seas.

Investigating the long-term variability of the Red Sea marine heatwaves and their relationship to different climate modes: focus on 2010 events in the northern basin - Manal Hamdeno; Aida Alvera-Azcárate; George Krokos; Ibrahim Hoteit (Ocean Science, 2024)
Marine heatwaves (MHWs) have been increasing in frequency, intensity, and duration worldwide, which poses a serious threat to marine ecosystems and fisheries. The Red Sea (RS), a semi-enclosed marginal sea, is highly vulnerable to climate change due to its small volume and slow rate of water renewal. Despite the importance of the RS, MHWs in this region remain poorly studied, and understanding of their spatial and temporal characteristics and forcing mechanisms is limited. This study examines MHWs in the RS over the last 4 decades (1982–2021) and their relationship to large-scale climate modes, with particular focus on the 2010 MHW event in the northern Red Sea (NRS). Analysis of sea surface temperature anomaly (SSTA) trends in the RS revealed a decadal variability, with the highest warming trends occurring alternately in the northern and southern regions. The RS has experienced a significant warming trend over the last 4 decades, which has intensified since 2016. This warming has led to an increase in the frequency and duration of MHWs in the region, with 46 % of events and 58 % of MHW days occurring only in the last decade. The RS exhibits a meridional gradient, with decreasing mean annual MHW intensity and duration but increasing mean annual MHW frequency from north to south. The annual MHW frequency in the NRS peaked in 2010, 2018, 2019, and 2021, while, in the Southern Red Sea (SRS), the highest frequency occurred in 1998 and from 2017 to 2021. The study also examined the correlation between MHWs and climate indices and found that the Atlantic Multidecadal Oscillation (AMO), the Indian Ocean Dipole (IOD), and the East Atlantic/West Russia pattern (EATL/WRUS) were the three dominant modes that correlated with SSTAs and MHWs in the region. The North Atlantic Oscillation (NAO) and the Oceanic Niño Index (ONI) showed weaker and less significant correlations. Finally, the authors conducted a case study of the 2010 MHW event in the NRS, which was the most intense and longest winter event of the year. Using a high-resolution ocean model and atmospheric reanalysis data, it was found that the MHW in late winter 2010 in the NRS extended to a depth of 120 m and was driven by a combination of atmospheric forcings, particularly an increase in air temperature (Tair) and humidity, possibly linked to reduced winds leading to reduced latent heat flux (LHF) and strong ocean warming, creating favourable conditions for MHWs to occur.

An assessment of equatorial Atlantic interannual variability in Ocean Model Intercomparison Project (OMIP) simulations - Arthur Prigent; Riccardo Farneti (Ocean Science, 2024)
The eastern equatorial Atlantic (EEA) seasonal cycle and interannual variability strongly influence the climate of the surrounding continents. It is thus crucial that models used in both climate predictions and future climate projections are able to simulate them accurately. In that context, the EEA monthly climatology and interannual variability are evaluated over the period 1985–2004 for models participating in the Ocean Model Intercomparison Project Phases 1 and 2 (OMIP1 and OMIP2). The main difference between OMIP1 and OMIP2 simulations is their atmospheric forcing: CORE-II and JRA55-do, respectively. Monthly climatologies of the equatorial Atlantic zonal wind, sea level anomaly, and sea surface temperature in OMIP1 and OMIP2 are comparable to reanalysis products. Yet, some discrepancies exist in both OMIP ensembles: the thermocline is too diffusive, and there is a lack of cooling during the development of the Atlantic cold tongue. The EEA interannual sea surface temperature variability during May–June–July in the OMIP1 ensemble mean is found to be 51 % larger (0.62 ± 0.04 °C) than that in the OMIP2 ensemble mean (0.41 ± 0.03 °C). Likewise, the May–June–July interannual sea surface height variability in the EEA is 33 % larger in the OMIP1 ensemble mean (0.02 ± 0.002 m) than in the OMIP2 ensemble mean (0.015 ± 0.002 m). Sensitivity experiments demonstrate that the discrepancies in interannual sea surface temperatures and sea surface height variabilities between OMIP1 and OMIP2 are mainly attributable to their wind forcings and, specifically, to their variability. While the April–May–June zonal wind variability in the western equatorial Atlantic is similar in both forcings, the zonal wind variability peaks in April for JRA55-do and in May for CORE-II.

The characteristics of tides and their effects on the general circulation of the Mediterranean Sea - Bethany McDonagh; Emanuela Clementi; Anna Chiara Goglio; Nadia Pinardi (Ocean Science, 2024)
The effects of tides on the Mediterranean Sea's general circulation, with a particular focus on the horizontal and vertical currents, are investigated using twin simulations with and without tides. Amplitudes of tides in the region are typically low, but an analysis of the potential and kinetic energy demonstrates that tides have effects across many spatial and temporal scales in the basin, including non-linear effects in short periods (less than 1 d) with high kinetic energy peaks at near-inertial basin modes and tidal frequencies. Internal tidal waves are also revealed below 100 m. Tides are found to amplify several basin modes of the Mediterranean Sea, broaden several tidal frequency energy spectra bands, and interact energetically with near-inertial waves. Tides increase the mixed layer depth in the Mediterranean Sea, particularly in the deep and intermediate water formation areas of the western Mediterranean Basin and eastern Mediterranean Basin. The addition of tides in the cases considered does also enhance Western Mediterranean Deep Water formation.

Deep learning for the super resolution of Mediterranean sea surface temperature fields - Claudia Fanelli; Daniele Ciani; Andrea Pisano; Bruno Buongiorno Nardelli (Ocean Science, 2024)
Sea surface temperature (SST) is one of the essential variables of the Earth's climate system. Being at the air–sea interface, SST modulates heat fluxes in and out of the ocean, provides insight into several upper and interior ocean dynamical processes, and is a fundamental indicator of climate variability potentially impacting the health of marine ecosystems. Its accurate estimation and regular monitoring from space is therefore crucial. However, even if satellite infrared/microwave measurements provide much better coverage than what is achievable from in situ platforms, they cannot sense the sea surface under cloudy and rainy conditions. Large gaps are present even in merged multi-sensor satellite products, and different statistical strategies, mostly based on optimal interpolation (OI) algorithms, have thus been proposed to obtain gap-free (L4) images. These techniques, however, filter out the signals below the space–time decorrelation scales considered, significantly smoothing most of the small mesoscale and submesoscale features. Here, deep learning models, originally designed for single-image super resolution (SR), are applied to enhance the effective resolution of SST products and the accuracy of SST gradients. SR schemes include a set of computer vision techniques leveraging convolutional neural networks to retrieve high-resolution data from low-resolution images. A dilated convolutional multi-scale learning network, which includes an adaptive residual strategy and implements a channel attention mechanism, is used to reconstruct features in SST data at 1/100° spatial resolution starting from 1/16° data over the Mediterranean Sea. The application of this technique shows an improvement in the high-resolution reconstruction, capturing small-scale features and providing a root-mean-squared-difference improvement of 0.02 °C with respect to the L3 ground-truth data.

Mechanisms and intraseasonal variability in the South Vietnam Upwelling, South China Sea: the role of circulation, tides, and rivers - Marine Herrmann; Thai To Duy; Patrick Marsaleix (Ocean Science, 2024)
Summer monsoon southwest wind induces the South Vietnam Upwelling (SVU) over four main areas along the southern and central Vietnamese coast: upwelling offshore of the Mekong shelf (MKU), along the southern and northern coasts (SCU and NCU), and offshore (OFU). Previous studies have highlighted the roles of wind and ocean intrinsic variability (OIV) in intraseasonal to interannual variability in the SVU. The present study complements these results by examining the influence of tides and river discharges and investigates the physical mechanisms involved in MKU functioning. MKU is driven by non-chaotic processes, explaining its negligible intrinsic variability. It is triggered first by the interactions of currents over marked topography. The surface convergence of currents over the southwestern slope of the Mekong shelf induces a downwelling of the warm northeastward alongshore current. It flows over the shelf and encounters a cold northwestward bottom current when reaching the northeastern slope. The associated bottom convergence and surface divergence lead to an upwelling of cold water, which is entrained further north by the surface alongshore current. Tides strengthen this circulation-topography-induced MKU through two processes. First, tidal currents weaken the current over the shallow coastal shelf by enhancing the bottom friction. This increases the horizontal velocity gradient and hence the resulting surface convergence and divergence and the associated downwelling and upwelling. Second, they reinforce the surface cooling upstream and downstream of the shelf through lateral and vertical tidal mixing. This tidal reinforcement explains 72 % of MKU intensity on average over the summer and is partly transmitted to SCU through advection. Tides do not significantly influence OFU and NCU intensity. Mekong waters slightly weaken MKU (by 9 % of the annual average) by strengthening the stratification but do not significantly influence OFU, NCU, and SCU. Last, tides and rivers do not modify the chronology of upwelling in the four areas.

Internal and forced ocean variability in the Mediterranean Sea - Roberta Benincasa; Giovanni Liguori; Nadia Pinardi; Hans von Storch (Ocean Science, 2024)
Two types of variability are discernible in the ocean: a response to the atmospheric forcing and the so-called internal/intrinsic ocean variability, which is associated with internal instabilities, nonlinearities, and the interactions between processes at different scales. Producing an ensemble of 20 multiyear ocean simulations of the Mediterranean Sea, initialized with different realistic initial conditions but using the same atmospheric forcing, the study examines the intrinsic variability in terms of its spatial distribution and seasonality. In general, the importance of the external forcing decreases with depth but dominates in extended shelves such as the Adriatic Sea and the Gulf of Gabes. In the case of temperature, the atmospheric forcing plays a major role in the uppermost 50 m of the water column during summer and the uppermost 100 m during winter. Additionally, intrinsic variability displays a distinct seasonal cycle in the surface layers, with a prominent maximum at around 30 m depth during the summer connected to the summer thermocline formation processes. Concerning current velocity, the internal variability has a significant influence at all depths.

An emerging pathway of Atlantic Water to the Barents Sea through the Svalbard Archipelago: drivers and variability - Kjersti Kalhagen; Ragnheid Skogseth; Till M. Baumann; Eva Falck; Ilker Fer (Ocean Science, 2024)
The Barents Sea, an important component of the Arctic Ocean, is experiencing changes in its ocean currents, stratification, sea ice variability, and marine ecosystems. Inflowing Atlantic Water (AW) is a key driver of these changes. As AW predominantly enters the Barents Sea via the Barents Sea Opening, other pathways remain relatively unexplored. Comparisons of summer climatology fields of temperature from the last century with those from 2000–2019 indicate warming in the Storfjordrenna trough and along two shallow banks, Hopenbanken and Storfjordbanken, within the Svalbard Archipelago. Additionally, they indicate shoaling of AW that extends further into the “channel” between the islands of Edgeøya and Hopen. This region emerges as a pathway enabling AW to enter the northwestern Barents Sea. Moreover, 1-year-long records from a mooring deployed between September 2018 and November 2019 at the saddle of this channel show the flow of Atlantic-origin waters into the Arctic domain of the northwestern Barents Sea. The average current is directed eastwards into the Barents Sea and exhibits significant variability throughout the year. Here, we investigate this variability on timescales ranging from hours to months. Wind forcing mediates currents, water exchange, and heat exchange through the channel by driving geostrophic adjustment to Ekman transport. The main drivers of the warm-water inflow and across-saddle transport of positive temperature anomalies include persistently strong semidiurnal tidal currents, intermittent wind-forced events, and wintertime warm-water intrusions forced by upstream conditions. We propose that similar topographic constraints near AW pathways may become more important in the future. Ongoing warming and shoaling of AW, coupled with changes in large-scale weather patterns, are likely to increase warm-water inflow and heat transport through the processes identified in this study.

Fusion of Lagrangian drifter data and numerical model outputs for improved assessment of turbulent dispersion - Sloane Bertin; Alexei Sentchev; Elena Alekseenko (Ocean Science, 2024)
Transport and dispersion processes in the ocean are crucial, as they determine the lifetime and fate of biological and chemical quantities drifting with ocean currents. Due to the complexity of the coastal ocean environment, numerical circulation models have difficulties to accurately simulate highly turbulent flows and dispersion processes, especially in highly energetic tidal basins such as the eastern English Channel. A method of improving the results of coastal circulation modeling and tracer dispersion in the Dover Strait is proposed. Surface current velocities derived from Lagrangian drifter measurements in November 2020 and May 2021 were optimally interpolated in time and space to constrain a high-resolution coastal circulation MARS model, with careful attention given to selecting ensemble members composing the model covariance matrix. The space–time velocity covariances derived from model simulations were utilized by the optimal interpolation algorithm to determine the most likely evolution of the velocity field under constraints provided by Lagrangian observations and their error statistics. The accuracy of the velocity field reconstruction was evaluated at each time step. The results of the fusion of model outputs with surface drifter velocity measurements show a significant improvement (by ∼ 50 %) of the model capability to simulate the drift of passive tracers in the Dover Strait. Optimized velocity fields were used to quantify the absolute dispersion in the study area. The implications of these results are important, as they can be used to improve existing decision-making support tool or design new tools for monitoring the transport and dispersion in a coastal ocean environment.

Internal-tide vertical structure and steric sea surface height signature south of New Caledonia revealed by glider observations - Arne Bendinger; Sophie Cravatte; Lionel Gourdeau; Luc Rainville; Clément Vic; Guillaume Sérazin; Fabien Durand; Frédéric Marin; Jean-Luc Fuda (Ocean Science, 2024)
In this study, we exploit autonomous underwater glider data to infer internal-tide dynamics south of New Caledonia, an internal-tide-generation hot spot in the southwestern tropical Pacific. By fitting a sinusoidal function to vertical displacements at each depth using a least-squares method, we simultaneously estimate diurnal and semidiurnal tides. Our analysis reveals regions of enhanced tidal activity, strongly dominated by the semidiurnal tide. To validate our findings, we compare the glider observations to a regional numerical simulation that includes tidal forcing. This comparison assesses the simulation's realism in representing tidal dynamics and evaluates the glider's ability to infer internal-tide signals and their signature in sea surface height (SSH). The glider observations and a pseudo glider, simulated using hourly numerical model output with identical sampling, exhibit similar amplitude and phase characteristics along the glider track. Existing discrepancies are in large part explained by tidal incoherence induced by eddy–internal-tide interactions. We infer the semidiurnal internal-tide signature in steric SSH by the integration of vertical displacements. Within the upper 1000 m, the pseudo glider captures roughly 78 % of the steric SSH total variance explained by the full water column signal. This value increases to over 90 % when projecting the pseudo glider's vertical displacements onto climatological baroclinic modes and extrapolating to full depth. Notably, the steric SSH from glider observations aligns closely with empirical estimates derived from satellite altimetry, highlighting the internal tide's predominant coherent nature during the glider's sampling.

Exploring water accumulation dynamics in the Pearl River estuary from a Lagrangian perspective - Mingyu Li; Alessandro Stocchino; Zhongya Cai; Tingting Zu (Ocean Science, 2024)
Investigating water accumulation is essential for understanding estuarine mass distribution and ecosystem management. In this study, we examined the water accumulation dynamics in the Pearl River estuary (PRE) from a Lagrangian perspective. Generally, there is a notable negative correlation between the horizontal velocity divergence (∇h⋅Vh) and the accumulation. Influenced by density fronts and velocity convergence, significant bottom-layer accumulation of particles was observed in the western part of the estuary and the Hong Kong waters during summer, whereas this accumulation moved landward in winter. Subregions with distinct accumulation patterns and interconnections were identified and combined with the trajectories. In summer, the western part of the estuary and the Macau waters exhibit significant net negative ∇h⋅Vh values, and strong density fronts are major accumulation targets, attracting particles from the entire estuary. Conversely, the eastern part of the estuary and Hong Kong waters exhibit significant westward motion, influencing the western side. In winter, particles are more likely to accumulate in their initial locations. The upper part of the estuary becomes a major accumulation area due to the obstructive density front and decreased river discharge. Tidal currents and river discharge mainly control water accumulation in the estuary by changing the mixing or current intensity. The weakening of tidal currents and river discharge induce intensified bottom intrusion and the landward movement of accumulation.

Continued warming of deep waters in the Fram Strait - Salar Karam; Céline Heuzé; Mario Hoppmann; Laura de Steur (Ocean Science, 2024)
The Fram Strait is the only deep gateway between the Arctic and the rest of the World Ocean, and it is thus a key region to understand how the deep Arctic will evolve. However, studies and data regarding the deep ocean are scarce, making it difficult to understand its role in the climate system. Here, we analyse oceanographic data obtained close to the Fram Strait sill depth of 2500 m by two long-term mooring locations (F11 and HG-FEVI) in the Fram Strait between 2010–2023 to investigate long-term changes in the hydrographic properties. For additional context, we compile hydrographic profile data from the 1980s for the adjacent basins: the Greenland Sea and the Eurasian Basin. At mooring F11 in the western Fram Strait, we find a clear seasonality, with increased Greenland Sea Deep Water (GSDW) presence during summer and increased Eurasian Basin Deep Water (EBDW) presence during winter. Evaluating long-term changes, we find a modest temperature increase of ∼ 0.1 °C for EBDW from the 1980s. For GSDW, south of the Fram Strait, we find a strong temperature increase of ∼ 0.4–0.5 °C for the same period. The different warming rates have led to GSDW becoming warmer than EBDW since ∼ 2017–2018. This means that the Greenland Sea is no longer a heat sink for the Arctic Ocean at depth but is rather a heat source. It is therefore possible that EBDW temperatures will increase faster in the future.

The Polar Front in the northwestern Barents Sea: structure, variability and mixing - Eivind H. Kolås; Ilker Fer; Till M. Baumann (Ocean Science, 2024)
In the northwestern Barents Sea the warm and salty Atlantic Water meets the cold and fresh Polar Water, forming a distinct thermohaline front, the Barents Sea Polar Front. Here we present the structure of the front, its variability and associated mixing using observations from two cruises conducted in October 2020 and February 2021 during the Nansen Legacy project in the region between the Hopen Trench and the Olga Basin. Ocean stratification, currents and turbulence data were obtained during seven ship transects across the Polar Front near 77° N, 30° E. These transects are complemented by four missions using ocean gliders, one of which was equipped with microstructure sensors to measure turbulence. Across the front, we observe warm (> 1 °C) and salty (> 35.0 g kg−1) Atlantic Water intruding below the colder (

Long-term changes in the ocean tide at Port Louis, Falkland Islands - Philip L. Woodworth (Ocean Science, 2024)
The historic tide gauge measurements at Port Louis in the Falkland Islands made by James Clark Ross in 1842 have been used to see whether there have been long-term changes in the ocean tide at that location. The conclusion is that there is no evidence for any significant change, which contrasts with tide gauge findings from other parts of the world over similar timescales. As by-products, the study has also been instructive in providing an example of how to obtain accurate tidal information from tabulations of high and low waters and from short tide gauge records.

Twenty-first century marine climate projections for the NW European shelf seas based on a perturbed parameter ensemble - Jonathan Tinker; Matthew D. Palmer; Benjamin J. Harrison; Enda O'Dea; David M. H. Sexton; Kuniko Yamazaki; John W. Rostron (Ocean Science, 2024)
The northwest European shelf (NWS) seas are environmentally and economically important, and an understanding of how their climate may change helps with their management. However, as the NWS seas are poorly represented in global climate models, a common approach is to dynamically downscale with an appropriate shelf sea model. We develop a set of physical marine climate projections for the NWS. We dynamically downscale 12 members of the HadGEM3-GC3.05 perturbed parameter ensemble (approximately 70 km horizontal resolution over Europe), developed for UKCP18, using the shelf sea model NEMO CO9 (7 km horizontal resolution). These are run under the RCP8.5 high-greenhouse-gas-emission scenario as continuous simulations over the period 1990–2098. We evaluate the simulations against observations in terms of tides, sea surface temperature (SST), surface and near-bed temperature and salinity, and sea surface height. These simulations represent the state of the art for NWS marine projections. We project an SST rise of 3.11 °C (± 2σ = 0.98 °C) and a sea surface salinity (SSS) freshening of −1.01 (± 2σ = 0.93; on the (unitless) practical salinity scale) for 2079–2098 relative to 2000–2019, averaged over the NWS (approximately bounded by the 200 m isobar and excluding the Norwegian Trench, the Skagerrak and Kattegat), a substantial seasonal stratification increase (23 d over the NWS seas), and a general weakening of the NWS residual circulation. While the patterns of NWS changes are similar to our previous projections, there is a greater warming and freshening that could reflect the change from the A1B emissions scenario to the RCP8.5 concentrations pathway or the higher climate sensitivity exhibited by HadGEM3-GC3.05. Off the shelf, south of Iceland, there is limited warming, consistent with a reduction in the Atlantic Meridional Overturning Circulation and associated northward heat transport. These projections have been publicly released, along with a consistent 200-year present-day control simulation, to provide an evidence base for climate change assessments and to facilitate climate impact studies. For example, we illustrate how the two products can be used to estimate climate trends, unforced variability and the time of emergence (ToE) of the climate signals. We calculate the average NWS SST ToE to be 2034 (with an 8-year range) and 2046 (with a 33-year range) for SSS. We also discuss how these projections can be used to describe NWS conditions under 2 and 4 °C global mean warming (compared with 1850–1900), as a policy-relevant exemplar use case.

Anomalous North Pacific subtropical mode water volume and density decrease in a recent stable Kuroshio Extension period from Argo observations - Jing Sheng; Cong Liu; Yanzhen Gu; Peiliang Li; Fangguo Zhai; Ning Zhou (Ocean Science, 2024)
North Pacific subtropical mode water (NPSTMW) is formed as a low-stratification water mass in the wintertime mixed layer south of the Kuroshio Extension (KE). In a recent period of 2018–2021, the KE jet was in a persistent stable dynamic state. But based on analysis of Argo observation, the mean volume of NPSTMW in the ventilation region dropped anomalously by ∼ 21 % during 2018–2021 relative to 2012–2015, when the KE jet was likewise stable. Moreover, the NPSTMW volume in a denser density range (approximately σθ>25.2 kg m−3) has started to decrease since 2018. The decreases in the NPSTMW subduction and formation rate are associated with anomalously shallow wintertime mixed-layer depth (MLD) and weak heat loss in the NPSTMW formation region. The decrease in air–sea heat exchange acts to weaken the vertical mixing and decrease the MLD, resulting in the weakening of subduction. The interannual variations in the air–sea heat exchange and wintertime MLD reflect the variability in the overlying atmosphere, which is correlated with a Pacific Decadal Oscillation (PDO) shift in 2018–2021. When the PDO shifted from its positive phase to a negative phase in the analysis period, the effects of local wind stress anomalies seemed to play an evident role in driving the variability in NPSTMW on interannual timescales. The MLD and heat loss change during the cold season in 2018–2021 were strongly coupled with the poleward shift of the westerlies – which cause weaker wintertime wind and easterly wind anomalies over the NPSTMW formation region. The declines in heat loss and southward Ekman transport, owing to the wind stress anomalies, further prohibit upper-ocean convection and mixed-layer deepening and cooling. Additionally, the insufficient development of wintertime MLD in 2018–2021 may also be associated with the significantly intensified preconditioning of near-surface stratification (

Observed change and the extent of coherence in the Gulf Stream system - Helene Asbjørnsen; Tor Eldevik; Johanne Skrefsrud; Helen L. Johnson; Alejandra Sanchez-Franks (Ocean Science, 2024)
By transporting warm and salty water poleward, the Gulf Stream system maintains a mild climate in northwestern Europe while also facilitating the dense water formation that feeds the deep ocean. The sensitivity of North Atlantic circulation to future greenhouse gas emissions seen in climate models has prompted an increasing effort to monitor the various ocean circulation components in recent decades. Here, we synthesize available ocean transport measurements from several observational programs in the North Atlantic and Nordic Seas, as well as an ocean state estimate (ECCOv4-r4), for an enhanced understanding of the Gulf Stream and its poleward extensions as an interconnected circulation system. We see limited coherent variability between the records on interannual timescales, highlighting the local oceanic response to atmospheric circulation patterns and variable recirculation timescales within the gyres. On decadal timescales, we find a weakening subtropical circulation between the mid-2000s and mid-2010s, while the inflow and circulation in the Nordic Seas remained stable. Differing decadal trends in the subtropics, subpolar North Atlantic, and Nordic Seas warrant caution in using observational records at a single latitude to infer large-scale circulation change.

New insights into the eastern subpolar North Atlantic meridional overturning circulation from OVIDE - Herlé Mercier; Damien Desbruyères; Pascale Lherminier; Antón Velo; Lidia Carracedo; Marcos Fontela; Fiz F. Pérez (Ocean Science, 2024)
The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the Earth's climate. However, there are few long time series of observations of the AMOC, and the study of the mechanisms driving its variability depends mainly on numerical simulations. Here, we use four ocean circulation estimates produced by different data-driven approaches of increasing complexity to analyse the seasonal to decadal variability of the subpolar AMOC across the Greenland–Portugal OVIDE (Observatoire de la Variabilité Interannuelle à DÉcennale) line since 1993. We decompose the MOC strength variability into a velocity-driven component due to circulation changes and a volume-driven component due to changes in the depth of the overturning maximum isopycnal. We show that the variance of the time series is dominated by seasonal variability, which is due to both seasonal variability in the volume of the AMOC limbs (linked to the seasonal cycle of density in the East Greenland Current) and to seasonal variability in the transport of the Eastern Boundary Current. The decadal variability of the subpolar AMOC is mainly caused by changes in velocity, which after the mid-2000s are partly offset by changes in the volume of the AMOC limbs. This compensation means that the decadal variability of the AMOC is weaker and therefore more difficult to detect than the decadal variability of its velocity-driven and volume-driven components, which is highlighted by the formalism that we propose.

Dynamical reconstruction of the upper-ocean state in the central Arctic during the winter period of the MOSAiC expedition - Ivan Kuznetsov; Benjamin Rabe; Alexey Androsov; Ying-Chih Fang; Mario Hoppmann; Alejandra Quintanilla-Zurita; Sven Harig; Sandra Tippenhauer; Kirstin Schulz; Volker Mohrholz; Ilker Fer; Vera Fofonova; et al. (Ocean Science, 2024)
This paper presents a methodological tool for dynamic reconstruction of the state of the ocean, based, as an example, on observations from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) experiment. The data used in this study were collected in the Amundsen Basin between October 2019 and January 2020. Analysing observational data to assess tracer field and upper-ocean dynamics is highly challenging when measurement platforms drift with the ice pack due to continuous drift speed and direction changes. We have equipped the new version of the coastal branch of the global Finite-volumE sea ice–Ocean Model (FESOM-C) with a nudging method. Model nudging was carried out assuming a quasi-steady state. Overall, the model can reproduce the lateral and vertical structure of the temperature, salinity, and density fields, which allows for projecting dynamically consistent features of these fields onto a regular grid. We identify two separate depth ranges of enhanced eddy kinetic energy located around two maxima in buoyancy frequency: the depth of the upper halocline and the depth of the warm (modified) Atlantic Water. Simulations reveal a notable decrease in surface layer salinity and density in the Amundsen Basin towards the north but no significant gradient from east to west. However, we find a mixed-layer deepening from east to west, with a 0.084 m km−1 gradient at 0.6 m km−1 standard deviation, compared to a weak deepening from south to north. The model resolves several stationary eddies in the warm Atlantic Water and provides insights into the associated dynamics. The model output can be used to further analyse the thermohaline structure and related dynamics associated with mesoscale and submesoscale processes in the central Arctic, such as estimates of heat fluxes or mass transport. The developed nudging method can be utilized to incorporate observational data from a diverse set of instruments and for further analysis of data from the MOSAiC expedition.

Anthropogenic CO 2 , air–sea CO 2 fluxes, and acidification in the Southern Ocean: results from a time-series analysis at station OISO-KERFIX (51° S–68° E) - Nicolas Metzl; Claire Lo Monaco; Coraline Leseurre; Céline Ridame; Gilles Reverdin; Thi Tuyet Trang Chau; Frédéric Chevallier; Marion Gehlen (Ocean Science, 2024)
The temporal variation of the carbonate system, air–sea CO2 fluxes, and pH is analyzed in the southern Indian Ocean, south of the polar front, based on in situ data obtained from 1985 to 2021 at a fixed station (50°40′ S–68°25′ E) and results from a neural network model that reconstructs the fugacity of CO2 (fCO2) and fluxes at monthly scale. Anthropogenic CO2 (Cant) is estimated in the water column and is detected down to the bottom (1600 m) in 1985, resulting in an aragonite saturation horizon at 600 m that migrated up to 400 m in 2021 due to the accumulation of Cant. At the subsurface, the trend of Cant is estimated at +0.53±0.01 µmol kg−1 yr−1 with a detectable increase in the trend in recent years. At the surface during austral winter the oceanic fCO2 increased at a rate close to or slightly lower than in the atmosphere. To the contrary, in summer, we observed contrasting fCO2 and dissolved inorganic carbon (CT) trends depending on the decade and emphasizing the role of biological drivers on air–sea CO2 fluxes and pH inter-annual variability. The regional air–sea CO2 fluxes evolved from an annual source to the atmosphere of 0.8 molC m−2 yr−1 in 1985 to a sink of −0.5 molC m−2 yr−1 in 2020. Over 1985–2020, the annual pH trend in surface waters of -0.0165±0.0040 per decade was mainly controlled by the accumulation of anthropogenic CO2, but the summer pH trends were modulated by natural processes that reduced the acidification rate in the last decade. Using historical data from November 1962, we estimated the long-term trend for fCO2, CT, and pH, confirming that the progressive acidification was driven by the atmospheric CO2 increase. In 59 years this led to a diminution of 11 % for both aragonite and calcite saturation state. As atmospheric CO2 is expected to increase in the future, the pH and carbonate saturation state will decrease at a faster rate than observed in recent years. A projection of future CT concentrations for a high emission scenario (SSP5-8.5) indicates that the surface pH in 2100 would decrease to 7.32 in winter. This is up to −0.86 lower than pre-industrial pH and −0.71 lower than pH observed in 2020. The aragonite undersaturation in surface waters would be reached as soon as 2050 (scenario SSP5-8.5) and 20 years later for a stabilization scenario (SSP2-4.5) with potential impacts on phytoplankton species and higher trophic levels in the rich ecosystems of the Kerguelen Islands area.

Exploring the tidal response to bathymetry evolution and present-day sea level rise in a channel–shoal environment - Robert Lepper; Leon Jänicke; Ingo Hache; Christian Jordan; Frank Kösters (Ocean Science, 2024)
Intertidal flats and salt marshes in channel–shoal environments are at severe risk of drowning under sea level rise (SLR) ultimately ceasing their function of coastal defense. Earlier studies indicated that these environments can be resilient against moderate SLR as their mean height is believed to correlate with tidal amplitude and mean sea level. Recent morphological analyses in the German Wadden Sea on the northwestern European continental shelf contradicted this assumption as mean tidal flat accretion surpassed relative SLR, indicating that nonlinear feedback between SLR, coastal morphodynamics, and tidal dynamics played a role. We explored this relationship in the German Wadden Sea's channel–shoal environment by revisiting the sensitivity of tidal dynamics to observed SLR and coastal bathymetry evolution over one nodal cycle (1997 to 2015) with a numerical model. We found a proportional response of tidal high and low water to SLR when the bathymetry was kept constant. In contrast, coastal bathymetry evolution caused a spatially varying hydrodynamic reaction with both increases and decreases in patterns of tidal characteristics within a few kilometers. An explorative assessment of potential mechanisms suggested that energy dissipation declined near the coast, which we related to a decreasing tidal prism and declining tidal energy import. Our study stresses the fact that an accurate representation of coastal morphology in hindcasts, nowcasts, and ensembles for bathymetry evolution to assess the impact of SLR is needed when using numerical models.

Combining neural networks and data assimilation to enhance the spatial impact of Argo floats in the Copernicus Mediterranean biogeochemical model - Carolina Amadio; Anna Teruzzi; Gloria Pietropolli; Luca Manzoni; Gianluca Coidessa; Gianpiero Cossarini (Ocean Science, 2024)
Biogeochemical-Argo (BGC-Argo) float profiles provide substantial information on key vertical biogeochemical dynamics and have been successfully integrated in biogeochemical models via data assimilation approaches. Although BGC-Argo assimilation results have been encouraging, data scarcity remains a limitation with respect to their effective use in operational oceanography. To address availability gaps in the BGC-Argo profiles, an observing system experiment (OSE) that combines a neural network (NN) and data assimilation (DA) was performed here. A NN was used to reconstruct nitrate profiles, starting from oxygen profiles and associated Argo variables (pressure, temperature, and salinity), while a variational data assimilation scheme (3DVarBio) was upgraded to integrate BGC-Argo and reconstructed observations in the Copernicus Mediterranean operational forecast system (MedBFM). To ensure the high quality of oxygen data, a post-deployment quality control method was developed with the aim of detecting and eventually correcting potential sensors drift. The Mediterranean OSE features three different set-ups: a control run without assimilation; a multivariate run with assimilation of BGC-Argo chlorophyll, nitrate, and oxygen; and a multivariate run that also assimilates reconstructed observations. The general improvement in the skill performance metrics demonstrated the feasibility of integrating new variables (oxygen and reconstructed nitrate). Major benefits have been observed with respect to reproducing specific biogeochemical-process-based dynamics such as the nitracline dynamics, primary production, and oxygen vertical dynamics. The assimilation of BGC-Argo nitrate corrects a generally positive bias of the model in most of the Mediterranean areas, and the addition of reconstructed profiles makes the corrections even stronger. The impact of enlarged nitrate assimilation propagates to ecosystem processes (e.g. primary production) at a basin-wide scale, demonstrating the importance of the assimilation of BGC-Argo profiles in forecasting the biogeochemical ocean state.

Impact of Hurricane Irma on coral reef sediment redistribution at Looe Key Reef, Florida, USA - Kimberly K. Yates; Zachery Fehr; Selena Johnson; David Zawada (Ocean Science, 2024)
Understanding event-driven sediment transport in coral reef environments is essential to assessing impacts on reef species, habitats, restoration, and mitigation, yet a global knowledge gap remains due to limited quantitative studies. Hurricane Irma made landfall in the Lower Florida Keys with sustained 209 km h−1 winds and waves greater than 8 m on 10 September 2017, directly impacting the Florida Reef Tract (FRT) and providing an opportunity to perform a unique comprehensive, quantitative assessment of its impact on coral reef structure and sediment redistribution. We used lidar and multibeam derived digital elevation models (DEMs) collected before and after the passing of Hurricane Irma over a 15.98 km2 area along the lower FRT including Looe Key Reef to quantify changes in seafloor elevation, volume, and structure due to storm impacts. Elevation change was calculated at over 4 million point locations across 10 habitat types within this study area for two time periods using data collected (1) approximately 1 year before the passing of Irma and 3 to 6 months following the storm's impact as well as (2) 3 to 6 months after and up to 16.5 months after the storm. Elevation change data were then used to generate triangulated irregular network (TIN) models in ArcMap to calculate changes in seafloor volume during each time period. Our results indicate that Hurricane Irma was primarily a depositional event that increased mean seafloor elevation and volume at this study site by 0.34 m and up to 5.4 Mm3, respectively. Sediment was transported primarily west-southwest (WSW) and downslope, modifying geomorphic seafloor features including the migration of sand waves and rubble fields, formation of scour marks in shallow seagrass habitats, and burial of seagrass and coral-dominated habitats. Approximately 16.5 months after Hurricane Irma (during a 13-month period between 2017 and 2019), net erosion was observed across all habitats with mean elevation change of −0.15 m and net volume change up to −2.46 Mm3. Rates of elevation change during this post-storm period were 1 to 2 orders of magnitude greater than decadal and multi-decadal rates of change in the same location, and changes showed erosion of approximately 50 % of sediment deposited during the storm event as seafloor sediment distribution began to re-equilibrate to non-storm sea-state conditions. Our results suggest that higher-resolution elevation change data collected over seasonal and annual time periods could enhance characterization and understanding of short-term and long-term rates and processes of seafloor change.

On the short-term response of entrained air bubbles in the upper ocean: a case study in the north Adriatic Sea - Alvise Benetazzo; Trygve Halsne; Øyvind Breivik; Kjersti Opstad Strand; Adrian H. Callaghan; Francesco Barbariol; Silvio Davison; Filippo Bergamasco; Cristobal Molina; Mauro Bastianini (Ocean Science, 2024)
Air bubbles in the upper ocean are generated mainly by waves breaking at the air–sea interface. As such, after the waves break, entrained air bubbles evolve in the form of plumes in the turbulent flow, exchange gas with the surrounding water, and may eventually rise to the surface. To shed light on the short-term response of entrained bubbles in different stormy conditions and to assess the link between bubble plume penetration depth, mechanical and thermal forcings, and the air–sea transfer velocity of CO2, a field experiment was conducted from an oceanographic research tower in the north Adriatic Sea. Air bubble plumes were observed using high-resolution echosounder data from an upward-looking 1000 kHz sonar. The backscatter signal strength was sampled at a high resolution, 0.5 s in time and 2.5 cm along the vertical direction. Time series profiles of the bubble plume depth were established using a variable threshold procedure applied to the backscatter strength. The data show the occurrence of bubbles organized into vertical plume-like structures, drawn downwards by wave-generated turbulence and other near-surface circulations, and reaching the seabed at 17 m depth under strong forcing. We verify that bubble plumes adapt rapidly to wind and wave conditions and have depths that scale approximately linearly with wind speed. Scaling with the wind–wave Reynolds number is also proposed to account for the sea-state severity in the plume depth prediction. Results show a correlation between measured bubble depths and theoretical air–sea CO2 transfer velocity parametrized with wind-only and wind/wave formulations. Further, our measurements corroborate previous results suggesting that the sinking of newly formed cold-water masses helps bring bubbles to greater depths than those reached in stable conditions for the water column. The temperature difference between air and sea seems sufficient for describing this intensification at the leading order of magnitude. The results presented in this study are relevant for air–sea interaction studies and pave the way for progress in CO2 gas exchange formulations.

The different dynamic influences of Typhoon Kalmaegi on two pre-existing anticyclonic ocean eddies - Yihao He; Xiayan Lin; Guoqing Han; Yu Liu; Han Zhang (Ocean Science, 2024)
Using multi-source observational data and GLORYS12V1 reanalysis data, we conduct a comparative analysis of different responses of two warm eddies, AE1 and AE2 in the northern South China Sea, to Typhoon Kalmaegi during September 2014. The findings of our research are as follows: (1) for horizontal distribution, the area and the sea surface temperature (SST) of AE1 and AE2 decrease by about 31 % (36 %) and 0.4 °C (0.6 °C). The amplitude, Rossby number (Ro = relative vorticity / Coriolis parameter) and eddy kinetic energy (EKE) of AE1 increase by 1.3 cm (5.7 %), 1.4×10-2 (20.6 %) and 107.2 cm2 s−2 (49.2 %) after the typhoon, respectively, while AE2 weakens and the amplitude, Rossby number and EKE decrease by 3.1 cm (14.6 %), 1.6×10-2 (26.2 %) and 38.5 cm2 s−2 (20.2 %), respectively. (2) In the vertical direction, AE1 demonstrates enhanced convergence, leading to an increase in temperature and a decrease in salinity above 150 m. The response below the mixed-layer depth (MLD) is particularly prominent (1.3 °C). In contrast, AE2 experiences cooling and a decrease in salinity above the MLD. Below the MLD, it exhibits a subsurface temperature drop and salinity increase due to the upwelling of cold water induced by the suction effect of the typhoon. (3) The disparity in the responses of the two warm eddies can be attributed to their different positions relative to Typhoon Kalmaegi. Under the influence of negative wind stress curl outside the maximum wind radius (Rmax) of the typhoon, triggering negative Ekman pumping velocity (EPV) and quasi-geostrophic adjustment of the eddy, the warm eddy AE1, with its center to the left of the typhoon's path, further enhances the converging sinking of the upper warm water, resulting in its intensification. On the other hand, the warm eddy AE2, situated closer to the center of the typhoon, weakens due to the cold suction caused by the strong positive wind stress curl within the typhoon's Rmax. The same polarity eddies may have different response to typhoons. The distance between eddies and typhoons, eddy intensity, and the background field need to be considered.

The Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength - Romain Caneill; Fabien Roquet; Jonas Nycander (Ocean Science, 2024)
The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper-ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn–winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas Current and East Australian Current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations in the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent.

Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26° N - Harry Bryden; Jordi Beunk; Sybren Drijfhout; Wilco Hazeleger; Jennifer Mecking (Ocean Science, 2024)
The Coupled Model Intercomparison Project (CMIP) allows the assessment of the representation of the Atlantic Meridional Overturning Circulation (AMOC) in climate models. While CMIP Phase 6 models display a large spread in AMOC strength, the multi-model mean strength agrees reasonably well with observed estimates from RAPID1, but this does not hold for the AMOC's various components. In CMIP Phase 6 (CMIP6), the present-day AMOC is characterized by a lack of lower North Atlantic Deep Water (lNADW) due to the small scale of Greenland–Iceland–Scotland Ridge overflow and too much mixing. This is compensated for by increased recirculation in the subtropical gyre and more Antarctic Bottom Water (AABW). Deep-water circulation is dominated by a distinct deep western boundary current (DWBC) with minor interior recirculation compared with observations. The future decline in the AMOC of 7 Sv by 2100 under a Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5) emission scenario is associated with decreased northward western boundary current transport in combination with reduced southward flow of upper North Atlantic Deep Water (uNADW). In CMIP6, wind stress curl decreases with time by 14 % so that wind-driven thermocline recirculation in the subtropical gyre is reduced by 4 Sv (17 %) by 2100. The reduction in western boundary current transport of 11 Sv is more than the decrease in wind-driven gyre transport, indicating a decrease over time in the component of the Gulf Stream originating from the South Atlantic.

Resemblance of the global depth distribution of internal-tide generation and cold-water coral occurrences - Anna-Selma van der Kaaden; Dick van Oevelen; Christian Mohn; Karline Soetaert; Max Rietkerk; Johan van de Koppel; Theo Gerkema (Ocean Science, 2024)
Internal tides are known to be an important source of mixing in the oceans, especially in the bottom boundary layer. The depth of internal-tide generation therefore seems important for benthic life and the formation of cold-water coral mounds, but internal-tide conversion is generally investigated in a depth-integrated sense. Using both idealized and realistic simulations on continental slopes, we found that the depth of internal-tide generation increases with increasing slope steepness and decreases with intensified shallow stratification. The depth of internal-tide generation also shows a typical latitudinal dependency related to Coriolis effects. Using a global database of cold-water corals, we found that, especially in Northern Hemisphere autumn and winter, the global depth pattern of internal-tide generation correlates (rautumn = 0.70, rwinter = 0.65, p

Persistent climate model biases in the Atlantic Ocean's freshwater transport - René M. van Westen; Henk A. Dijkstra (Ocean Science, 2024)
The Atlantic Meridional Overturning Circulation (AMOC) is considered to be one of the most dangerous climate tipping elements. The salt–advection feedback plays an important role in AMOC tipping behaviour, and its strength is strongly connected to the freshwater transport carried by the AMOC at 34° S, below indicated by FovS. Available observations have indicated that FovS has a negative sign for the present-day AMOC. However, most climate models of the Coupled Model Intercomparison Project (CMIP, phase 3 and phase 5) have an incorrect FovS sign. Here, we analyse a high-resolution and a low-resolution version of the Community Earth System Model (CESM) to identify the origin of these FovS biases. Both CESM versions are initialised from an observed ocean state, and FovS biases quickly develop under fixed pre-industrial forcing conditions. The most important model bias is a too fresh Atlantic Surface Water, which arises from deficiencies in the surface freshwater flux over the Indian Ocean. The second largest bias is a too saline North Atlantic Deep Water and arises through deficiencies in the freshwater flux over the Atlantic Subpolar Gyre region. Climate change scenarios branched from the pre-industrial simulations have an incorrect FovS upon initialisation. Most CMIP phase 6 models have similar biases to those in the CESM. Due to the biases, the value of FovS is not in agreement with available observations, and the strength of the salt advection feedback is underestimated. Values of FovS are projected to decrease under climate change, and their response is also dependent on the various model biases. To better project future AMOC behaviour, an urgent effort is needed to reduce biases in the atmospheric components of current climate models.

Surface factors controlling the volume of accumulated Labrador Sea Water - Yavor Kostov; Marie-José Messias; Herlé Mercier; David P. Marshall; Helen L. Johnson (Ocean Science, 2024)
We explore historical variability in the volume of Labrador Sea Water (LSW) using ECCO, an ocean state estimate configuration of the Massachusetts Institute of Technology general circulation model (MITgcm). The model's adjoint, a linearization of the MITgcm, is set up to output the lagged sensitivity of the water mass volume to surface boundary conditions. This allows us to reconstruct the evolution of LSW volume over recent decades using historical surface wind stress, heat, and freshwater fluxes. Each of these boundary conditions contributes significantly to the LSW variability that we recover, but these impacts are associated with different geographical fingerprints and arise over a range of time lags. We show that the volume of LSW accumulated in the Labrador Sea exhibits a delayed response to surface wind stress and buoyancy forcing outside the convective interior of the Labrador Sea at important locations in the North Atlantic Ocean. In particular, patterns of wind and surface density anomalies can act as a “traffic controller” and regulate the North Atlantic Current's (NAC's) transport of warm and saline subtropical water masses that are precursors for the formation of LSW. This propensity for a delayed response of LSW to remote forcing allows us to predict a limited yet substantial and significant fraction of LSW variability at least 1 year into the future. Our analysis also enables us to attribute LSW variability to different boundary conditions and to gain insight into the major mechanisms that contribute to volume anomalies in this deep water mass. We point out the important role of key processes that promote the formation of LSW in both the Irminger and Labrador seas: buoyancy loss and preconditioning along the NAC pathway and in the Iceland Basin, the Irminger Sea, and the Nordic Seas.

Influence of stratification and wind forcing on the dynamics of Lagrangian residual velocity in a periodically stratified estuary - Fangjing Deng; Feiyu Jia; Rui Shi; Shuwen Zhang; Qiang Lian; Xiaolong Zong; Zhaoyun Chen (Ocean Science, 2024)
Wind and stratification play pivotal roles in shaping the structure of the Lagrangian residual velocity (LRV). However, the intricate dynamics by which wind and stratification modify the LRV remain poorly studied. This study derives numerical solutions of LRV components and eddy viscosity subcomponents to elucidate the dynamics within the periodically stratified Pearl River estuary. The vertical shear cross-estuary LRV (uL) is principally governed by the interplay among the eddy viscosity component (uLtu), the barotropic component (uLba), and the baroclinic component (uLgr) under stratified conditions. During neap tides, southwesterly winds notably impact uL by escalating uLtu by an order of magnitude within the upper layer. This transforms the eastward flow dominated by uLtu under wind influence into a westward flow dominated by uLba in upper shoal regions without wind forcing. The along-estuary LRV exhibits a gravitational circulation characterized by upper-layer outflow engendered by a barotropic component (vLba) and lower-layer inflow predominantly driven by a baroclinic component (vLgr). The presence of southwesterly winds suppresses along-estuary gravitational circulation by diminishing the magnitude of vLba and vLgr. The contributions of vLba and vLgr are approximately equal, while the ratio between uLba and uLgr (uLtu) fluctuates within the range of 1 to 2 in stratified waters. Under unstratified conditions, LRV exhibits a lateral shear structure due to differing dominant components compared to stratified conditions. In stratified scenarios, the eddy viscosity component of LRV is predominantly governed by the turbulent mean component, while it succumbs to the influence of the tidal straining component in unstratified waters.

Exploring the relationship between sea ice and phytoplankton growth in the Weddell Gyre using satellite and Argo float data - Clara Celestine Douglas; Nathan Briggs; Peter Brown; Graeme MacGilchrist; Alberto Naveira Garabato (Ocean Science, 2024)
Some of the highest rates of primary production across the Southern Ocean occur in the seasonal ice zone (SIZ), making this a prominent area of importance for both local ecosystems and the global carbon cycle. There, the annual advance and retreat of ice impacts light and nutrient availability, as well as the circulation and stratification, thereby imposing a dominant control on phytoplankton growth. In this study, the drivers of variability in phytoplankton growth between 2002–2020 in the Weddell Gyre SIZ were assessed using satellite net primary production (NPP) products alongside chlorophyll-a and particulate organic carbon (POC) data from autonomous biogeochemical floats. Although the highest daily rates of NPP are consistently observed in the continental shelf region (water depths shallower than 2000 m), the open-ocean region's larger size and longer ice-free season mean that it dominates biological carbon uptake within the Weddell Gyre, accounting for 93 %–96 % of the basin's total annual NPP. Variability in the summer maximum ice-free area is the strongest predictor of inter-annual variability in total NPP across the Weddell Gyre, with greater ice-free area resulting in greater annual NPP, explaining nearly half of the variance (R2=42 %). In the shelf region, the return of sea ice cover controls the end of the productive season. In the open ocean, however, both satellite NPP and float data show that a decline in NPP occurs before the end of the ice-free season (∼ 80 to 130 d after sea ice retreat). Evidence of concurrent increases in float-observed chlorophyll-a and POC suggest that later in the summer season additional factors such as micro-nutrient availability or top-down controls (e.g. grazing) could be limiting NPP. These results indicate that in a warmer and more ice-free Weddell Gyre, notwithstanding compensating changes in nutrient supply, NPP is likely to be enhanced only up to a certain limit of ice-free days.

Tipping of the double-diffusive regime in the southern Adriatic Pit in 2017 in connection with record high-salinity values - Felipe L. L. Amorim; Julien Le Meur; Achim Wirth; Vanessa Cardin (Ocean Science, 2024)
In double-diffusive mixing, whenever salinity and temperature decrease with depth, the water column is either unstable or predisposed to a state called salt fingering (SF), which exhibits increased vertical mixing. Analysis of a high-frequency time series of thermohaline data measured at the EMSO-E2M3A regional facility in the southern Adriatic Pit (SAP) from 2014 to 2019 reveals that in the south Adriatic, SF is the dominant regime. The same time series shows the presence of a very saline core of the Levantine Intermediate Water that penetrated with unprecedented strength during the winter of 2016/17 at around 550 dbar and even higher-salinity water above. The effect of strong heat loss at the surface during that winter allowed deep convection to transport this high-salinity water from the intermediate to the deep layers within the pit. This resulted in an increased predisposition to SF throughout the water column. In the subsurface layer (350 to 550 dbar) the increase is from 27 % to 72 % of observations. We observe an alteration of vertical stratification throughout the water column during the winter of 2016/17 from a stratified water column to an almost homogeneous water column down to 700 dbar, with no return in the following years.

Investigating extreme marine summers in the Mediterranean Sea - Dimitra Denaxa; Gerasimos Korres; Emmanouil Flaounas; Maria Hatzaki (Ocean Science, 2024)
The Mediterranean Sea (MS) has undergone significant surface warming, particularly pronounced during summers and associated with devastating impacts on marine life. Alongside the ongoing research on warming trends and marine heatwaves (MHWs), here we address the importance of understanding anomalously warm conditions also on the seasonal timescale. We propose the concept of extreme marine summers (EMSs) and investigate their characteristics in the MS, using sea surface temperature (SST) reanalysis data spanning 1950–2020. We define EMSs at a particular location, as the summers with a mean summer SST exceeding the 95th percentile. A marine summer may become extreme, under various SST substructures. Results show that, in most of the basin, EMSs are formed primarily due to the warmer summer days being warmer than normal. Areas where the warmest (coldest) part of the SST distribution is more variable experience EMSs primarily due to the warmest (coldest) part of the distribution being anomalously warm. MHWs occurring within EMSs are more intense, longer lasting, and more frequent than usual mainly in the northern MS regions. These enhanced MHW conditions occur mainly within the warmest part of the SST distribution. By means of temporal coverage of MHW conditions, a more pronounced occurrence of MHWs in EMSs is found for the central and eastern basin where up to 55 % of MHW days over 1950–2020 fall within EMSs. The role of air–sea heat fluxes in driving EMSs is quantified through a newly proposed metric. Results suggest that surface fluxes primarily drive EMSs in the northern half of the MS, while oceanic processes play a major role in southern regions. Upper-ocean preconditioning also contributes to the formation of EMSs. Finally, a detrended dataset was produced to examine how the SST multi-decadal variability affects the studied EMS features. Despite leading to warmer EMSs basin-wide, the multi-decadal signal does not significantly affect the dominant SST substructures during EMSs. Results also highlight the fundamental role of latent heat flux in modulating the surface heat budget during EMSs, regardless of the long-term trends.

Machine learning methods to predict sea surface temperature and marine heatwave occurrence: a case study of the Mediterranean Sea - Giulia Bonino; Giuliano Galimberti; Simona Masina; Ronan McAdam; Emanuela Clementi (Ocean Science, 2024)
Marine heatwaves (MHWs) have significant social and ecological impacts, necessitating the prediction of these extreme events to prevent and mitigate their negative consequences and provide valuable information to decision-makers about MHW-related risks. In this study, machine learning (ML) techniques are applied to predict sea surface temperature (SST) time series and marine heatwaves in 16 regions of the Mediterranean Sea. ML algorithms, including the random forest (RForest), long short-term memory (LSTM), and convolutional neural network (CNN), are used to create competitive predictive tools for SST. The ML models are designed to forecast SST and MHWs up to 7 d ahead. For each region, we performed 15 different experiments for ML techniques, progressively sliding the training and the testing period window of 4 years from 1981 to 2017. Alongside SST, other relevant atmospheric variables are utilized as potential predictors of MHWs. Datasets from the European Space Agency Climate Change Initiative (ESA CCI SST) v2.1 and the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis from 1981 to 2021 are used to train and test the ML techniques. For each area, the results show that all the ML methods performed with minimum root mean square errors (RMSEs) of about 0.1 °C at a 1 d lead time and maximum values of about 0.8 °C at a 7 d lead time. In all regions, both the RForest and LSTM consistently outperformed the CNN model across all lead times. LSTM has the highest predictive skill in 11 regions at all lead times. Importantly, the ML techniques show results similar to the dynamical Copernicus Mediterranean Forecasting System (MedFS) for both SST and MHW forecasts, especially in the early forecast days. For MHW forecasting, ML methods compare favorably with MedFS up to 3 d lead time in 14 regions, while MedFS shows superior skill at 5 d lead time in 9 out of 16 regions. All methods predict the occurrence of MHWs with a confidence level greater than 50 % in each region. Additionally, the study highlights the importance of incoming solar radiation as a significant predictor of SST variability along with SST itself.

Biophysical coupling of seasonal chlorophyll- a bloom variations and phytoplankton assemblages across the Peninsula Front in the Bransfield Strait - Marta Veny; Borja Aguiar-González; Ángeles Marrero-Díaz; Tania Pereira-Vázquez; Ángel Rodríguez-Santana (Ocean Science, 2024)
This study investigates the spatio-temporal variations in the chlorophyll-a (chl-a) blooms in the Bransfield Strait (BS) at a climatological scale (1998–2018). We propose that suitable monitoring of these blooms can be achieved through remotely sensed observations only if the BS is divided following the Peninsula Front (PF), which ultimately influences the phytoplankton assemblage. Our analysis is based on characterizing climatological fields of sea surface temperature (SST), air temperature, sea ice coverage, chl-a concentrations and wind stress, guided by synoptic novel and historical in situ observations which reveal two niches for phytoplankton assemblage: the Transitional Bellingshausen Water (TBW) and Transitional Weddell Water (TWW) pools. The TBW pool features stratified, less saline, warmer waters with shallow mixed layers, while the TWW pool features well-mixed, saltier, and colder waters. We identify that the 0.6 °C isotherm corresponds to the summertime climatological PF location, effectively dividing the BS into two different scenarios. Furthermore, the 0.5 mg m−3 chl-a isoline aligns well with the 0.6 °C isotherm, serving as a threshold for chl-a blooms of the highest concentrations around the South Shetland Islands. For the first time, these thresholds enable the monthly climatological descriptions of the two blooms developing in the BS on both sides of the PF. We think this approach underscores the potential of combining SST and chl-a data to monitor the year-to-year interplay of the chl-a blooms occurring in the TBW and TWW pools contoured by the PF.

Effects of sea level rise and tidal flat growth on tidal dynamics and geometry of the Elbe estuary - Tara F. Mahavadi; Rita Seiffert; Jessica Kelln; Peter Fröhle (Ocean Science, 2024)
Future global mean sea level rise (SLR) will affect coastlines and estuaries in the North Sea and therefore also coastal protection structures, unique local ecosystems and important waterways. SLR will not only raise water levels but also influence tidal dynamics and morphodynamics, which is why the tidal flats of the Wadden Sea can grow to a certain extent with SLR. Investigations on the effects of climate-change-induced SLR and the related potential bathymetric changes inside of estuaries form an important basis for identifying vulnerabilities and developing appropriate adaptation strategies. To analyse the influence of potential SLR and tidal flat elevation scenarios on the tidal dynamics in the Elbe estuary, we used a highly resolved hydrodynamic numerical model of the German Bight. The analysis results show increasing tidal range in the Elbe estuary solely due to SLR. They also reveal strongly varying changes with different tidal flat growth scenarios: while tidal flat elevation up to the mouth of the estuary can cause tidal range to decrease relative to SLR alone, tidal flat elevation in the entire estuary can lead to an increase in tidal range relative to SLR alone. Further analyses show how the geometric parameters of the Elbe estuary are changing due to SLR and tidal flat elevation. We discuss how these changes in estuarine geometry can provide an explanation for the changes in tidal range.

Drivers of Laptev Sea interannual variability in salinity and temperature - Phoebe A. Hudson; Adrien C. H. Martin; Simon A. Josey; Alice Marzocchi; Athanasios Angeloudis (Ocean Science, 2024)
Eurasian rivers provide a quarter of total fresh water to the Arctic, maintaining a persistent fresh layer that covers the surface Arctic Ocean. This freshwater export controls Arctic Ocean stratification, circulation, and basin-wide sea ice concentration. The Lena River supplies the largest volume of runoff and plays a key role in this system, as runoff outflows into the Laptev Sea as a particularly shallow plume. Previous in situ and modelling studies suggest that local wind forcing is a driver of variability in Laptev sea surface salinity (SSS) but there is no consensus on the roles of Lena River discharge and sea ice cover in contributing to this variability or on the dominant driver of variability. Until recently, satellite SSS retrievals were insufficiently accurate for use in the Arctic. However, retreating sea ice cover and continuous progress in satellite product development have significantly improved SSS retrievals, giving satellite SSS data true potential in the Arctic. In this region, satellite-based SSS is found to agree well with in situ data (r>0.8) and provides notable improvements compared to the reanalysis product used in this study (r>0.7) in capturing patterns and variability observed in in situ data. This study demonstrates a novel method of identifying the dominant drivers of interannual variability in Laptev Sea dynamics within reanalysis products and testing if these relationships appear to hold in satellite-based SSS, sea surface temperature (SST) data, and in situ observations. The satellite SSS data firmly establish what is suggested by reanalysis products and what has previously been subject to debate due to the limited years and locations analysed with in situ data; the zonal wind is the dominant driver of offshore or onshore Lena River plume transport. The eastward wind confines the plume to the southern Laptev Sea and drives alongshore transport into the East Siberian Sea, and westward wind drives offshore plume transport into the northern Laptev Sea. This finding is affirmed by the strong agreement in SSS pattern under eastward and westward wind regimes in all reanalyses and satellite products used in this study, as well as with in situ data. The pattern of SST also varies with the zonal wind component and drives spatial variability in sea ice concentration.

Dependency of simulated tropical Atlantic current variability on the wind forcing - Kristin Burmeister; Franziska U. Schwarzkopf; Willi Rath; Arne Biastoch; Peter Brandt; Joke F. Lübbecke; Mark Inall (Ocean Science, 2024)
The upper wind-driven circulation in the tropical Atlantic Ocean plays a key role in the basin-wide distribution of water mass properties and affects the transport of heat, freshwater, and biogeochemical tracers such as oxygen or nutrients. It is crucial to improve our understanding of its long-term behaviour, which largely relies on model simulations and applied forcing due to sparse observational data coverage, especially before the mid-2000s. Here, we apply two different forcing products, the Coordinated Ocean-ice Reference Experiments (CORE) v2 and the Japanese 55-year Reanalysis (JRA55-do) surface dataset, to a high-resolution ocean model. Where possible, we compare the simulated results to long-term observations. We find large discrepancies between the two simulations regarding the wind and current field. In the CORE simulation, strong, large-scale wind stress curl amplitudes above the upwelling regions of the eastern tropical North Atlantic seem to cause an overestimation of the mean and seasonal variability in the eastward subsurface current just north of the Equator. The wind stress curl of JRA55-do forcing shows much finer structures, and the JRA55-do simulation is in better agreement with the mean and intraseasonal fluctuations in the subsurface current found in observations. The northern branch of the South Equatorial Current flows westward at the surface just north of the Equator. On interannual to decadal timescales, it shows a high correlation of R=0.9 with the zonal wind stress in the CORE simulation but only a weak correlation of R=0.35 in the JRA55-do simulation. We also identify similarities between the two simulations. The strength of the eastward-flowing North Equatorial Counter Current located between 3 and 10° N covaries with the strength of the meridional wind stress just north of the Equator on interannual to decadal timescales in the two simulations. Both simulations present a comparable mean, seasonal cycle and trend of the eastward off-equatorial subsurface current south of the Equator but underestimate the current strength by half compared to observations. In both simulations, the eastward-flowing Equatorial Undercurrent weakened between 1990 and 2009. In the JRA simulation, which covers the modern period of observations, the Equatorial Undercurrent strengthened again between 2008 to 2018, which agrees with observations, although the simulation underestimates the strengthening by over a third. We propose that long-term observations, once they have reached a critical length, need to be used to test the quality of wind-driven simulations. This study presents one step in this direction.

Fjord circulation permits a persistent subsurface water mass in a long, deep mid-latitude inlet - Laura Bianucci; Jennifer M. Jackson; Susan E. Allen; Maxim V. Krassovski; Ian J. W. Giesbrecht; Wendy C. Callendar (Ocean Science, 2024)
Fjords are deep nearshore zones that connect watersheds and oceans, typically behaving as an estuary. In some fjords, strong katabatic winds in winter (also known as Arctic outflow wind events) can lead to cooling and reoxygenation of subsurface waters, with effects lasting until the following autumn, as observed in 2019 in Bute Inlet, British Columbia, Canada. We used high-resolution, three-dimensional ocean model summer simulations to investigate the mechanisms allowing for the persistence of these cool, oxygen-rich subsurface conditions in Bute Inlet. The slow residual circulation underneath the brackish outflow (and consequent slow advection) in this long, deep fjord is a main reason why the cold subsurface water mass stays in place until conditions change in autumn (i.e., start of stronger wind mixing and reduced freshwater forcing). Another mechanism is a positive feedback provided by the presence of this subsurface water mass, since it further reduces the already weak residual circulation. These findings are applicable to any similar long, deep fjord that experiences katabatic winds in winter, and they could have implications not only for the preservation of water masses but other possible subsurface features (e.g., pollutant spills, planktonic larvae). Furthermore, the identification of mechanisms that permit persistent cold and oxygenated conditions is key to understanding potential areas of ecological refugia in a warming and deoxygenating ocean.

Contribution of satellite sea surface salinity to the estimation of liquid freshwater content in the Beaufort Sea - Marta Umbert; Eva De Andrés; Maria Sánchez; Carolina Gabarró; Nina Hoareau; Veronica González-Gambau; Aina García-Espriu; Estrella Olmedo; Roshin P. Raj; Jiping Xie; Rafael Catany (Ocean Science, 2024)
The hydrography of the Arctic Ocean has experienced profound changes over the last 2 decades. The sea ice extent has declined by more than 10 % per decade, and its liquid freshwater content has increased mainly due to glaciers and sea ice melting. Further, new satellite retrievals of sea surface salinity (SSS) in the Arctic might contribute to better characterizing the freshwater changes in cold regions. Ocean salinity and freshwater content are intimately related such that an increase (decrease) in one entails a decrease (increase) in the other. In this work, we evaluate the freshwater content in the Beaufort Gyre using surface salinity measurements from the satellite radiometric mission Soil Moisture and Ocean Salinity (SMOS) and TOPAZ4b reanalysis salinity at depth, estimating the freshwater content from 2011 to 2019 and validating the results with in situ measurements. The results highlight the underestimation of the freshwater content using reanalysis data in the Beaufort Sea and a clear improvement in the freshwater content estimation when adding satellite sea surface salinity measurements in the mixed layer. The improvements are significant, with up to a 70 % reduction in bias in areas near the ice melting. Our research demonstrates how remotely sensed salinity can assist us in better monitoring the changes in the Arctic freshwater content and understanding key processes related to salinity variations that cause density differences with potential to influence the global circulation system that regulates Earth's climate.

Predictability of marine heatwaves: assessment based on the ECMWF seasonal forecast system - Eric de Boisséson; Magdalena Alonso Balmaseda (Ocean Science, 2024)
Marine heatwaves (MHWs), defined as prolonged period of extremely warm sea surface temperature (SST), have been receiving a lot of attention in the past decade as their frequency and intensity increase in a warming climate. This paper investigates the extent to which the seasonal occurrence and duration of MHWs can be predicted with the European Centre for Medium-Range Weather Forecast (ECMWF) operational seasonal forecast system. The prediction of the occurrence of MHW events, the number of MHW days per season, and their intensity and spatial extent are derived from seasonal SST forecasts and evaluated against an observation-based SST analysis using both deterministic and probabilistic metrics over the 1982–2021 period. Forecast scores show useful skill in predicting the occurrence of MHWs globally for the two seasons following the starting date. The skill is the highest in the El Niño region, the Caribbean, the wider tropics, the north-eastern extra-tropical Pacific, and southwest of the extra-tropical basins. The skill is not as good for other midlatitude eastern basins nor for the Mediterranean, with the forecast system being able to represent the low-frequency modulation of MHWs but showing poor skill in predicting the interannual variability of the MHW characteristics. Linear trend analysis shows an increase in MHW occurrence at a global scale, which the forecasts capture well.

Numerical investigation of interaction between anticyclonic eddy and semidiurnal internal tide in the northeastern South China Sea - Liming Fan; Hui Sun; Qingxuan Yang; Jianing Li (Ocean Science, 2024)
We investigate the interaction between an anticyclonic eddy (AE) and semidiurnal internal tide (SIT) on the continental slope of the northeastern South China Sea (SCS), using a high spatiotemporal resolution numerical model. Two key findings are as follows: first, the AE promotes energy conversion from low-mode to higher-mode SIT. Additionally, production terms indicate that energy is also transferred from the SIT field to the eddy field at an average rate of 3.0 mW m−2 (accounting for 7 % of the incoming energy flux of SIT when integrated over the eddy diameter). Second, the AE can modify the spatial distribution of tidal-induced dissipation by refracting, scattering, and reflecting low-mode SIT. The phase and group velocities of the SIT are significantly influenced by the eddy field, resulting in a northward or southward shift in the internal tidal rays. These findings deepen our understanding of the complex interactions between AE and SIT, as well as their impacts on energy conversion, wave propagation, and coastal processes.

Linking satellites to genes with machine learning to estimate phytoplankton community structure from space - Roy El Hourany; Juan Pierella Karlusich; Lucie Zinger; Hubert Loisel; Marina Levy; Chris Bowler (Ocean Science, 2024)
Ocean color remote sensing has been used for more than 2 decades to estimate primary productivity. Approaches have also been developed to disentangle phytoplankton community structure based on spectral data from space, in particular when combined with in situ measurements of photosynthetic pigments. Here, we propose a new ocean color algorithm to derive the relative cell abundance of seven phytoplankton groups, as well as their contribution to total chlorophyll a (Chl a) at the global scale. Our algorithm is based on machine learning and has been trained using remotely sensed parameters (reflectance, backscattering, and attenuation coefficients at different wavelengths, plus temperature and Chl a) combined with an omics-based biomarker developed using Tara Oceans data representing a single-copy gene encoding a component of the photosynthetic machinery that is present across all phytoplankton, including both prokaryotes and eukaryotes. It differs from previous methods which rely on diagnostic pigments to derive phytoplankton groups. Our methodology provides robust estimates of the phytoplankton community structure in terms of relative cell abundance and contribution to total Chl a concentration. The newly generated datasets yield complementary information about different aspects of phytoplankton that are valuable for assessing the contributions of different phytoplankton groups to primary productivity and inferring community assembly processes. This makes remote sensing observations excellent tools to collect essential biodiversity variables (EBVs) and provide a foundation for developing marine biodiversity forecasts.

Unsupervised classification of the northwestern European seas based on satellite altimetry data - Lea Poropat; Dani Jones; Simon D. A. Thomas; Céline Heuzé (Ocean Science, 2024)
From generating metrics representative of a wide region to saving costs by reducing the density of an observational network, the reasons to split the ocean into distinct regions are many. Traditionally, this has been done somewhat arbitrarily using the bathymetry and potentially some artificial latitude–longitude boundaries. We use an ensemble of Gaussian mixture models (GMMs, unsupervised classification) to separate the complex northwestern European coastal region into classes based on sea level variability observed by satellite altimetry. To reduce the dimensionality of the data, we perform a principal component analysis on 27 years of observations and use the spatial components as input for the GMM. The number of classes or mixture components is determined by locating the maximum of the silhouette score and by testing several models. We use an ensemble approach to increase the robustness of the classification and to allow the separation into more regions than a single GMM can achieve. We also vary the number of empirical orthogonal function (EOF) maps and show that more EOFs result in a more detailed classification. With three EOFs, the area is classified into four distinct regions delimited mainly by bathymetry. Adding more EOFs results in further subdivisions that resemble oceanic fronts. To achieve a more detailed separation, we use a model focused on smaller regions, specifically the Baltic Sea, North Sea, and the Norwegian Sea.

Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially to well-mixed main estuary - Zhongyuan Lin; Guang Zhang; Huazhi Zou; Wenping Gong (Ocean Science, 2024)
Salt intrusion in estuaries has been exacerbated by climate change and human activities. Previous studies have primarily focused on salt intrusion in the mainstem of estuaries, whereas those in sub-estuaries (those that branch off their main estuaries) have received less attention. During an extended La Niña event from 2021 to 2022, a sub-estuary (the East River estuary) alongside the Pearl River estuary, China, experienced severe salt intrusions, posing a threat to the freshwater supply in the surrounding area. Observations revealed that maximum salinities in the main estuary typically preceded spring tides, exhibiting significant asymmetry in salinity rise and fall over a fortnightly timescale. In contrast, in the upstream region of the sub-estuary, the variation in salinity was in phase with that of the tidal range, and the rise and fall of the salinity were more symmetrical. Inspired by these observations, we employed idealized numerical models and analytical solutions to investigate the underlying physics behind these behaviors. It was discovered that under normal dry conditions (with a river discharge of 1500 m3 s−1 at the head of the main estuary), the river–tide interaction and change in horizontal dispersion accounted for the in-phase relationship between the salinity and tidal range in the upstream region of the sub-estuary. Under extremely dry conditions (i.e., a river discharge of 500 m3 s−1 at the head of the main estuary), salinity variations were in phase with those of the tidal range in the middle as well as the upstream region of the sub-estuary. The variation in salinity in the main estuary along with those in salt dispersion and freshwater influx inside the sub-estuary collectively influenced salinity variation in the well-mixed sub-estuary. These findings have important implications for water resource management and salt intrusion prevention in the catchment area.

Ensemble analysis and forecast of ecosystem indicators in the North Atlantic using ocean colour observations and prior statistics from a stochastic NEMO–PISCES simulator - Mikhail Popov; Jean-Michel Brankart; Arthur Capet; Emmanuel Cosme; Pierre Brasseur (Ocean Science, 2024)
This study is anchored in the H2020 SEAMLESS project (https://www.seamlessproject.org, last access: 29 January 2024), which aims to develop ensemble assimilation methods to be implemented in Copernicus Marine Service monitoring and forecasting systems, in order to operationally estimate a set of targeted ecosystem indicators in various regions, including uncertainty estimates. In this paper, a simplified approach is introduced to perform a 4D (space–time) ensemble analysis describing the evolution of the ocean ecosystem. An example application is provided, which covers a limited time period in a limited subregion of the North Atlantic (between 31 and 21∘ W, between 44 and 50.5∘ N, between 15 March and 15 June 2019, at a 1/4∘ and a 1 d resolution). The ensemble analysis is based on prior ensemble statistics from a stochastic NEMO (Nucleus for European Modelling of the Ocean)–PISCES simulator. Ocean colour observations are used as constraints to condition the 4D prior probability distribution. As compared to classic data assimilation, the simplification comes from the decoupling between the forward simulation using the complex modelling system and the update of the 4D ensemble to account for the observation constraint. The shortcomings and possible advantages of this approach for biogeochemical applications are discussed in the paper. The results show that it is possible to produce a multivariate ensemble analysis continuous in time and consistent with the observations. Furthermore, we study how the method can be used to extrapolate analyses calculated from past observations into the future. The resulting 4D ensemble statistical forecast is shown to contain valuable information about the evolution of the ecosystem for a few days after the last observation. However, as a result of the short decorrelation timescale in the prior ensemble, the spread of the ensemble forecast increases quickly with time. Throughout the paper, a special emphasis is given to discussing the statistical reliability of the solution. Two different methods have been applied to perform this 4D statistical analysis and forecast: the analysis step of the ensemble transform Kalman filter (with domain localization) and a Monte Carlo Markov chain (MCMC) sampler (with covariance localization), both enhanced by the application of anamorphosis to the original variables. Despite being very different, the two algorithms produce very similar results, thus providing support to each other's estimates. As shown in the paper, the decoupling of the statistical analysis from the dynamical model allows us to restrict the analysis to a few selected variables and, at the same time, to produce estimates of additional ecological indicators (in our example: phenology, trophic efficiency, downward flux of particulate organic matter). This approach can easily be appended to existing operational systems to focus on dedicated users' requirements, at a small additional cost, as long as a reliable prior ensemble simulation is available. It can also serve as a baseline to compare with the dynamical ensemble forecast and as a possible substitute whenever useful.

New insights into the Weddell Sea ecosystem applying a quantitative network approach - Tomás I. Marina; Leonardo A. Saravia; Susanne Kortsch (Ocean Science, 2024)
Network approaches can shed light on the structure and stability of complex marine communities. In recent years, such approaches have been successfully applied to study polar ecosystems, improving our knowledge on how they might respond to ongoing environmental changes. The Weddell Sea is one of the most studied marine ecosystems outside the Antarctic Peninsula in the Southern Ocean. Yet, few studies consider the known complexity of the Weddell Sea food web, which in its current form comprises 490 species and 16 041 predator–prey interactions. Here we analysed the Weddell Sea food web, focusing on the species and trophic interactions that underpin ecosystem structure and stability. We estimated the strength for each interaction in the food web, characterised species position in the food web using unweighted and weighted food web properties, and analysed species' roles with respect to the stability of the food web. We found that the distribution of the interaction strength (IS) at the food web level is asymmetric, with many weak interactions and few strong ones. We detected a positive relationship between species median IS and two unweighted properties (i.e. trophic level and the total number of interactions). We also found that only a few species possess key positions in terms of food web stability. These species are characterised by high median IS, a middle to high trophic level, a relatively high number of interactions, and middle to low trophic similarity. In this study, we integrated unweighted and weighted food web information, enabling a more complete assessment of the ecosystem structure and function of the Weddell Sea food web. Our results provide new insights, which are important for the development of effective policies and management strategies, particularly given the ongoing initiative to implement a marine protected area (MPA) in the Weddell Sea.

Water properties and bottom water patterns in hadal trench environments - Jessica Kolbusz; Jan Zika; Charitha Pattiaratchi; Alan Jamieson (Ocean Science, 2024)
We examine baseline water properties and bottom water patterns in hadal trench environments across the Southern Ocean, Indian Ocean, and western Pacific. Significant differences are identified in the South Fiji Basin and surrounding the Philippine Sea, primarily due to the movement of cold Lower Circumpolar Deep Water along topographic features, highlighting the importance of a trench's geospatial position. We present the first hydrographic profiles in the Java Trench, warranting further research. Salinity increases with increasing depth for profiles over 10 000 dbar, with potential causes including instrumentation error, internal mixing, and saline pore water expulsion. These hadopelagic variations are crucial for assessing climate change impacts, especially regarding Antarctic Bottom Water. The study underscores the importance of incorporating these adiabatic conditions to gain insights into ecological biodiversity, alongside the indispensable baseline conditions presented, which are crucial for future oceanographic research across multiple disciplines.

Characterization of physical properties of a coastal upwelling filament with evidence of enhanced submesoscale activity and transition from balanced to unbalanced motions in the Benguela upwelling region - Ryan P. North; Julia Dräger-Dietel; Alexa Griesel (Ocean Science, 2024)
We combine high-resolution in situ data (acoustic Doppler current profiler (ADCP), Scanfish, and surface drifters) and remote sensing to investigate the physical characteristics of a major filament observed in the Benguela upwelling region. The 30–50 km wide and about 400 km long filament persisted for at least 40 d. Mixed-layer depths were less than 40 m in the filament and over 60 m outside of it. Observations of the Rossby number Ro from the various platforms provide the spatial distribution of Ro for different resolutions. Remote sensing focuses on geostrophic motions of the region related to the mesoscale eddies that drive the filament formation and thereby reveals |Ro| 1 for scales smaller than ∼500 m. This suggests that filaments are hotspots of ageostrophic dynamics, pointing to the presence of a forward energy cascade. The different dynamics indicated by our Ro analysis are confirmed by horizontal kinetic energy wavenumber spectra, which exhibit a power law k−α with α∼5/3 for wavelengths 2π/k smaller than a transition scale of 15 km, supporting significant submesoscale energy at scales smaller than the first baroclinic Rossby radius (Ro1∼30 km). The detected transition scale is smaller than those found in regions with less mesoscale eddy energy, consistent with previous studies. We found evidence for the processes which drive the energy transfer to turbulent scales. Positive Rossby numbers 𝒪(1) associated with cyclonic motion inhibit the occurrence of positive Ertel potential vorticity (EPV) and stabilize the water column. However, where the baroclinic component of EPV dominates, submesoscale instability analysis suggests that mostly gravitational instabilities occur and that symmetric instabilities may be important at the filament edges.

Altered Weddell Sea warm- and dense-water pathways in response to 21st-century climate change - Cara Nissen; Ralph Timmermann; Mathias van Caspel; Claudia Wekerle (Ocean Science, 2024)
The transport of water masses with ocean circulation is a key component of the global climate system. In this context, the Filchner Trough in the southern Weddell Sea is critical, as it is a hotspot for the cross-shelf-break exchange of Dense Shelf Water and Warm Deep Water. We present results from Lagrangian particle tracking experiments in a global-ocean–sea-ice model (FESOM-1.4) which includes ice-shelf cavities and has eddy-permitting resolution on the southern Weddell Sea continental shelf. With backward and forward experiments, we assess changes between a present-day and a future (SSP5-8.5) time slice in the origin of waters reaching the Filchner Ice Shelf front and the fate of waters leaving it. We show that particles reaching the ice-shelf front from the open ocean originate from 173 % greater depths by 2100 (median; 776 m as compared to 284 m for the present day), while waters leaving the cavity towards the open ocean end up at 35 % shallower depths (550 m as compared to 850 m for the present day). Pathways of water leaving the continental shelf increasingly occur in the upper ocean, while the on-shelf flow of waters that might reach the ice-shelf cavity, i.e., at deeper layers, becomes more important by 2100. Simultaneously, median transit times between the Filchner Ice Shelf front and the continental shelf break decrease (increase) by 6 (9.5) months in the backward (forward) experiments. In conclusion, our study demonstrates the sensitivity of regional circulation patterns in the southern Weddell Sea to ongoing climate change, with direct implications for ice-shelf basal melt rates and local ecosystems.

Transport dynamics in a complex coastal archipelago - Elina Miettunen; Laura Tuomi; Antti Westerlund; Hedi Kanarik; Kai Myrberg (Ocean Science, 2024)
The Archipelago Sea (in the Baltic Sea) is characterised by thousands of islands of various sizes and steep gradients of the bottom topography. Together with the much deeper Åland Sea, the Archipelago Sea acts as a pathway to the water exchange between the neighbouring basins, Baltic proper and Bothnian Sea. We studied circulation and water transports in the Archipelago Sea using a new configuration of the NEMO 3D hydrodynamic model that covers the Åland Sea–Archipelago Sea region with a horizontal resolution of around 500 m. The results show that currents are steered by the geometry of the islands and straits and the bottom topography. Currents are highest and strongly aligned in the narrow channels in the northern part of the area, with the directions alternating between south and north. In more open areas, the currents are weaker with wider directional distribution. During our study period of 2013–2017, southward currents were more frequent in the surface layer. In the bottom layer, in areas deeper than 25 m, northward currents dominated in the southern part of the Archipelago Sea, while in the northern part southward and northward currents were more evenly represented. Due to the variation in current directions, both northward and southward transports occur. During our study period, the net transport in the upper 20 m layer was southward. Below 20 m depth, the net transport was southward at the northern edge and northward at the southern edge of the Archipelago Sea. There were seasonal and inter-annual variations in the transport volumes and directions in the upper layer. Southward transport was usually largest in spring and summer months, and northward transport was largest in autumn and winter months. The transport dynamics in the Archipelago Sea show different variabilities in the north and south. A single transect cannot describe water transport through the whole area in all cases. Further studies on the water exchange processes between the Baltic proper and the Bothnian Sea through the Archipelago Sea would benefit from using a two-way nested model set-up for the region.

Internal tides off the Amazon shelf – Part 1: The importance of the structuring of ocean temperature during two contrasted seasons - Fernand Assene; Ariane Koch-Larrouy; Isabelle Dadou; Michel Tchilibou; Guillaume Morvan; Jérôme Chanut; Alex Costa da Silva; Vincent Vantrepotte; Damien Allain; Trung-Kien Tran (Ocean Science, 2024)
The impact of internal and barotropic tides on the vertical and horizontal temperature structure off the Amazon River was investigated during two highly contrasted seasons (AMJ: April–May–June; ASO: August–September–October) over a 3-year period from 2013 to 2015. Twin regional simulations, with and without tides, were used to highlight the general effect of tides. The findings reveal that tides have a cooling effect on the ocean from the surface (∼ 0.3 ∘C) to above the thermocline (∼ 1.2 ∘C), while warming it up below the thermocline (∼ 1.2 ∘C). The heat budget analysis indicates that the vertical mixing is the dominant process driving temperature variations within the mixed layer, while it is associated with both horizontal and vertical advection to explain temperature variations below. The increased mixing in the simulations including tides is attributed to breaking of internal tides (ITs) on their generation sites over the shelf break and offshore along their propagation pathways. Over the shelf, mixing is driven by the dissipation of the barotropic tides. In addition, the vertical terms of the heat budget equation exhibit wavelength patterns typical of mode-1 IT. The study highlights the key role of tides and particularly how IT-related vertical mixing shapes the ocean temperature off the Amazon. Furthermore, we found that tides impact the interactions between the upper ocean interface and the overlying atmosphere. They contribute significantly to increasing the net heat flux between the atmosphere and the ocean, with a notable seasonal variation from 33.2 % in AMJ to 7.4 % in ASO seasons. This emphasizes the critical role of tidal dynamics in understanding regional-scale climate.

Assessing the drift of fish aggregating devices in the tropical Pacific Ocean - Philippe F. V. W. Frankemölle; Peter D. Nooteboom; Joe Scutt Phillips; Lauriane Escalle; Simon Nicol; Erik van Sebille (Ocean Science, 2024)
The tropical Pacific Ocean is characterized by its dominant zonal flow, strong climate dependence on the El Niño–Southern Oscillation (ENSO) and abundant tuna stocks. Tuna fisheries in the West and Central Pacific Ocean accounted for 55 % of the world-wide tuna catch in 2019 and are one of the main sources of income in many Pacific island nations. One of the dominant fishing methods in this region relies on the use of drifting fish aggregating devices (dFADs): rafts with long underwater appendages (on average 50 m deep) that aggregate fish. Although currents such as the North Equatorial Countercurrent (NECC) and South Equatorial Current (SEC) in the tropical Pacific Ocean vary strongly with ENSO, little is known about the impact of this variability in flow on dFAD dispersion. In this study, virtual Lagrangian particles are tracked for the period 2006 to 2021 over the domain in a 3D hydrodynamic model and are advected in simulations with only surface flow, as well as simulations using a depth-averaged horizontal flow over the upper 50 m, representing virtual dFADs. Zonal displacements, eddy-like behaviour and ENSO variability are then studied for both types of virtual particles. It was found that virtual particles advected by surface flow only are displaced up to 35 % farther than virtual dFADs subjected to a depth-averaged flow, but no other major differences were found in dispersion patterns. The strongest correlations between ENSO and virtual dFAD dispersion for the assessed variables were found in the West Pacific Ocean, with Pearson correlation coefficients of up to 0.59 for virtual dFAD displacement. Connections between ENSO and eddy-like behaviour were found in the western part of the SEC, indicating more circulation and meandering during El Niño. These findings may be useful for improving sustainable deployment strategies during ENSO events and understanding the ocean processes driving the distribution of dFADs.

Technical note: Extending sea level time series for the analysis of extremes with statistical methods and neighbouring station data - Kévin Dubois; Morten Andreas Dahl Larsen; Martin Drews; Erik Nilsson; Anna Rutgersson (Ocean Science, 2024)
Extreme sea levels may cause damage and the disruption of activities in coastal areas. Thus, predicting extreme sea levels is essential for coastal management. Statistical inference of robust return level estimates critically depends on the length and quality of the observed time series. Here, we compare two different methods for extending a very short (∼ 10-year) time series of tide gauge measurements using a longer time series from a neighbouring tide gauge: linear regression and random forest machine learning. Both methods are applied to stations located in the Kattegat Basin between Denmark and Sweden. Reasonable results are obtained using both techniques, with the machine learning method providing a better reconstruction of the observed extremes. By generating a set of stochastic time series reflecting uncertainty estimates from the machine learning model and subsequently estimating the corresponding return levels using extreme value theory, the spread in the return levels is found to agree with results derived by more physically based methods.

Integration of microseism, wavemeter buoy, HF radar and hindcast data to analyze the Mediterranean cyclone Helios - Alfio Marco Borzì; Vittorio Minio; Raphael De Plaen; Thomas Lecocq; Salvatore Alparone; Salvatore Aronica; Flavio Cannavò; Fulvio Capodici; Giuseppe Ciraolo; Sebastiano D'Amico; Danilo Contrafatto; Giuseppe Di Grazia; et al. (Ocean Science, 2024)
In this work, we study a Mediterranean cyclone, Helios, which took place during 9–11 February 2023 in the southeastern part of Sicily and Malta, by a multiparametric approach combining microseism results with sea state and meteorological data provided by wavemeter buoy, HF radar, hindcast maps and satellite SEVIRI images. The sub-tropical system Helios caused heavy rainfall, strong wind gusts and violent storm surges with significant wave heights greater than 5 m. We deal with the relationships between such a system and the features of microseism (the most continuous and ubiquitous seismic signal on Earth) in terms of spectral content, space–time variation of the amplitude and source locations tracked by means of two methods (amplitude-based grid search and array techniques). By comparing the location of the microseism sources and the area affected by significant storm surges derived from sea state data, we note that the microseism location results are in agreement with the real position of the storm surges. In addition, we are able to obtain the seismic signature of Helios using a method that exploits the coherence of continuous seismic noise. Hence, we show how an innovative monitoring system of the Mediterranean cyclones can be designed by integrating microseism information with other techniques routinely used to study meteorological phenomena.

Observations of strong turbulence and mixing impacting water exchange between two basins in the Baltic Sea - Julia Muchowski; Martin Jakobsson; Lars Umlauf; Lars Arneborg; Bo Gustafsson; Peter Holtermann; Christoph Humborg; Christian Stranne (Ocean Science, 2023)
Turbulent diapycnal mixing is important for the estuarine circulation between basins of the Baltic Sea as well as for its local ecosystems, in particular with regard to eutrophication and anoxic conditions. While the interior of the basins is overall relatively calm, stratified flow over steep bathymetric features is known as a source of strong turbulent mixing. Yet, current in situ observations often cannot capture the spatio-temporal development of dynamic and intermittent turbulent mixing related to overflows over rough bathymetry. We present observational oceanographic data together with openly accessible high-resolution bathymetry from a prototypical sill and an adjacent deep channel in the sparsely sampled Southern Quark located in the Åland Sea, connecting the northern Baltic Proper with the Bothnian Sea. Our data were acquired during two 1-week cruises on R/V Electra in February–March 2019 and 2020. We collected high-resolution broadband acoustic observations of turbulent mixing together with in situ microstructure profiler measurements, and current velocities from acoustic Doppler current profilers. We found that a temporally reversing non-tidal stratified flow over the steep bathymetric sill created a dynamic and extremely energetic environment. The observed flow reversed during both cruises on timescales of a few days. Saltier, warmer, and less oxygenated deep water south of the sill was partly blocked, the reversing flow was at times hydraulically controlled with hydraulic jumps occurring on both sides of the sill, and high spatial variability occurred in the surface layer on small scales. Dissipation rates of turbulent kinetic energy, vertical turbulent diffusivities, and vertical salt flux rates were increased by 3–4 orders of magnitude in the entire water column in the vicinity of the sill compared to reference stations not directly influenced by the overflow with average dissipation rates near the sill between 10−7 and 10−6 W kg−1, average vertical diffusivities of 0.001 m2 s−1 in the halocline and up to 0.1 m2 s−1 below the halocline, and average vertical salt flux rates around 0.01 g m−2 s−1 in the halocline and between 0.1 and 1 g m−2 s−1 below the halocline. We suggest, based on acoustic observations and in situ measurements, that the underlying mechanism for the highly increased mixing across the halocline is a combination of shear and topographic lee waves breaking at the halocline interface. We anticipate that the resulting deep- and surface-water modification in the Southern Quark directly impacts exchange processes between the Bothnian Sea and the northern Baltic Proper and that the observed mixing is likely important for oxygen and nutrient conditions in the Bothnian Sea.

Southern Weddell Sea surface freshwater flux modulated by icescape and atmospheric forcing - Lukrecia Stulic; Ralph Timmermann; Stephan Paul; Rolf Zentek; Günther Heinemann; Torsten Kanzow (Ocean Science, 2023)
Sea ice formation dominates surface salt forcing in the southern Weddell Sea. Brine rejected in the process of sea ice production results in the production of High Salinity Shelf Water (HSSW) that feeds the global overturning circulation and fuels the basal melt of the adjacent ice shelf. The strongest sea ice production rates are found in coastal polynyas, where steady offshore winds promote divergent ice movement during the freezing season. We used the Finite Element Sea ice–ice shelf–Ocean Model (FESOM) forced by output from the regional atmospheric model COSMO-CLM (CCLM) with 14 km horizontal resolution to investigate the role of polynyas for the surface freshwater flux of the southern Weddell Sea (2002–2017). The presence of stationary icescape features (i.e., fast-ice areas and grounded icebergs) can influence the formation of polynyas and, therefore, impact sea ice production. The representation of the icescape in our model is included by prescribing the position, shape and temporal evolution of a largely immobile ice mélange formed between the Filchner–Ronne Ice Shelf (FRIS) and a major grounded iceberg based on satellite data. We find that 70 % of the ice produced on the continental shelf of the southern Weddell Sea is exported from the region. While coastal polynyas cover 2 % of the continental shelf area, sea ice production within the coastal polynyas accounts for 17 % of the overall annual sea ice production (1509 km3). The largest contributions come from the Ronne Ice Shelf and Brunt Ice Shelf polynyas and polynyas associated with the ice mélange. Furthermore, we investigate the sensitivity of the polynya-based ice production to the (i) representation of the icescape and (ii) regional atmospheric forcing. Although large-scale atmospheric fields determine the sea ice production outside polynyas, both the treatment of the icescape and the regional atmospheric forcing are important for the regional patterns of sea ice production in polynyas. The representation of the ice mélange is crucial for the simulation of polynyas westward/eastward of it, which are otherwise suppressed/overestimated. Compared to using ERA-Interim reanalysis as an atmospheric forcing data set, using CCLM output reduces polynya-based ice production over the eastern continental shelf due to weaker offshore winds, yielding a more realistic polynya representation. Our results show that the location and not just the strength of the sea ice production in polynyas is a relevant parameter in setting the properties of the HSSW produced on the continental shelf, which in turn affects the basal melting of the Filchner–Ronne Ice Shelf.

Long-term eddy modulation affects the meridional asymmetry of the halocline in the Beaufort Gyre - Jinling Lu; Ling Du; Shuhao Tao (Ocean Science, 2023)
Against the background of wind-forcing change along with Arctic sea ice retreat, the mesoscale processes undergoing distinct variation in the Beaufort Gyre (BG) region are increasingly important to oceanic transport and energy cascades, and these changes subsequently put oceanic stratification into a new state. Here, the varying number and strength of eddies in the central Canada Basin (CB) and Chukchi–Beaufort continental slope are obtained based on mooring observations (2003–2018), altimetry measurements (1993–2019), and reanalysis data (1980–2020). In this paper, the variability in the BG halocline, representing the adjustment of stratification in the upper layer, is shown in order to analyse how variability occurs under changing mesoscale processes. We find that over almost the last 2 decades the halocline depth has deepened by ∼ 40 m in the south of the central gyre, while that in the north has deepened by ∼ 70 m according to multiple datasets. Surrounding the central gyre, the asymmetry of the halocline, with much steeper and deeper isopycnals over the southern continental slope, reduced after 2014. In the meantime, eddy activities in the upper layer from the southern margin of the BG to the abyssal plain have been enhanced. Moreover, the convergence of the eddy lateral flux has increased as the halocline structures on either side, which is at least 120 km from the central gyre, have reached a nearly identical and stable regime. It has been clarified that long-term dynamic eddy modulation through eddy fluxes, facilitating the freshwater redistribution, affects the meridional asymmetry of the BG halocline. Our results provide a better understanding of the eddy modulation processes and their influence on the halocline structure.

Uncertainties and discrepancies in the representation of recent storm surges in a non-tidal semi-enclosed basin: a hindcast ensemble for the Baltic Sea - Marvin Lorenz; Ulf Gräwe (Ocean Science, 2023)
Extreme sea level events, such as storm surges, pose a threat to coastlines around the globe. Many tide gauges have been measuring the sea level and recording these extreme events for decades, some for over a century. The data from these gauges often serve as the basis for evaluating the extreme sea level statistics, which are used to extrapolate sea levels that serve as design values for coastal protection. Hydrodynamic models often have difficulty in correctly reproducing extreme sea levels and, consequently, extreme sea level statistics and trends. In this study, we generate a 13-member hindcast ensemble for the non-tidal Baltic Sea from 1979 to 2018 using the coastal ocean model GETM (General Estuarine Transport Model). In order to cope with mean biases in maximum water levels in the simulations, we include both simulations with and those without wind-speed adjustments in the ensemble. We evaluate the uncertainties in the extreme value statistics and recent trends of annual maximum sea levels. Although the ensemble mean shows good agreement with observations regarding return levels and trends, we still find large variability and uncertainty within the ensemble (95 % confidence levels up to 60 cm for the 30-year return level). We argue that biases and uncertainties in the atmospheric reanalyses, e.g. variability in the representation of storms, translate directly into uncertainty within the ensemble. The translation of the variability of the 99th percentile wind speeds into the sea level elevation is in the order of the variability of the ensemble spread of the modelled maximum sea levels. Our results emphasise that 13 members are insufficient and that regionally large ensembles should be created to minimise uncertainties. This should improve the ability of the models to correctly reproduce the underlying extreme value statistics and thus provide robust estimates of climate change-induced changes in the future.

Delayed-mode reprocessing of in situ sea level data for the Copernicus Marine Service - Jue Lin-Ye; Begoña Pérez Gómez; Alejandro Gallardo; Fernando Manzano; Marta de Alfonso; Elizabeth Bradshaw; Angela Hibbert (Ocean Science, 2023)
The number of tide gauges providing coastal sea level data has significantly increased in recent decades. They help in the issue of coastal hazard warnings, in the forecasting (indirectly through models) of storm surges and tsunamis, and in operational oceanography applications. These data are automatically quality controlled in near-real time in the Copernicus Marine Service. A new initiative seeks to provide delayed-mode reprocessed data for the Copernicus Marine Service by developing a new product and upgrading the software used in its automated quality control. Several new modules, such as buddy checking or the detection of attenuated data, are implemented. The new product was launched in November of 2022. The entire reprocessing is discussed in detail. An example of the information that can be extracted from the delayed-mode reprocessed product is also given.

A thermodynamic potential of seawater in terms of Absolute Salinity, Conservative Temperature, and in situ pressure - Trevor J. McDougall; Paul M. Barker; Rainer Feistel; Fabien Roquet (Ocean Science, 2023)
A thermodynamic potential is derived for seawater as a function of Conservative Temperature, Absolute Salinity and pressure. From this thermodynamic potential, all the equilibrium thermodynamic properties of seawater can be found, just as all these thermodynamic properties can be found from the TEOS-10 (the International Thermodynamic Equation of Seawater – 2010; IOC et al., 2010) Gibbs function (which is a function of in situ temperature, Absolute Salinity, and pressure). Present oceanographic practice in the Gibbs SeaWater Oceanographic Toolbox uses a polynomial expression for specific volume (and enthalpy) in terms of Conservative Temperature (as well as of Absolute Salinity and pressure), whereas the relationship between in situ temperature and Conservative Temperature is based on the Gibbs function. This mixed practice introduces (numerically small) inconsistencies and superfluous conversions between variables. The proposed thermodynamic potential of seawater, being expressed as an explicit function of Conservative Temperature, overcomes these small numerical inconsistencies, and in addition, the new approach allows for greater computational efficiency in the evaluation of sea surface temperature from Conservative Temperature. It is also shown that when using Conservative Temperature, the thermodynamic information in enthalpy is independent of that contained in entropy. This contrasts with the cases where either in situ temperature or potential temperature is used. In these cases, a single thermodynamic potential serves the important purpose of avoiding having to impose a separate consistency requirement between the functional forms of enthalpy and entropy.

Equatorial wave diagnosis for the Atlantic Niño in 2019 with an ocean reanalysis - Qingyang Song; Hidenori Aiki (Ocean Science, 2023)
The propagation of equatorial waves is essential for the onset of Atlantic Niño, but diagnosing waves with ocean reanalysis or in situ data remains a challenge. This study uses an ocean reanalysis to diagnose the wave energy transfer route during the 2019 event. The climatological values and the anomaly in 2019 at each grid point are decomposed into the first four baroclinic modes based on their local density profiles. The decomposed geopotential can well reproduce the displacement of the thermocline during the event. Wave energy flux is calculated by means of a group-velocity-based scheme. In addition to detecting wind-forced Kelvin waves and reflected Rossby waves, the wave energy flux reveals another possible energy transfer route along the western boundary, where some off-equatorial wave energy can excite coastally trapped Kelvin waves and transfer back to the equatorial Atlantic. Five transects are selected, across which the passing wave energy fluxes in 2019 are integrated. The results suggest that the Kelvin waves in the third and fourth mode are locally forced, while the wave energy in the second mode is more likely from the off-equatorial region. Therefore, in the autumn of 2019, the second-mode Kelvin waves can deepen the thermocline ahead of other modes from September, serving to precondition the Niño event.

Modeling the interannual variability in Maipo and Rapel river plumes off central Chile - Julio Salcedo-Castro; Antonio Olita; Freddy Saavedra; Gonzalo S. Saldías; Raúl C. Cruz-Gómez; Cristian D. De la Torre Martínez (Ocean Science, 2023)
River plumes have a direct influence on coastal environments, impacting coastal planktonic and benthic communities, including fishery resources. In general, the main drivers of river plume dynamics are the river discharge and the alongshore wind stress, whereas the tides and topography play a secondary role. In central Chile, rivers flowing into the eastern Pacific have a relatively short path on land, with a high slope and a mixed snow–rain regime. This study aims to understand the interannual variability in the plumes of the Maipo and Rapel rivers in the coastal/shelf area off central Chile and their influence on local ocean dynamics. We used the Coastal and Regional Ocean Community (CROCO) model, with 1 km horizontal resolution and 20 sigma levels, to simulate the ocean dynamics for the period 2003–2011. The results show that the plume's area coverage and coastal ocean salinity are strongly correlated with the river discharges. The predominant northeastward winds control the plumes' orientation toward the northwest. However, episodes of southeastward winds in winter can reverse the plumes' direction, promoting their attachment to the coast and southward transport. Results also show a salification trend linked to the severe droughts hitting central Chile during the studied period. This salification determines a change in local dynamics which could be more frequent in future scenarios of climate change with a significant lack of rain and river discharges along central Chile.

Constraining an eddy energy dissipation rate due to relative wind stress for use in energy budget-based eddy parameterisations - Thomas Wilder; Xiaoming Zhai; David Munday; Manoj Joshi (Ocean Science, 2023)
A geostrophic eddy energy dissipation rate due to the interaction of the large-scale wind field and mesoscale ocean currents, or relative wind stress, is derived here for use in eddy energy budget-based eddy parameterisations. We begin this work by analytically deriving a relative wind stress damping term and a baroclinic geostrophic eddy energy equation. The time evolution of this analytical eddy energy in response to relative wind stress damping is compared directly with a baroclinic eddy in a general circulation model for both anticyclones and cyclones. The dissipation of eddy energy is comparable between each model and eddy type, although the numerical model diverges from the analytical model at around day 150, likely due to the presence of non-linear baroclinic processes. A constrained dissipation rate due to relative wind stress is then proposed using terms from the analytical eddy energy budget. This dissipation rate depends on the potential energy of the eddy thermocline displacement, which also depends on eddy length scale. Using an array of ocean datasets, and computing two forms for the eddy length scale, a range of values for the dissipation rate are presented. The analytical dissipation rate is found to vary from 0.25 to 4 times that of a constant dissipation rate employed in previous studies. The dissipation rates are generally enhanced in the Southern Ocean but smaller in the western boundaries. This proposed dissipation rate offers a tool to parameterise the damping of total eddy energy in coarse resolution global climate models and may have implications for a wide range of climate processes.

Response of the Arctic sea ice–ocean system to meltwater perturbations based on a one-dimensional model study - Haohao Zhang; Xuezhi Bai; Kaiwen Wang (Ocean Science, 2023)
A one-dimensional coupled sea ice–ocean model is used to investigate how the Arctic Ocean stratification and sea ice respond to changes in meltwater. In the control experiments, the model is capable of accurately simulating seasonal changes in the upper-ocean stratification structure compared with observations, and the results suggest that ocean stratification is important for ice thickness development during the freezing season. The sensitivity experiments reveal the following: (1) a decrease in meltwater release weakens ocean stratification and creates a deeper, higher-salinity mixed layer. (2) Meltwater reduced ice melting by 17 % by strengthening ocean stratification. (3) The impact of meltwater released during the previous melting season on ice growth in winter depends on the strength of stratification. After removing all the meltwater during the summer, ice formation in areas with strong stratification increased by 12 % during the winter, while it decreased by 43 % in areas with weak stratification. (4) In some areas of the Nansen Basin where stratification is nearly absent, the warm Atlantic Water can reach the ice directly in early spring, leading to early melting of the sea ice in winter if all meltwater is removed from the model. These findings contribute to our understanding of the complex interactions between ocean stratification, meltwater and sea ice growth and have important implications for climate models and future change prediction in the Arctic.

Observations and modeling of tidally generated high-frequency velocity fluctuations downstream of a channel constriction - Håvard Espenes; Pål Erik Isachsen; Ole Anders Nøst (Ocean Science, 2023)
We investigate data from an acoustic Doppler current profiler deployed in a constricted ocean channel showing a tidally dominated flow with intermittent velocity extrema during outflow from the constriction but not during inflow. A 2D numerical ocean model forced by tides is used to examine the spatial flow structure and underlying dynamical processes. We find that flow-separation eddies generated near the tightest constriction point form a dipole pair which propagates downstream and drives the observed intermittent flow variability. The eddies, which are generated by an along-channel adverse pressure gradient, spin up for some time near the constriction until they develop local low pressures in their centers that are strong enough to modify the background along-channel pressure gradient significantly. When the dipole has propagated some distance away from the constriction, the conditions for flow separation are recovered, and new eddies are formed.

Relative dispersion and kinematic properties of the coastal submesoscale circulation in the southeastern Ligurian Sea - Pierre-Marie Poulain; Luca Centurioni; Carlo Brandini; Stefano Taddei; Maristella Berta; Milena Menna (Ocean Science, 2023)
An array of Lagrangian instruments (more than 100 drifters and a profiling float) were deployed for several days in the coastal waters of the southeastern Ligurian Sea to characterize the near-surface circulation at the submesoscale (

Southern Ocean warming and Antarctic ice shelf melting in conditions plausible by late 23rd century in a high-end scenario - Pierre Mathiot; Nicolas C. Jourdain (Ocean Science, 2023)
How much Antarctic ice shelf basal melt rates can increase in response to global warming remains an open question. Here we describe the response of the Southern Ocean and ice shelf cavities to an abrupt change to high-end atmospheric conditions plausible by the late 23rd century under the SSP5-8.5 scenario. To achieve this objective, we first present and evaluate a new 0.25∘ global configuration of the NEMO (Nucleus for European Modelling of the Ocean NEMO System Team, 2019) ocean and sea ice model. Our present-day simulations demonstrate good agreement with observational data for key variables such as temperature, salinity, and ice shelf melt rates, despite the remaining difficulties to simulate the interannual variability in the Amundsen Sea. The ocean response to the high-end atmospheric perturbation includes a strengthening and extension of the Ross and Weddell gyres and a quasi-disappearance of sea ice, with a subsequent decrease in production of High Salinity Shelf Water and increased intrusion of warmer water onto the continental shelves favoured by changes in baroclinic currents at the shelf break. We propose to classify the perturbed continental shelf as a “warm–fresh shelf”. This induces a substantial increase in ice shelf basal melt rates, particularly in the coldest seas, with a total basal mass loss rising from 1180 to 15 700 Gt yr−1 and an Antarctica averaged melt rate increasing from 0.8 to 10.6 m yr−1. In the perturbed simulation, most ice shelves around Antarctica experience conditions that are currently found in the Amundsen Sea, while the Amundsen Sea warms by 2 ∘C. These idealised projections can be used as a base to calibrate basal melt parameterisations used in long-term ice sheet projections.

Impact of surface and subsurface-intensified eddies on sea surface temperature and chlorophyll a in the northern Indian Ocean utilizing deep learning - Yingjie Liu; Xiaofeng Li (Ocean Science, 2023)
Mesoscale eddies, including surface-intensified eddies (SEs) and subsurface-intensified eddies (SSEs), significantly influence phytoplankton distribution in the ocean. Nevertheless, due to the sparse in situ data, understanding of the characteristics of SSEs and their influence on chlorophyll a (Chl a) concentration is still unclear. Consequently, the study utilized a deep learning model to extract SEs and SSEs in the northern Indian Ocean (NIO) from 2000 to 2015, using satellite-derived sea surface height (SSH) and sea surface temperature (SST) data. The analysis revealed that SSEs accounted for 39 % of the total eddies in the NIO, and their SST signatures exhibited opposite behaviour compared to SEs. Furthermore, by integrating ocean colour remote-sensing data, the study investigated the contrasting impacts of SEs and SSEs on Chl a concentration in two basins of the NIO, the Arabian Sea (AS) and the Bay of Bengal (BoB), known for their disparate biological productivity. In the AS, SEs induced Chl a anomalies that were 2 to 3 times higher than those caused by SSEs. Notably, there were no significant differences in Chl a anomalies induced by the same type of eddies between summer and winter. In contrast, the BoB exhibited distinct seasonal variations, where SEs induced slightly higher Chl a anomalies than SSEs during the summer, while substantial differences were observed during the winter. Specifically, subsurface-intensified anticyclonic eddies (SSAEs) led to positive Chl a anomalies, contrasting the negative anomalies induced by surface-intensified anticyclonic eddies (SAEs) with comparable magnitudes. Moreover, while both subsurface-intensified cyclonic eddies (SSCEs) and surface-intensified cyclonic eddies (SCEs) resulted in positive Chl a anomalies during winter in the BoB, the magnitude of SSCEs was only one-third of that induced by SCEs. Besides, subsurface Chl a induced by SSAEs (SSCEs) is ∼0.1 mg m−3 greater (less) than that caused by SAEs (SCEs) in the upper 30 (50) m using Biogeochemical Argo profiles. The distinct Chl a between SEs and SSEs can be attributed to their contrasting subsurface structures revealed by Argo profiles. Compared to SAEs (SCEs), SSAEs (SSCEs) enhance (decrease) production via the convex (concave) of the isopycnals that occur around the mixed layer. The study provides a valuable approach to investigating subsurface eddies and contributes to a comprehensive understanding of their influence on chlorophyll concentration.

Short-term prediction of the significant wave height and average wave period based on the variational mode decomposition–temporal convolutional network–long short-term memory (VMD–TCN–LSTM) algorithm - Qiyan Ji; Lei Han; Lifang Jiang; Yuting Zhang; Minghong Xie; Yu Liu (Ocean Science, 2023)
The present work proposes a prediction model of significant wave height (SWH) and average wave period (APD) based on variational mode decomposition (VMD), temporal convolutional networks (TCNs), and long short-term memory (LSTM) networks. The wave sequence features were obtained using VMD technology based on the wave data from the National Data Buoy Center. Then the SWH and APD prediction models were established using TCNs, LSTM, and Bayesian hyperparameter optimization. The VMD–TCN–LSTM model was compared with the VMD–LSTM (without TCN cells) and LSTM (without VMD and TCN cells) models. The VMD–TCN–LSTM model has significant superiority and shows robustness and generality in different buoy prediction experiments. In the 3 h wave forecasts, VMD primarily improved the model performance, while the TCN had less of an influence. In the 12, 24, and 48 h wave forecasts, both VMD and TCNs improved the model performance. The contribution of the TCN to the improvement of the prediction result determination coefficient gradually increased as the forecasting length increased. In the 48 h SWH forecasts, the VMD and TCN improved the determination coefficient by 132.5 % and 36.8 %, respectively. In the 48 h APD forecasts, the VMD and TCN improved the determination coefficient by 119.7 % and 40.9 %, respectively.

Statistical analysis of dynamic behavior of continental shelf wave motions in the northern South China Sea - Junyi Li; Tao He; Quanan Zheng; Ying Xu; Lingling Xie (Ocean Science, 2023)
This study aims to analyze statistical behavior of the continental shelf wave motions, including continental shelf waves (CSWs) and arrested topographic waves (ATWs), in the northern South China Sea. The baseline consists of tide-gauge data from stations Kanmen, Xiamen, Shanwei, and Zhapo as well as along-track sea level anomaly (SLA) data derived from multiple satellite altimeters from 1993 to 2020. The subtidal signals propagating along the coast with periods shorter than 40 d and phase speeds of about 10 m s−1 are interpreted as CSWs. The cross-shelf structure of along-track SLAs indicates that Mode 1 of CSWs is the predominant component trapped in the area shallower than about 200 m. The amplitudes of CSWs reach a maximum of 0.6 m during July–September and a minimum of 0.2 m during April–June. The inter-seasonal and seasonal signals represent ATWs. The amplitudes of ATWs reach 0.10 m during October–December, twice that during July–September. These observations can be well interpreted in the framework of linear wave theory. The cross-shelf structures of CSWs and ATWs derived from along-track SLAs illustrate that the methods are suitable for observing dynamic behavior of the CSWs.

On the drivers of regime shifts in the Antarctic marginal seas, exemplified by the Weddell Sea - Verena Haid; Ralph Timmermann; Özgür Gürses; Hartmut H. Hellmer (Ocean Science, 2023)
Recent studies have found evidence for a potential future tipping point, when the density of Antarctic continental shelf waters, specifically in the southern Weddell Sea, will allow for the onshore flow of warm waters of open ocean origin. A cold-to-warm regime shift in the adjacent ice shelf cavities entails a strong enhancement of ice shelf basal melt rates and could trigger instabilities in the ice sheet. From a suite of numerical experiments, aimed to force such a regime shift on the continental shelf, we identified the density balance between the shelf waters formed by sea ice production and the warmer water at the shelf break as the defining element of a tipping into a warm state. In our experiments, this process is reversible but there is evidence for hysteresis behaviour. Using HadCM3 20th-century output as atmospheric forcing, the resulting state of the Filchner–Ronne cavity depends on the initial state. In contrast, ERA Interim forcing pushes even a warm-initialized cavity into a cold state, i.e. it pushes the system back across the reversal threshold to the cold side. However, it turns out that for forcing data perturbations of a realistic magnitude, a unique and universal recipe for triggering a regime shift in Antarctic marginal seas was not found; instead, various ocean states can lead to an intrusion of off-shelf waters onto the continental shelf and into the cavities. Whether or not any given forcing or perturbation yields a density imbalance and thus allows for the inflow of warm water depends on the complex interplay between bottom topography, mean ocean state, sea ice processes, and atmospheric conditions.

Regional mapping of energetic short mesoscale ocean dynamics from altimetry: performances from real observations - Florian Le Guillou; Lucile Gaultier; Maxime Ballarotta; Sammy Metref; Clément Ubelmann; Emmanuel Cosme; Marie-Helène Rio (Ocean Science, 2023)
For over 25 years, satellite altimetry has provided invaluable information about the ocean dynamics at many scales. In particular, gridded sea surface height (SSH) maps allow us to estimate the mesoscale geostrophic circulation in the ocean. However, conventional interpolation techniques rely on static optimal interpolation schemes, hence limiting the estimation of non-linear dynamics at scales not well sampled by altimetry (i.e., below 150–200 km at mid-latitudes). To overcome this limitation in the resolution of small-scale SSH structures (and thus small-scale geostrophic currents), a back-and-forth nudging algorithm combined with a quasi-geostrophic model, a technique called BFN-QG, has been successfully applied on simulated SSH data in observing system simulation experiments (OSSEs). The result is a significant reduction in interpolation error and an improvement in the space–time resolutions of the experimental gridded product compared to those of operational products. In this study, we propose that the BFN-QG be applied to real altimetric SSH data in a highly turbulent region spanning a part of the Agulhas Current. The performances are evaluated within observing system experiments (OSEs) that use independent data (such as independent SSH, sea surface temperature and drifter data) as ground truth. By comparing the mapping performances to the ones obtained with operational products, we show that the BFN-QG improves the mapping of short, energetic mesoscale structures and associated geostrophic currents both in space and time. In particular, the BFN-QG improves (i) the spatial effective resolution of the SSH maps by a factor of 20 %, (ii) the zonal and (especially) the meridional geostrophic currents, and (iii) the prediction of Lagrangian transport for lead times up to 10 d. Unlike the results obtained in the OSSEs, the OSEs reveal more contrasting performances in low-variability regions, which are discussed in the paper.

The Mediterranean Forecasting System – Part 1: Evolution and performance - Giovanni Coppini; Emanuela Clementi; Gianpiero Cossarini; Stefano Salon; Gerasimos Korres; Michalis Ravdas; Rita Lecci; Jenny Pistoia; Anna Chiara Goglio; Massimiliano Drudi; Alessandro Grandi; Ali Aydogdu; et al. (Ocean Science, 2023)
The Mediterranean Forecasting System produces operational analyses and reanalyses and 10 d forecasts for many essential ocean variables (EOVs), from currents, temperature, salinity, and sea level to wind waves and pelagic biogeochemistry. The products are available at a horizontal resolution of 1/24∘ (approximately 4 km) and with 141 unevenly spaced vertical levels. The core of the Mediterranean Forecasting System is constituted by the physical (PHY), the biogeochemical (BIO), and the wave (WAV) components, consisting of both numerical models and data assimilation modules. The three components together constitute the so-called Mediterranean Monitoring and Forecasting Center (Med-MFC) of the Copernicus Marine Service. Daily 10 d forecasts and analyses are produced by the PHY, BIO, and WAV operational systems, while reanalyses are produced every ∼ 3 years for the past 30 years and are extended (yearly). The modelling systems, their coupling strategy, and their evolutions are illustrated in detail. For the first time, the quality of the products is documented in terms of skill metrics evaluated over a common 3-year period (2018–2020), giving the first complete assessment of uncertainties for all the Mediterranean environmental variable analyses.