Characteristics of ocean mesoscale eddies in the Canadian Basin from a high resolution pan-Arctic model

Abstract

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

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