Mesoscale dynamics of an intrathermocline eddy in the Canary Eddy Corridor

Abstract

High-resolution observations of an intrathermocline eddy were conducted in November 2022 within the Canary Eddy Corridor. Formed in early summer 2022, this mature mesoscale eddy exhibited a vertical extent of 550 m, with its core centered at 110 m depth, and a segmented horizontal structure comprising a 23 km radius solid-body core surrounded by a 47 km wide outer ring. Propagating southwestward at 4.5 kmd-1, its motion was consistent with the phase speed of a first-mode baroclinic Rossby wave. The eddy's rotational dynamics featured a 3.9 d inner-core rotation period shaped by stratification, leading to the formation of distinct rotational layers. Rossby number estimates (maximum of -0.7 at the center and -0.5 on average) and low core potential vorticity (∼10-11 m-1s-1, 90 % lower than surrounding values) revealed a regime dominated by planetary rotation, yet with a dynamically significant centripetal contribution - suggestive of a cyclogeostrophic momentum balance - and strong water mass isolation. Burger numbers, ranging from 1.27 to 0.14 (length-scale-based) and from 0.21 to 0.69 (energy-based), underscored the role of stratification and buoyancy forces in shaping the eddy's vertical structure. The eddy carried available heat and salt anomalies of 6.550×1018 J and 0.015×1012 kg, driving heat and salt (freshwater equivalent) fluxes of 4.60×1012 W and 0.42×109 kgs-1 (-0.012 Sv, where 1SvCombining double low line106 m3s-1), highlighting its role in transporting coastal upwelling waters into the ocean interior. The intrathermocline nature of the Bentayga eddy appears to have developed during its growth phase, likely driven by surface convergence linked to the interaction with upwelling filaments and subsequent isopycnal deepening as it propagated offshore. Low dissolved oxygen concentrations (100-110 μmolkg-1) and low apparent oxygen utilization (20-30 μmolkg-1) within the eddy core support the hypothesis of recent trapping of surface-derived upwelled waters. Over the course of its year-long lifespan, the eddy experienced intrinsic instabilities and eddy-to-eddy interactions, ultimately decaying by early summer 2023. The distinct properties of this eddy, together with the apparent variability among similar features in the Canary Eddy Corridor, underscore the need for expanded quasi-synoptic high-resolution studies. Comprehensive observational programs and advanced numerical simulations are essential to better understand the role of intrathermocline eddies (ITEs) as zonal pathways for heat, salt, and biogeochemical properties within regional ocean circulation.