Marine Mammal-Based Observations of Subsurface-Intensified Eddies in the Seasonally Sea Ice-Covered Southern Ocean

Abstract

Submesoscale coherent vortices (SCVs) are long-lived subsurface-intensified eddies that advect heat, salt, and biogeochemical tracers throughout the ocean. Previous observations indicate that SCVs are abundant in the Arctic because sea ice suppresses surface-intensified mesoscale structures. Regional observational and modeling studies have indicated that SCVs may be similarly prevalent beneath Antarctic sea ice, but there has been no previous systematic attempt to observe these eddies. This study presents the discovery of eddies in the Southern Ocean's seasonally sea ice-covered region using the Marine Mammals Exploring the Oceans Pole to Pole (MEOP) hydrographic measurements. Eddies are identified via a novel algorithm that utilizes anomalies in spice, isopycnal separation, and dynamic height along MEOP seal tracks. This algorithm is tested and calibrated by simulating the MEOP seal tracks using output from a 1/48$°$ global ocean/sea ice model, in which subsurface eddies are independently identified via the Okubo–Weiss parameter. Approximately 60 detections of cyclonic and over 100 detections of anticyclonic SCVs are identified, with typical dynamic height anomalies of $\sim 0.05m^2{s}^{-2}$, core depths of $\sim 200m$, and vertical half-widths of $\sim 100m$, similar to their Arctic counterparts. The eddies exhibit a pronounced geographical asymmetry: cyclones are exclusively observed in the open ocean, while $\sim$90% of the anticyclones are located on the continental shelf, consistent with injection of low-potential vorticity waters by surface buoyancy loss. These findings provide a first observational characterization of eddies in the seasonally ice-covered Southern Ocean, which will serve as a basis for future investigation of their role in near-Antarctic circulation and tracer transport.