Submesoscales in the Antarctic Circumpolar Current

The Southern Ocean is home to the eastward flowing Antarctic Circumpolar Current (ACC), one of the earth's major current systems. The ACC plays a central role in the global ocean overturning circulation and is a hot-spot for the exchange of water between the ocean surface and interior. Submesoscale eddies and fronts with scales between 1-10 km are regularly observed in the upper ocean and are known to be associated with enhanced vertical velocity that aids the subduction of surface waters. Despite their importance in other regions, comparatively little is known about submesoscales in the Southern Ocean.

Here, we will present new observations and high-resolution numerical simulations of submesoscales in the Southern Ocean. Observations were made as part of the SMILES project from April-May 2015 east of Drake Passage in the Scotia Sea. During the cruise, a large northward meander of the Antarctic Circumpolar Current (ACC) was sampled at high resolution using towed bodies, surface drifters, microstructure, and dye release. Satellite imagery indicates the presence of submesoscale ‘wrinkles’ along the sharp temperature gradient associated with the ACC. Observations using a moving vessel profiler (MVP) reveal rich three-dimensional structure associated with these features. A high resolution regional model, initialized from a concurrent state estimate, captures many of the observed features and allows us to quantify the energy and vertical velocity associated with submesoscales.

A linear stability analysis and a non-hydrostatic process study model suggest that submesoscale dynamics are fundamentally altered by the strong jet associated with the ACC. Growing submesoscale disturbances along a sharp temperature front are transformed into submesoscale Rossby waves which propagate upstream relative to the eastward jet. Unlike their counterparts in slower currents, the submesoscale waves do not destroy the underlying front. The submesoscale Rossby waves are associated with very large vertical velocities which lead to subduction of surface water.