Ubiquity and seasonality of deep submesoscales in the Southern Ocean revealed by elephant seals

Ocean submesoscale fronts with a size of 1-20 km have been extensively documented within the surface mixed layer but little is known about their existence below it. Here, we analyze a historical record of 133 598 dives performed by instrumented Southern Elephant Seals from 2014 to 2020 in the Indian sector of the Southern Ocean with a sub-km resolution. Results show that submesoscale motions are intensified below the mixed layer and are present year-round from the surface down to depths of at least 500 m. They exhibit a seasonal cycle with an austral spring (Sep-Oct-Nov) intensification and a weakening in fall (Mar-Apr-May). Results also show that deep submesoscales follow the seasonality of surface eddy kinetic energy derived from altimetry, suggesting that they are generated by mesoscale eddies (with a size > 50 km). Since submesoscales are known to be associated with strong vertical velocities, these results suggest that vertical exchanges of heat and nutrient at depth may be more important than commonly assumed.