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. We analyze a 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. Here, we 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-Nov) intensification and a weakening in fall (Mar-May). These deep submesoscales follow the seasonality of surface eddy kinetic energy derived from altimetry, suggesting that they are generated by mesoscale eddies. 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.