Patrice Klein (Caltech, United States); Christophe Guinet (CEBC CNRS, France)

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 an extensive record of 133 598 dives performed by instrumented Southern Elephant Seals over several years 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).
These deep submesoscales have a weak signature on sea surface temperature and salinity and conventional altimetry. However, they are detectable in high-resolution SWOT sea surface height due their significant steric height signal at submesoscale. Results show that these deep submesoscales are generated by subsurface intensified eddies (with a size > 50 km) and follow their seasonality. 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. Overall, these results highlight the potential of SWOT for detecting subsurface features and quantifying their role in ocean transport.