Ageostrophic dynamics of deep submesoscale fronts and their interaction with internal gravity waves

Submesoscale ocean processes, characterized by order one Rossby and Richardson numbers, are typically associated with energetic frontogenesis, driving upward vertical heat flux and leading to enhanced restratification. These ageostrophic submesoscales (<30 km) are currently thought to be mainly confined to the ocean surface mixed layer, while the ocean interior is commonly assumed to be in quasi-geostrophic equilibrium. Here, a realistic numerical simulation in the Antarctic Circumpolar Current, with a 1/48° horizontal resolution and tidal forcing, is used to demonstrate that the ocean interior departs from the quasi-geostrophic regime down to depths of 900 m, i.e., well below the mixed-layer. Results highlight that, contrary to the classical paradigm, the ocean interior is strongly ageostrophic. The relative vorticity field, from the surface down to 900 m, comprises numerous vortices and filaments with large Rossby numbers that are associated with strong lateral gradients of buoyancy. These deep submesoscales fronts drive significant upward vertical heat and buoyancy fluxes, highlighting the unexpected presence of deep oceanic restratification and suggesting that a significant conversion of PE into KE is occurring in the ocean interior. Finally, wavenumber-frequency spectra demonstrate that enhanced vertical velocity and tracers flux fields associated to these deep ageostrophic fronts are affected by internal gravity waves below the permanent thermocline but not above it. This last result points to the importance of energy exchanges between balanced motions and internal gravity waves in the ocean interior, with potential major consequence for the larger-scale ocean circulation.