Submesoscale frontal waves and instabilities driven by sheared flows

Submesoscale processes at the frontal region are frequently observed in the form of submesoscale fronts, eddies, and filaments due to a variety of submesoscale instabilities. Previous studies tended to emphasize baroclinic conversions (e.g. mixed-layer instability) and few efforts have focused on the contribution of barotropic instability. In the present study, a series of idealized numerical simulations are implemented to examine how submesoscale structures are generated and evolve on conditions of different (horizontal and vertical) flow shear. The results show that barotropic instability would slow down or even restrain the growth of propagating submesoscale frontal waves, but instead, generate slower, larger phase-locked frontal waves. These waves with large amplitude significantly advect the denser, colder water to the warmer, lighter side. This provides active available potential energy and subsequently causes strong buoyancy production. In that context, barotropic instability also greatly contributes to the generation of submesoscale features, restratification, and vertical transport in the upper ocean. But this process cannot be identified in most time-averaged analyses. In this paper, the energy variation with time is also studied based on the spatial mean by Reynolds decomposition, the results of which suggest that the energy cascade paths caused by baroclinic instability and barotropic instability are different, but their contributions to downscale energy transition are equally important.