Baroclinic Mixed Layer Instabilities in the Presence of Convection

Submesoscale surface fronts have been suggested to regulate the exchange of heat and carbon between the surface and interior ocean. Recent observations from the western subtropical North Atlantic show that submesoscale turbulence undergoes a seasonal cycle at scales of order 10 km: submesoscale fronts and currents are strong in deep winter mixed layers and weak in the summertime seasonal thermocline. This suggests that baroclinic mixed layer instabilities energize submesoscale flows in winter and have the potential to exchange tracers between the mixed layer and the permanent thermocline. By themselves, however, baroclinic instabilities quickly restratify the mixed layer and exhaust the potential energy that fuels them. To allow the instabilities to continuously inject energy into the submesoscale range, the restratification must be countered by atmospheric forcing, which induces small-scale turbulence and keeps the mixed layer unstratified. It is unclear whether this fast small-scale turbulence can damp out the slower baroclinic instabilities or whether baroclinic instabilities are unaffected. We study the interaction between baroclinic instabilities and small-scale turbulence in a set of idealized numerical simulations that resolve both processes. We find that baroclinic instabilities are remarkably resilient to the presence of small-scale turbulence and emerge even if vigorous convection induced by surface cooling keeps the mixed layer unstratified. Baroclinic instabilities grow, sharpen fronts, and render convection slantwise along these fronts. Implications for the seasonal cycle of submesoscale turbulence will be discussed.