Interaction of Shallow, Salinity-Stratified Fronts with Surface Cooling: Results from Submesoscale-Permitting Simulations

We study submesoscale instabilities in shallow, salinity-stratified layers using submesoscale-permitting simulations. The initial velocity and density fields are in thermal-wind balance, with the lateral buoyancy gradient in the model chosen to match the same in recent high-resolution measurements at salinity fronts in the central Bay of Bengal. We explore the onset and subsequent spatio-temporal evolution of the submesoscale instabilities, both with and without surface forcing. In the forced simulations, we examine the effects of different types of surface forcing: winds (upfront and downfront), heating and freshwater fluxes. The simulation configuration is periodic in the frontal direction with rigid, no-slip boundaries in the across-front direction. The finest lateral resolution in our suite of simulations is 250 m, fine enough to permit baroclinic-instability modes and partially resolve select symmetric-instability modes. The unforced simulations show the generation of numerous pycnostads immediately beneath the shallow O(1-10 m) mixed layers. The pycnostads, anticyclonic and anomalously low in Ertel potential vorticity (PV), are generated by isopycnal subduction of low-PV water near the surface. The lateral and vertical scales of the pycnostads are O(1-10 km) and O(1-10 m), respectively, similar to those seen in wintertime observations during weak winds. The frontal subduction also leads to the formation of subsurface temperature inversions, a defining feature of the upper Bay in the winter. The forced simulations share some of the above features, but also present interesting differences. The relative vorticity in the presence of downfront winds can attain values as high as 20f (f is the Coriolis parameter), unlike the unforced simulations where the peak values are O(f). Additional simulations with similar initial frontal gradients and downfront winds, but at higher latitudes (42N), do not show such large relative vorticities. We are presently investigating to what extent such sharp lateral gradients in the velocity, documented repeatedly in observations, result from frontal processes at the submesoscales.