Summer Monsoon winds and density fronts: Results from a submesoscale-permitting model

We present results from idealized process studies, motivated by recent observations in the Bay of Bengal during the summer monsoon. The observations of temperature, salinity and velocity provide the first detailed picture of the submesoscale variability of the Bay and suggest an active role for O(1-10km) submesoscale frontal processes in setting the vertical stratification of the upper Bay. Using a non-hydrostatic model that permits submesoscales, we simulate shallow, O(5-30m) mixed-layer fronts forced by upfront winds, the prevalent configuration during the summer monsoon. We force the simulations with winds and heat fluxes using the MERRA reanalysis. The simulations allow us to explore in detail the competing tendencies of the wind forcing to mix and to promote restratification through the Ekman advection of lighter over heavier water. The simulations show rich variability at the submesoscales and a complex vertical structure in the upper tens of meters. The interaction of winds with the sharp frontal gradients generates barrier layers and multiple stratification peaks, consistent with observations. This underscores the need for inclusion of these O(1-10km) processes in larger scale simulations of the Bay. The widespread occurrence of shallow, O(10m) tropical boundary layers raises questions about the applicability of existing parameterizations developed for deep mid-latitude mixed layer fronts forced by down front winds. We also perform simulations with idealized rain histories to study the effect of weak and strong rain events on the upper-ocean stratification. We document the sensitivity of our simulated results to the subgrid turbulence parameterization, using both the K-profile parameterization and more complex second-order closure models.