Fresh water input as driver of submesoscale variability in coastal waters: The case of the Gulf of Mexico

With an ensemble of numerical simulations and using the Gulf of Mexico as test bed, we investigate the dynamics at the ocean submesoscale in regions characterized by large density gradients that are not (or not only) associated to stirring and straining by mesoscale structures, but are ‘externally’ forced or supplied through river water fluxes. We show that the coastal areas where lateral density gradients are externally supplied are ‘hotspots’ of submesoscale activities throughout the year, independently of the mixed layer state. During the summer season the lateral density gradients induced at the ocean surface by the river inflow fuel frontogenesis, otherwise precluded by the shallow mixed layer depth. In winter, on the other hand, freshwater forcing limits, to some degree, the strength of frontogenesis and mixed layer instabilities, by increasing stratification near the surface and reducing the amplitude of the mixed layer deepening in response to the atmospheric forcing.