The role of turbulence in strong submesoscale fronts of the Bay of Bengal

Abstract

The evolution of turbulence in a shallow submesoscale front is investigated using high-resolution large-eddy simulations (LES). The model front has uniquely large cross-front temperature and salinity differences motivated by recent observations in the Bay of Bengal. The temperature and salinity differences are increased in a parametric study to examine their effects on the evolution of turbulence. Shear turbulence induced by the initially geostrophic jet causes the front to slump and an ageostrophic secondary circulation (ASC) develops. The ASC is composed of two rotating currents: a surface gravity current (SGC) with cold fresh water above a counter-current (CC) jet with warm saline water. Virtual moorings on the two sides of the front are used to depict the evolution of stratification, shear, turbulent stresses, turbulent heat and salt fluxes, as well as turbulent kinetic energy (TKE) budget terms. A barrier layer with thermal inversion develops on both sides of the front while the mixed layer depth becomes shallower across the front. Turbulent production driven by vertical shear is the dominant source of TKE on both sides of the front and horizontal shear production also contributes significantly to the TKE generation. The turbulent stresses, heat and salt fluxes are enhanced on both sides of the front with subsurface warming in the thermal inversion layers. As the cross-front temperature and salinity differences increase among the cases, the ASC intensifies, and the currents and the turbulence become stronger.