Ageostrophic Secondary Circulation at a Submesoscale Front and the Formation of Gravity Currents

Abstract

AbstractLarge-eddy simulations are performed to investigate the development of the ageostrophic secondary circulation (ASC) and associated transport in a submesoscale front. Based on the observations in the northern Bay of Bengal and in the Pacific cold tongue, the model front has a large cross-front density difference that is partially compensated with lateral temperature and salinity gradients. Vertical stratification is varied in different cases to explore its effect on the ASC. The evolution of the ASC differs with stratification. When the front is unstratified, shear instabilities, which develop from the geostrophic shear, cause the front to slump. Cold water from the light side propagates across the front on the surface, while warm water from the dense side spreads in the opposite direction at depth. In cases with stratifications, a shear layer driven by the cross-front pressure gradient forms at the surface to initiate the ASC. Shear-driven turbulence associated with the enhanced shear in the layer causes the front to slump, and the development of the ASC onward is similar to the unstratified case. Irrespective of the initial stratification of the strong fronts simulated here, the surface layer evolves into a gravity current. The ASC is composed of the surface gravity current and a countercurrent that are separated by a middle layer with enhanced stratification and a thermal inversion. Turbulent dissipation is enhanced at the nose of the gravity current and in a sheared region somewhat behind the leading edge of the countercurrent. The gravity current propagates at a speed proportional to the buoyancy difference across the front in the case with no stratification.