Turbulence at a symmetrically unstable front

Recent observations in the Bay of Bengal show persistent lateral salinity and temperature gradients in the upper ocean over a wide range of horizontal scales. It is of interest to understand how the lateral density gradient and vertical stratification combine to affect the sea surface temperature, turbulent transport and restratification. The present study uses high-resolution three-dimensional large-eddy simulations (LES) to investigate the evolution of turbulence and mixing associated with the onset of symmetric instability (SI) at a geostrophic front with Ri = 0.5. A compensated front with lateral salinity and temperature gradient in thermal wind balance is simulated, and the mechanisms and the energetic pathways leading to turbulence and mixing are analyzed. The stratified front is shear-stable based on the Ri criterion; however, it is susceptible to symmetric instability. When SI develops, bands of cross-front velocity form along the isopycnals, grow to finite-amplitude and trigger secondary Kelvin-Helmholtz (KH) shear instability. KH rollers break down into localized patches of turbulence and mix the surface layer. After the onset of KH instability, energetic large-scale modes, depicted by interleaving bands of strong and weak lateral density gradient, persist. These bands with strong lateral gradient continue to trigger KH instability and turbulence. Analysis of the energy budget shows that the available potential energy associated with the lateral density causes the mean kinetic energy of the geostrophic current to increase as well as the kinetic energy of the large-scale fluctuations. The sources of turbulent kinetic energy are shear production and positive buoyancy flux. The amount of mixing and the evolution of the stratification, geostrophic current and lateral density gradient are quantified as a function of frontal strength.