Turbulent mixing in a marginally-unstable shear layer

Large-eddy simulation (LES) is used to investigate turbulence in a marginally-unstable stratified shear layer motivated by the observation of deep-cycle turbulence in the upper Equatorial Undercurrents. The initial condition consists of uniform shear and stratification profiles, and a constant gradient Richardson number Rig = 0.25. A constant wind stress forces turbulence in the upper surface layer. Cases with different shear and stratification are simulated to elucidate the effects of shear and stratification on the deep-cycle turbulence. In all cases, a thin layer of enhanced shear rate descends from the surface into the stratified shear layer. As the layer deepens, it causes the local Rig to become less than 0.25, and shear instabilities develop into turbulence. Turbulent kinetic energy (TKE) in the shear layer is higher in cases with stronger shear, and it can be significantly larger than the TKE generated by the surface wind. Cases with stronger shear also have higher shear production, buoyancy flux, and dissipation rate in the TKE budget. The shear production is the largest term in the budget showing that the mean kinetic energy local to the shear layer is the main source of energy for turbulent mixing.