Large-Eddy Simulations of Stabilizing Effects Induced by Opposing Eulerian Shear and Stokes Drift Shear in an Idealized Ocean Surface Boundary Layer

Abstract Ocean surface waves strongly modulate vertical turbulent mixing in the ocean surface boundary layer. When they are aligned with wind-driven Eulerian shear, Craik–Leibovich instability occurs, resulting in the formation of Langmuir turbulence that strongly enhances vertical mixing. By the same mechanism, ocean surface waves can also stabilize the water column and suppress boundary layer turbulence when they are in the opposite direction as the Eulerian shear. Here, we demonstrate this stabilizing effect induced by opposing Eulerian shear and Stokes drift shear in large-eddy simulations (LESs) under idealized homogeneous surface cooling and no-wind conditions. Rolls of convection form under the competing effects of destabilizing surface cooling and stabilizing wave-induced stratification. The latter depends on the alignment of Eulerian shear and Stokes drift shear, resulting in roll structures aligned perpendicular to the Stokes drift. In addition, the intensity of turbulence is significantly reduced as compared to the case of pure convection. Such a stabilizing effect of wave-induced stratification has yet to be incorporated in wave-driven mixing parameterizations and may lead to potential improvements. Using this idealized test case, we also demonstrate the effect of assuming down-Eulerian shear mixing versus down-Lagrangian shear mixing in the subgrid-scale scheme by comparing two LES models. While such an effect may be hidden in strongly wind-forced cases, it results in completely different solutions in this idealized case by changing the boundary condition for the mean flow. Therefore, care should be taken when designing and interpreting LES with misaligned currents and waves.