On the effect of surface gravity waves on mixing in the oceanic mixed layer

We apply a one-dimensional mixed layer model, based on second moment closure of turbulence, to study the effects of surface gravity waves on mixing in the oceanic mixed layer. The turbulent kinetic energy injected near the surface by breaking waves, and the kinetic energy input from Langmuir circulations that may exist in the presence of surface gravity waves, are both parameterized and included in the turbulence model. As expected, the wave breaking elevates both the turbulent kinetic energy and its dissipation rate in the upper few meters, well above the classical values expected from similarity theory for shear layers adjacent to a boundary. While there is a significant impact on mixed layer properties near the surface, wave breaking-induced turbulence decays rapidly with distance from the surface and hence the overall effects on the mixed layer are small. On the other hand, the energy input to turbulence from Langmuir cells elevates the turbulent kinetic energy and mixing throughout the mixed layer, and is therefore more effective in deepening the mixed layer. While the changes in sea surface temperature (SST) brought about by the inclusion of Langmuir cells are rather small on diurnal time scales, they can be appreciable over seasonal time scales. Nevertheless, these SST changes are well within the uncertainties in the modeled SST resulting from an imperfect knowledge of the air–sea fluxes used to drive the mixed layer models.