Contribution of coherent vortices such as Langmuir cells to wind‐driven surface layer mixing

Abstract

The wind blowing over the water surface causes, even for low wind speeds, Langmuir circulation in addition to shear‐generated turbulence. Both of these mechanisms mix the upper layer of oceans and lakes, but since the mixing efficiency of Langmuir cells is unknown, their relevance to mixed layer deepening is still an open question. In order to estimate the contribution to mixing by Langmuir vortex cells relative to shear‐induced mixing, we employ results on entrainment rate obtained from laboratory experiments with turbulent Taylor vortex cells. These are coherent horizontal vortices analogous to Langmuir cells. To relate the two, we define a surface friction velocity u* that would be necessary to drive cells of strength equivalent to the Taylor vortices. It is then shown that up to a friction Richardson number of Ri* ≈ 50, layer deepening is predominantly caused by shear‐generated turbulence, whereas for Ri* > 50, the contribution by turbulent coherent vortices (Langmuir cells) dominates the mixing process. For Richardson numbers Ri* > 120, the entrainment rate decreases, but there is no criterion for the arrest of mixing by turbulent Langmuir cells as was previously suggested. The present results confirm observations that shear‐generated turbulence dominates during initial layer deepening under relatively weak buoyancy effects and that subsequently Langmuir cell mixing dominates the mixed layer deepening. The entrainment rates obtained from laboratory experiments are discussed and predict values for the mixed layer deepening that are in good agreement with in situ observations.