Turbulence Reduction and the Langmuir Circulation
In this study, we expand previous large eddy simulation (LES) modeling investigations of Langmuir turbulence (LT) to real ocean conditions using field observations collected under the multi-platform field campaign “Coupled Air-Sea Processes and Electromagnetic (EM) ducting Research (CASPER-East)”. The measurement site has strong local variabilities of temperature and salinity and experienced large variations in wind forcing and several cooling events.
While LT enhances the turbulence in the water column and deepens the mixed layer during most of the simulation period, being consistent with previous studies, significant reduction in turbulent intensity is observed in the simulation with Stokes drift compared to that without Stokes drift during a one-day period, in contradiction to previous findings. Two main reasons are found to contribute to reducing the turbulence: the large misalignment between the wind and surface gravity waves and the interaction of LT with deep convection. The large wind-wave misalignment not only reduces the turbulence in the water column and traps it in a shallower surface layer, but also traps the momentum in shallower surface layer, thus producing large surface mean currents and consequently leads to further reduction of the turbulent intensity. During the cooling event, strong upwelling induced by LT at the base of the mixed layer counteracts on the downwelling associated with the deep convection and reduces the total turbulence level in the water column.