Nonlocal transport due to Langmuir circulation in a coastal ocean

We present observations and simulations of large scale velocity structures associated with turbulent boundary layer dynamics of a coastal ocean. Special purpose acoustic Doppler current profiler measurements revealed that such structures were frequently present, in spite of obstructing tidal currents and misaligned wind and wave fields. Large eddy simulation results are only consistent with these observations if the Langmuir circulation (LC) effect due to wave-current interaction is included in the model. Thus, model results indicate that the observed large scale velocity structures are due to LC. Based on these simulations, we examine the shift of energetics and transport from a local regime for purely shear-driven turbulence to a nonlocal regime for turbulence with LC due to coherent large-scale motions that span the whole water column. Without LC, turbulent kinetic energy (TKE) dissipation rates approximately balance TKE shear production, consistent with solid wall boundary layer turbulence. This stands in contrast to the LC case for which the vertical TKE transport plays a dominant role in the TKE balance. Conditional averages argue that large scale LC coherent velocity structures extract only a small fraction of energy from the wave field, but receive most of their energy input from the Eulerian shear production. The analysis of scalar fields and Lagrangian particles demonstrates that the vertical transport is significantly enhanced with LC, but that small scale mixing may be reduced. In the presence of LC, vertical scalar fluxes may be up-gradient, violating a common assumption in oceanic boundary layer turbulence parameterizations.