Roles of breaking waves and Langmuir circulation in the surface boundary layer of a coastal ocean

Breaking waves and Langmuir circulation are two important turbulent processes in the wind-driven upper ocean. To investigate their roles in generating turbulence in the surface boundary layer of a coastal ocean, a large eddy simulation model is used to simulate the turbulence measurements collected at the Martha's Vineyard Coastal Observatory's Air-Sea Interaction Tower, during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) experiment in 2003. The model provides reasonable predictions for the vertical profiles of vertical velocity variance, turbulent kinetic energy (TKE), energy dissipation rates, and heat flux. It shows breaking waves dominating turbulence generation near the ocean surface and turbulent large eddies characteristic of Langmuir circulation deeper in the water column. Diagnostic analysis of TKE budget in the model shows a dominant balance between turbulent transport and dissipation near the surface and a dominant balance between shear production and dissipation at deeper depths. Although the Stokes production is a significant term in the TKE budget balance near the surface, it is smaller than shear production. The turbulent transport is large in the near-surface zone and is still significant in the region affected by Langmuir circulation. These results are in agreement with a conclusion inferred from a recent analysis of the near-surface turbulence measurements at the CBLAST site.