AN OCEAN LARGE-EDDY SIMULATION OF LANGMUIR CIRCULATIONS AND CONVECTION IN THE SURFACE MIXED-LAYER

Numerical experiments were performed using a three-dimensional large-e ddy simulation model of the ocean surface mixed layer that includes th e Craik-Leibovich vortex force [Craik 1977; Leibovich 1977] to paramet erize the interaction of surface waves with mean currents. Results fro m the experiments show that the vortex force generates Langmuir circul ations that can dominate vertical mixing. The simulated vertical veloc ity fields show linear, small-scale, coherent structures near the surf ace that extend downwind across the model domain. In the interior of t he mixed layer, scales of motion increase to eddy sizes that are rough ly equivalent to the mixed-layer depth. Cases with the vortex force ha ve stronger circulations near the surface in contrast to cases with on ly heat flux and wind stress, particularly when the heat flux is posit ive. Calculations of the velocity variance and turbulence dissipation rates for cases with and without the vortex force, surface cooling, an d wind stress indicate that wave-current interactions are a dominant m ixing process in the upper mixed layer. Heat flux calculations show th at the entrainment rate at the mixed-layer base can be up to two times greater when the vortex force is included. In a case with reduced win d stress, turbulence dissipation rates remained high near the surface because of the vortex force interaction with preexisting inertial curr ents. In deep mixed layers (similar to 250 m) the simulations show tha t Langmuir circulations can vertically transport water 145 m during co nditions of surface heating. Observations of turbulence dissipation ra tes and the vertical temperature structure support the model results.