Ocean Surface Mixed Layer

The upper region of the ocean typically exhibits of a surface mixed layer with a thickness of a few to several hundreds meters. This mixed layer is a key component in studies of climate, biological productivity and marine pollution. It is the link between the atmosphere and deep ocean and directly affects the air-sea exchange of heat, momentum and gases. Moreover, turbulent flows in the mixed layer affect biological productivity by controlling both the supply of nutrients to the upper sunlit layer and the light exposure of phytoplankton.

Several processes contribute to turbulent mixing in the mixed layer. Thermal convection can be generated by the ocean losing heat through longwave back radiation or evaporative cooling. The shear generated in wind-driven currents can produce Kelvin-Helmholtz billows. The interaction between surface waves and wind-driven shear current also produces Langmuir circulation, consisting of counter-rotating vortices with their axes aligned roughly in the wind direction.

As a part of CBLAST program funded by the Office of Naval Research, I am using Large Eddy Simulation (LES) models to investigate how large eddies affect the deepening of the ocean surface mixed layer and how they affect the air-sea momentum and heat fluxes. The following diagrams show examples of LES simulation results.

Langmuir Circulation	Thermal Convection

  

Wave-driven Langmuir circulation, buoyancy-driven thermal convection and shear-driven Kelvin-Helmholtz billows are three dominant large eddies in the ocean surface mixed layer. We are examining how they compete to generate turbulence in an initially well-mixed layer. By nondimensionalizing the LES equations, we have identified two controlling dimensionless numbers: (1) Hoenikker number Ho (Li & Garrett, 1995, JPO) is a ratio of buoyancy force to vortex force; (2) turbulent Langmuir number La_t (McWilliams et al. 1997, JFM) is a ratio of the water friction velocity to the Stokes drift velocity.

We have explored the transition from shear-dominated turbulence to Langmuir-dominated turbulence in the absence of surface heat fluxes (see the following figure). In the LES experiments we conducted, the wind speed ranges between 5 and 15 m/s, the surface wave height varies from 0.5 to 4 m while the dominant wave length is kept at 60 m. The vertical velocity variance is a key measure of the turbulence field and represents the amount of kinetic energy available for mixing the water column. As wave forcing is added and La_t decreases, it becomes larger and reaches a maximum at a lower depth (left figure). The right figure shows the depth-averaged (within the mixed layer) vertical velocity variances as a function of La_t. The far-right end point represents a pure shear turbulence case. The vertical velocity variance shows little variation until La_t drops below about 0.7. However, it increases rapidly as La_t further decreases. This shows a separation between the shear-dominated and Langmuir-dominated turbulence. These results are encouraging and suggest that a small set of distinguishing flow metrics exist to characterize complex turbulence field.

In this ONR project, I am collaborating with Prof. Chris Garrett at the University of Victoria, Canada, Drs. Bob Weller, Jim Edson, John Trowbridge at Woods Hole Oceanographic Institution and Dr. Eric DAsaro at the University of Washington.