Deepening of the ocean mixed layer by Langmuir and shear turbulence

A problem of critical importance to air-sea interaction is how turbulent large eddies erode the stratification and redistribute water properties in the ocean surface layer. Using the LES model, we have examined how wind-driven shear and Langmuir turbulence penetrate into a linearly stratified water. We identified two controlling dimensionless parameters: (1) turbulent Langmuir number which is the ratio of water friction velocity to the Stokes drift velocity; (2) a Richardson number which compares the strength of stabilizing buoyancy force to the wind shear. We ran the LES model for a wide range of buoyancy frequency and examined how the turbulent large eddies erode the stratification and generate a surface mixed layer. We then compared the numerical results at two different values of turbulent Langmuir number: one representing Langmuir turbulence and one representing shear turbulence. In Langmuir turbulence, the mixed-layer deepening occurs through direct engulfment of stratified water into the mixed layer. In shear turbulence, the deepening occurs through the shear instability. Both deepening processes can be interpreted in terms of a Froude number representing a competition between vertical mixing and stabilizing stratification force. The mixed-layer deepening stops when the required potential energy exceeds the available kinetic energy.

To interpret turbulence measurements collected at CBLAST-low site, we also investigated the mixed-layer deepening in shallow water. We examined how turbulence generated in the bottom boundary layer interacts with turbulence in the surface mixed layer. In addition to surface forcing due to wind stress, waves and heat flux, we impose an oscillating body force to generate a tidal current. We carried out numerical runs to investigate the stratification conditions under which the surface and bottom boundary layers are coupled or decoupled.