HR: 16:15h
AN: OS52G-08    [PDF]
TI: Deepening of the Mixed Layer Into a Linearly Stratified Water by Langmuir and Shear Turbulence
AU: * Li, M
EM: mingli@hpl.umces.edu
AF: University of Maryland CES, Horn Point Lab. 2020 Horn Point Road P.O. Box 775, Cambridge, MD 21613 United States
AB: A problem of critical importance to air-sea interaction is how the turbulent large eddies erode the stratification and redistribute water properties in the ocean surface layer. Using the LES model, we have examined how shear and Langmuir turbulence erode 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 the other representing shear turbulence. In Langmuir turbulence, vigorous mixing quickly generates a surface mixed layer, as cold water is engulfed from below by upwelling plumes. Because of the effective momentum and tracer transport, both the mean velocity and mean temperature profiles become nearly uniform within the mixed layer. In shear turbulence, the deepening of the mixed layer is slower and is caused by Kelvin-Kelmholtz billows. Significant shear remains in the mean velocity profile. The mean temperature profile also shows a more gradual change across the mixed-layer base. These numerical experiments point to two different mechanisms for mixed-layer deepening. In Langmuir turbulence, the deepening occurs through direct engulfment of stratified water into the mixed layer. In shear turbulence, the deepening occurs through the shear instability. The LES results are used to test two different parameterization scheme: one based on the PWP bulk Richardson number criterion and another one based on the 2D DNS results of Li \& Garrett (1997).
DE: 4504 Air/sea interactions (0312)
DE: 4568 Turbulence, diffusion, and mixing processes
SC: Ocean Sciences [OS]
MN: 2004 Ocean Sciences Meeting