RESEARCH> Upper Ocean Dynamics

Upper ocean dynamics and air-sea interaction

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.

Figure 1. A schematic diagram of physical processes occurring near the air-sea interface

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.

Thermal Convection Langmuir Circulation

Figure 2. LES simulations of thermal convection and Langmuir circulation.

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 have examined 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 Lat (McWilliams et al. 1997, JFM) is a ratio of the water friction velocity to the Stokes drift velocity. We have explored low-order turbulence statistics in the Lat and Ho parameter space for a wide range of atmospheric forcing conditions and construct a regime diagram to differentiate buoyancy-, shear- and wave-driven turbulence. All three types of turbulent flows are anisotropic but show different orderings of turbulence intensities: vertical > (downwind, crosswind) in convective turbulence; downwind>crosswind>vertical in shear turbulence; crosswind>vertical>downwind in Langmuir turbulence. These orderings of turbulence intensities can be explained by examining the turbulence energy production in three directions.

Buoyancy production in the vertical direction dominates turbulence generation in convective turbulence, whereas shear production in the downwind direction dominates turbulence generation in shear-driven turbulence. In Langmuir turbulence, however, Stokes production due to surface waves generates turbulence energy in both crosswind and vertical directions (Figure 3). Turbulence in the wind-driven upper ocean shows a transition from shear to Langmuir turbulence as Lat decreases. A fully-developed sea state corresponds to Lat=0.3 and lies within the Langmuir regime. Vertical turbulence intensity in Langmuir turbulence is about two times larger than that in shear turbulence and falls into the range observed in the upper ocean. Hence the wind-driven upper ocean will be dominated by Langmuir turbulence under typical sea state conditions. Transition from Langmuir to convective turbulence occurs around Ho=O(1), which is much greater than Ho=O(0.01) obtained using typical heat fluxes and wind speeds.

Figure 3. The depth-averaged vertical velocity variance as a function of Lat, showing a transition from shear to Langmuir turbulence. When Lat >0.7, it is nearly constant so that turbulence intensity is scaled by the friction velocity only. When Lat <0.7, it increases rapidly with decreasing Lat, showing additional turbulence generation due to the Stokes drift. The shaded grey box indicates the range of vertical velocity variances observed in the upper ocean over deep water while a fully-developed sea corresponds to Lat=0.3 (from Li et al., 2005).

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. We have examined how Langmuir and shear turbulence interact with stratified fluid and cause the deepening of the surface mixed layer. Figure 4 shows a comparison between two numerical experiments. Run 1 (Figures 4a-c) corresponds to Lat =0.34 typical of Langmuir turbulence. Vigorous mixing due to Langmuir eddies quickly generates a surface mixed layer. As shown by uplifting temperature contours, stratified water is being engulfed into the surface layer by upwelling plumes in Langmuir turbulence. Run 2 (Figures 4d-f) corresponds to Lat =1.76 typical of shear turbulence. Turbulent mixing now appears to be generated by Kelvin-Helmholtz billows near the base of the mixed layer. The comparison shown in Figure 4 points to two different mechanisms for the mixed-layer deepening: engulfment of stratified water by Langmuir turbulence and localized overturning by shear turbulence.

Figure 4. Deepening of the mixed layer into linearly stratified water by Langmuir (a-c) and shear (d-f) turbulence. Vertical/downwind velocity distribution (a/d) and contours of temperature (b, e) in a crosswind section, vertical profiles of mean temperature (c, f). In Langmuir turbulence, upwelling plumes engulf stratified water into the mixed layer. In shear turbulence, Kelvin-Kelmholtz billows cause the deepening of the mixed layer.

Related Publications:

Li, M., C. Garrett and E. Skyllingstad. 2005. A regime diagram for classifying turbulent large eddies in the upper ocean. Deep-Sea Res. I, 52, 259-278.

Li, M. 2004. Deepening of the ocean mixed layer by Langmuir and shear turbulence. Proceedings of American Meteorological Society 16th symposium on boundary layer turbulence. 11.10, 5 pp.

Farmer, D.M., S. Vagle and M. Li. 2001. Bubble and temperature fields in Langmuir circulation. Fluid Mechanics and the Environment: Dynamical Approaches, Edited by John L. Lumley, 91-105.

Garrett, C., M. Li and D.M. Farmer. 2000. The connection between bubble size spectra and energy dissipation rates in the upper ocean, J. Phys. Oceanogr., 30, 2163-2171.

Colbo, K. and M. Li. 1999. Parameterizing particle dispersion in Langmuir circulation. J. Geophys. Res., 104, 26059-26068.

Li, M. and C. Garrett. 1998. Large eddies in the surface mixed layer and their effects on mixing, dispersion and biological cycling. In Physical Processes in Lakes and Oceans (AGU series on Coastal and Estuarine Studies), edited by J. Imberger, 61-86.

Li, M. and C. Garrett. 1997. Mixed-layer deepening due to Langmuir circulation. J. Phys. Oceanogr., 27, 121-132.

Li, M., K. Zahariev and C. Garrett. 1995. Role of Langmuir circulation in the deepening of the ocean surface mixed layer. Science, 270, 1955-1957.

Li, M. and C. Garrett. 1995. Is Langmuir circulation driven by surface waves or surface cooling? J. Phys. Oceanogr., 25, 64-76.

Farmer, D.M. and M. Li. 1995. Patterns of bubble clouds organized by Langmuir circulation. J. Phys. Oceanogr., 25, 1426-1440.

Li, M. and C. Garrett. 1993. Cell merging and the jet/downwelling ratio in Langmuir circulation. J. Mar. Res. 51, 737-769.