|
|
|
|
Upper ocean dynamics and
air-sea interaction
Figure 1. A schematic diagram of physical processes occurring near the
air-sea interface.
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.
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.
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.