Ocean mixing
Small-scale mixing processes, including wind- and buoyancy-driven mixing in the ocean surface layer, instabilities driven by large-scale shear and internal waves in the ocean interior, double-diffusive interleaving, and frictionally-driven processes at the bottom boundary, are an important component of the large-scale ocean climate, providing cross-isopycnal fluxes of heat, salt and other tracers. These processes occur on scales much smaller than the climate model grid-scale, and so must be parameterized (represented in terms of resolved scales) in climate simulations. Our goal is to develop efficient physically-based parameterizations of these processes, using a combination of high resolution simulations which resolve the predominant scales of the mixing, and information from laboratory experiments, theory and ocean observations.
Some key areas under investigation at GFDL include:
Mixing driven by large-scale shear: Because the ocean is stratified away from the top and bottom boundaries, work is required to move fluid in the vertical. In the interior, a major source of energy for this mixing is shear associated with large-scale currents. Representing such mixing is tricky, as parameterizations that work for regions like the equatorial undercurrent may not work for shear mixing associated with overflows. One key issue is representing the length scales of turbulence. Laura Jackson, Bob Hallberg and Sonya Legg have used high-resolution simulations to develop a new parameterization of this process that has been incorporated in our isopycnal climate model.
Overflows: Like massive waterfalls, tens of kilometers across, overflows carry cold dense waters from marginal seas in polar regions into the abyssal ocean, mixing and entraining ambient fluid. Mixing helps determine the paths and properties of such overflows. Because coarse-resolution models have trouble representing overflows (see papers here and here), a number of GFDL scientists have been involved with investigating such flows in range of models as part of a Climate Process Team on Gravity Current Entrainment, also described here.
Topographically controlled mixing: Mixing can be generated as a result of tidal
flow over ridges and sea-mounts. Motivated by the Hawaii Ocean Mixing Experiment, Sonya Legg has been looking at the generation of mixing by tides over the Hawaiian Ridge.
We are also interested in understanding how this mixing impacts the climate system (particularly the ocean overturning) and global biogeochemical cycling. Mixing affects the pathways of upwelling of deep water, with impacts on biological cycling, global radiocarbon distributions, and the uptake of anthropogenic carbon dioxide.
Scientists involved in this work:
Anand Gnanadesikan (impacts of mixing)
Robert Hallberg (shear mixing, mixed layers, overflows)
Matt Harrison (impacts of mixing in coupled models)
Sonya Legg (process studies of shear mixing, overflows)
Links
Link to Climate Process Team on Gravity Current Entrainment
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