2001: Oceans: The Ocean Down to Its Last Eddie
To further our understanding of the climate system we make use of computer models which seek to simulate its behavior under both present conditions and other possible ones future and past. It is not possible, however, to explicitly represent all the important physical processes which govern the climate in such a computer simulation due to the limitations on the abilities of computing machines for the forseeable future. In order for the models to be realistic it is nonetheless necessary to include these processes via parameterizations. Processes which occur on scales too small to be resolved in a simulation, but nevertheless have significant effects on the larger scales which are resolved, must be included by parameterizing their behavior in terms of the large scales.
For example the mixing of momentum, heat, salt and other tracers in the ocean component of the climate system is largely brought about by chaotic motions on scales beneath the resolution of the corresponding component of the computer model, the Ocean General Circulation Model. The mixing can be divided into diapycnal, that across surfaces of constant density, and isopycnal, along the surfaces of constant density. The isopycnal mixing is several orders of magnitude greater than the diapycnal. The turbulence group applies turbulence theory to the development of parameterizations for both types of mixing.
While the diapycnal mixing involves mainly more traditional roughly isotropic turbulence at scales <~1 meter, the isopycnal mixing is mainly by mesoscale eddies which extend vertically over the ~5km depth of the ocean, but horizontally a much greater distance ~100 km. The mesoscale eddies are large enough that, while they cannot presently be resolved in realistic ocean simulations run over the long time scales necessary for climate research, they have been resolved in ocean models run for shorter times recently using supercomputers.
Our project for this summer is to make comparisons of data obtained from such high resolution OGCM runs with the models used to parameterize the mixing by mesoscale eddies in the coarser resolution OGCMs feasible for use in climate studies. The models for the parameterization will include the Gent McWilliams model[Gent, P.R., and J.C. McWilliams, 1990, JPO Vol.20], which has already seen wide use and produced significant improvements [Large, W.G., G. Danabasoglu, S.C. Doney and J.C. McWilliams, 1997, JPO Vol.27], but also been found to yield some major discrepancies with eddy resolving simulation results [Bryan, Dukowicz, and Smith, 1999, JPO Vol.29; M. Solevev, P.H. Stone and P. Malanotte-Rizzoli, submitted JPO], and a new model derived from turbulence theory [V.M. Canuto, M.S. Dubovikov and M.I. Syrkin, submitted JPO] being developed by our group at GISS . They will compare the high resolution results with predictions made by binning the high resolution data to coarse scale and then applying the models for the mesoscale eddy parameterization to it. Fields to be compared will include the bolus velocity, which represents an effective largescale advection contributed by the mesoscale eddies, and the divergence of the product of this velocity with the distance between isopycnal surfaces. They will produce visualizations of these quantities as functions of depth and for different horizontal positions. Their results will help to evaluate the parameterizations and provide guidance as to which parameterization should be used to attain the most realistic results in coarse grid OGCM and coupled climate models.
