GM vs Biharmonic Ocean Mixing in the Arctic

Elizabeth Hunke, LANL

As part of the Arctic Ocean Model Intercomparison Project, the LANL ice-ocean modeling team completed two 55-year, global, ice-ocean simulations forced with atmospheric reanalysis data for 1948-2002. These two simulations differ only in the parameterization used for lateral mixing of tracers (potential temperature and salinity) in the ocean, but the resulting circulation and kinetic energy of the simulated oceans are very different, particularly at high latitudes. The differences can be traced to two effects, (1) scale selectivity, in which the Laplacian form of the Gent and McWilliams (GM) parameterization damps wave energy more quickly than the biharmonic mixing formulation, and (2) grid dependence of the diffusion coefficient, which appears in the biharmonic formulation but not in GM, and is particularly important at high latitudes where the grid scale decreases dramatically on the sphere. We conclude that, in order to maintain consistent suppression of numerical noise while allowing for a more energetic circulation in regions of finer grid spacing, future global simulations using the GM parameterization should include a diffusivity scaling factor given by the square root of the grid cell area.