On a recent parameterization of mesoscale eddies

Abstract

A recent parameterization of mesoscale eddies by Gent and McWilliams (GMc) represents their effects as advective and diffusive fluxes along isopycnals. The form chosen for the added transport velocity due to eddies flattens isopycnals as in baroclinic instability but implicitly assumes purely viscous dissipation of the available potential energy released. If, however, the energy dissipation occurs in the ocean interior due to a process such as internal wave breaking, it is likely to cause diapycnal mixing. The implied diffusivity is large in a frontal situation, but the analysis of the spindown equation for a quasigeostrophic front shows that it causes only small changes in the frontal evolution. The spindown equation also permits analysis of the relative importance of various terms describing subgrid-scale fluxes of momentum and buoyancy, and may be interpreted in terms of Eliassen-Palm fluxes. Another possibility for the dissipation of the eddy energy that is generated from the mean available potential energy in the GMc mechanism involves air-sea interaction and subsequent water mass modification, but this is also clearly diabatic across mean isopycnals. The GMc parameterization does accomplish diabatic transfer across mean isopycnals near the surface due to the boundary conditions on the advective eddy flux, though it is not clear that this is the same as if the effect of air-sea interaction on the eddies were treated explicitly. The cross-frontal volume flux must be compatible with the buoyancy budget. In the case of the Southern Ocean, this may require the net meridional circulation cell to be weak if the air-sea buoyancy flux is small.