On a recent parameterization of mesoscale eddies
J. Phys. Oceanogr., 26, 3, 406-411, 1996.
A recent (GMc) parameterization of mesoscale eddies represents their
effects as advective and diffusive fluxes along isopycnals (Gent and
McWilliams 1990; Gent et al. 1995). 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 we show, by analysis of the spindown equation
for a quasi-geostrophic front, that it causes only small changes in the
frontal evolution. The spindown equation also permits analysis of the relative
importance of various terms describing sub-grid-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.
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