Simulated Dynamic Balances for Mid-Ocean Mesoscale Eddies

Abstract

Eulerian potential vorticity and heat balances calculated for a multi-layered quasi-geostrophic model are shown to be consistent with those inferred from the MODE data. Above the thermocline the balances are the result of the turbulent cascade of relative vorticity which tends to separate the first moments of the enstrophy and energy spectra. The vorticity balance is dominated by the passive advection of the small-scale contributions to the relative vorticity by the energy-containing scales. By either transferring to the Lagrangian frame or focusing on the energy-containing scales this advective effect is suppressed and one observes a nearly linear response by relative vorticity and vertical vortex stretching to changes in planetary vorticity. In the bottom layer there are topographic Rossby waves superposed on a time-mean flow along f/H contours. Both of these effects ?detune? the bottom layers from those above the thermocline to inhibit the cascade to barotropic flow which otherwise occurs in quasi-geostrophic turbulence. The intermediate layers have a balance which is a mixture of those described above.