Generation of strong mesoscale eddies by weak ocean gyres

The generation of strong mesoscale variability through instability of the large-scale circulation in the interior of oceanic gyres is addressed. While previous studies have shown that eddies generated from weakly sheared zonal flows are generally weak, the present results demonstrate that weakly sheared meridional flows typical of wind-forced gyres can generate very strong mesoscale variability. Meridional flows are effective at generating strong eddies because the reduced influence of the planetary vorticity gradient allows the potential energy stored in the zonal potential vorticity gradient to be converted to eddy kinetic energy. A simple scaling theory based on a balance between turbulent cascade and baroclinic energy production yields an estimate of the equilibration amplitude of the eddy kinetic energy. Nonlinear quasi-geostrophic model calculations configured in both a periodic meridional channel and a wind-driven subtropical gyre agree well with the scaling theory.