Generation of Strong Mid-Ocean Eddies from Nonzonal Mean Flows

The plausibility of local baroclinic instability as a generation mechanism for mid-ocean mesoscale eddies is examined with a two-layer, quasi-geostrophic (QG) model forced by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated through bottom Ekman friction. We seek a possible explanation of the fact that the kinetic energy of mid-ocean eddies is much larger than that of the mean gyre flow. Mid-ocean gyre flows are typically so weak that it might seem that they would be stabilized by planetary beta. However, we argue that the inherent nonzonal nature of mid-ocean mean flows allows them to generate baroclinic instability much more readily than would zonal mean flows.

Therefore we present one of the first systematic studies of geostrophic turbulence forced by nonzonal mean flows. We find that when shear-induced mean potential vorticity (PV) gradients are small compared to the planetary gradient (beta), energy is indeed a strong function of the angle the mean flow presents to the east-west direction. The eddy field in this regime consists of strong zonal jets whose width is apparently not set by the Rhines scale.

The anisotropy of these jets is unrealistically large for the mid-ocean eddy field. Accordingly, we next examine isotropy as a function of the ratio of planetary to shear-induced potential vorticity gradients. We find that isotropic eddies can be generated when the angle between the layer mean potential vorticity gradients approaches 90 degrees, which occurs when shear-induced and planetary gradients are comparable. Potential vorticity maps indicate that this may be the appropriate regime for the mid-ocean.