Baroclinically Unstable Geostrophic Turbulence in the Limits of Strong and Weak Bottom Ekman Friction: Application to Midocean Eddies

Abstract

This paper examines the plausibility of mesoscale eddy generation through local baroclinic instability of weak midocean gyre flows. The main tool is a statistically steady, two-layer quasigeostrophic turbulence model driven by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated by bottom Ekman friction. A wide range of friction strengths is investigated. In the weakly damped limit, flow is nearly barotropic, and the horizontal length scale of barotropic energy increases with decreasing friction, consistent with previous studies. The strongly damped limit, explored here for the first time, is equivalent barotropic (lower-layer velocities are nearly zero) and features an increase in the horizontal scale of potential energy with increasing friction. Current-meter data suggest that midocean eddies lie between the barotropic and equivalent barotropic limits. In accord with this suggestion, the moderately damped regime of the model compares well to observations of eddy amplitude, vertical structure, and horizontal scale, especially when stratification is surface intensified. A review of pertinent observations suggests that mesoscale eddies may indeed lie in the moderately damped limit. These arguments are first developed in f-plane simulations. Previous studies of beta-plane turbulence have had eastward mean flows, and in this case eddy energy has little sensitivity to friction. However, midocean gyre flows are generally nonzonal, and this nonzonality appears to be a significant factor in the production of energetic eddies. Beta-plane turbulence driven by nonzonal mean flows is sensitive to bottom friction, such that moderate damping is required for model eddies to compare well to observations, as on the f plane. A heuristic argument is presented in support of this similarity.