Generation of mid-ocean eddies : the local baroclinic instability hypothesis

Abstract

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, Explanations are sought for two ob- served features of mid-ocean eddies: 1) substantial energy is retained in the baro- clinic mode and in the associated deformation radius (Rd ) scale, and 2) the ratio of eddy to mean kinetic energy is much larger than one, The tendency of QG to cascade energy into the barotropic mode and into scales larger than Rd can be counteracted when stratification is surface-trapped, for then the baroclinic mode is weakly damped, and hence enhanced, Numerical experiments are performed with both surface-trapped and uniform stratification to quantify this, Experiments with equal Ekman frictions in the two layers are also performed for purposes of contrast, Interpretation is aided with an inequality derived from the energy and enstrophy equations, The inequality forbids the simultaneous retention of substantial energy in the baroclinic mode and in scales near Rd when Ekman friction is symmetric, but points towards surface-trapped stratification and bottom- trapped friction as an environment in which both of these can be achieved, The dissertation also contains a systematic study of geostrophic turbulence forced by nonzonal flows, Narrow zonal jets emerge when shear-induced mean potential vorticity (PV) gradients are small compared to the planetary gradient (f3), and energy is a strong function of the angle shear presents to the east-west direction, When shear-induced PV gradients are comparable to f3, and the mean shear has a westward component, fields of monopolar vortices form and persist, Energy is asym- metric between fields of cyclones and anticyclones, Such asymmetry was commonly thought not to occur in QG, but is shown here to be introduced by the nonzonal basic state, In both jet and vortex regimes, eddy energy can be much larger than mean kinetic energy, contrary to the expectation that f3 stabilizes weak shear flows,