Vortex formation by unstable oceanic jets

Abstract

In a two-layer quasi-geostrophic model, we investigate the mixed barotropic-baroclinic instability of a thin zonal jet. Given the jet velocity profile at the surface, the deformation radius and layer depths, the three parameters governing the instability are the ratio of layerwise maximum velocities (U2 /U1 ), the planetary vorticity gradient (Beta-effect), and the wavenumber (k) of the perturbation. First, simple criteria provide thresholds in this parameter space for the onset of instability. Growth rates of monochromatic perturbations are then computed and their most unstable wavelengths are compared with oceanic observations. With a nonlinear numerical model, high-Reynolds-number evolutions of the perturbed jet are computed in the (U2 /U1 , Beta) space: wave breaking and vortex formation occur at small (Beta, while for larger values of Beta, the perturbation equilibrates into meanders. Multiple waves are also observed. Instability is minimum when the jet is confined in the upper layer (U2 /U1 =0). Finally, vortex formation by this unstable jet is specifically studied for a localized perturbation.