Vortex expulsion by a zonal coastal jet on a transverse canyon

Abstract

Recent observations of subsurface-intensified, alongshore slope-currents have shown vortex formation over a transverse canyon, in the Gulf of Cadiz. To analyze this process, we idealize this situation to a zonal coastal jet, with piecewise-constant potential-vorticity, flowing over a meridionally sloping bottom. We analytically calculate the linear barotropic and baroclinic stability of the flow in the quasi-geostrophic framework (in the absence of the canyon). Several physical and geometrical cases are considered. The effect of an additional transverse canyon is simulated numerically using the two-dimensional contour-surgery code. It is shown that a double strip of vorticity is linearly unstable and a very shallow canyo is sufficient to provoke wave growth on the vorticity interfaces; these waves nonlinearly sat rate into a dipolar vortex; a single active vorticity region is linearly stable, but a deep enough ca yon can trigger various nonlinear responses (waves, filaments, vortex detachment, turbulence). The relevance of such an idealized model to real oceanic cases is finally discussed, its shortcomings are highlighted, and possible improvements are suggested for futur work.