Potential vorticity "crises", adverse pressure gradients, and western boundary current separation

The vorticity dynamics involved in western boundary current separation are investigated in a depth-averaged barotropic single-gyre circulation forced by a spatially uniform wind stress curl in a circular basin on a beta-plane. The mechanism of separation is of interest in this simple model because it lacks most of the features (such as a change in sign of the wind stress curl, collision with another western boundary current, outcropping of isopycnals or an abrupt change in bottom topography or boundary shape) often associated with boundary current separation. It has been suggested that a "crisis" due to insufficient recovery of potential vorticity Q in the outer boundary current outflow can result in separation. However, the numerical results and analysis presented here demonstrate that under no-slip boundary conditions the opposite "crisis" occurs in the viscous sublayer of the western boundary current, where fluid columns acquire more Q than they lost in the interior. The outflow must, therefore, adopt a configuration which dissipates this excess Q before fluid elements return to the interior flow. It is shown that under strongly nonlinear conditions sufficient viscous dissipation of Q can only be obtained when the outflow separates from the boundary; this flow structure is also associated with an "adverse" ageostrophic pressure gradient along the boundary. Under the free-slip boundary condition the cyclonic sublayer is absent, so there is no "crisis" of excess Q and the separation behavior is markedly different.