Why western boundary currents are diffusive: A link between bottom pressure torque and bolus velocity

The bottom pressure torque is known to vanish in the interior ocean but to play a dominant role in the western boundary layer in balancing the planetary vorticity on spatial scales larger than the Rossby radius of deformation. In this study, the appearance of the bottom pressure torque and thus any deviation of wind-driven flow from classical Sverdrup balance is locally related in steady state to non-zero bolus velocity and/or friction, under the assumption that horizontal density advection is small compared to the lifting of isopycnals. To first order approximation, the vortex stretching by the vertical bolus velocity is related to the bottom pressure torque. The bolus vortex stretching becomes a significant term in the barotropic vorticity budget of the western boundary layer and is formally equivalent to bottom friction as in the classical models of the wind-driven gyre circulation.