Adiabatic Eastern boundary currents

The dynamics of the eastern boundary current of a high-resolution, idealized model of the oceanic circulation are analyzed and interpreted in terms of residual mean theory. In this framework it is clear that the eastern boundary current is adiabatic and inviscid. Nevertheless, the time averaged potential vorticity is not conserved along averaged streamlines, because of the divergence of Eliassen-Palm fluxes, associated with buoyancy and momentum eddy fluxes. In particular, eddy fluxes of buoyancy completely cancel the mean downwelling or upwelling, so that there is no net diapycnal residual transport. The eddy momentum flux acts like a drag on the mean velocity, opposing the acceleration due to the eddy buoyancy flux: in the potential vorticity budget this results in a balance between the divergences of eddy relative vorticity and buoyancy fluxes, which leads to a baroclinic eastern boundary current whose horizontal scale is the Rossby deformation radius, and whose vertical extent depends on the eddy buoyancy transport, the Coriolis parameter and the mean surface buoyancy distribution.