Density-driven flow along an eastern-boundary continental margin
in an inviscid, variable-density layer model


Surface currents along the eastern boundries of ocean basins are usually directed equatorward, being driven by equatorward winds. There is however, a prominent poleward current against the prevailing equatorward winds along the west coast of Australia, called the Leeuwin Current. It is suspected to be driven by the surface poleward density gradient, which is particularly strong in the South Indian Ocean.

To study the dynamics of the Leeuwin Current, we obtain steady, analytic solutions to a variable-density, two-layer model on a beta-plane without friction in a semi-infinite domain x <x_e and y <y_n<0 (Southern Hemisphere). The upper-layer density field is externally prescribed to increase poleward, providing a thermohaline forcing for the system, while the lower layer is assumed to be motionless with a uniform density. When the upper layer does not intersect the bottom, planetary Rossby waves eliminate all pressure gradients.

When it does, topographic Rossby waves trap the pressure gradient on the slope. As a result, a broad eastward flow is generated in the offshore region and it bends poleward where the upper layer intersects the bottom, forming a poleward eastern-boundary current. The transport of this current is insensitive to the bottom slope and is approximately proportional to the square of the upper layer thickness.