Thermohaline forcing of eastern boundary currents: with application to the circulation off the west coast of Australia

Abstract

The linear, viscid, continuously stratified model of McCreary (1981) is extended to allow for thermohaline forcing by a specified, longitudinally independent, surface density field ρ_s. When the ocean is unbounded and ρ_s is steady, the density field is altered throughout the water column by vertical diffusion. If ρ_s increases poleward, the resulting pressure field slopes downward toward the pole in the upper water column, and there is an associated eastward geostrophic current. This interior current forces downwelling at an eastern ocean boundary, and generates a poleward surface coastal current and an equatorward undercurrent. For realistic choices of model parameters the coastal circulation is as strong as, and opposite in direction to, that caused by a typical equatorward wind stress τ^y. When ρ_s oscillates at the annual cycle, the unbounded flowis confined to a surface boundary layer. The coastal circulation is qualitatively quasi-steady, but also has characteristics of a vertically propagating Kelvin wave. One solution is forced by an idealized representation of the observed ρ_s and τ_y fields off the west coast of Australia. This solution compares well with observations of the oceanic circulation there. In particular, there is a surface coastal jet (the model Leeuwin Current) that flows against the wind. This successful comparison suggests that the mean circulation in the region is significantly forced by ρ_s, whereas the annual variability is strongly forced by τ_y.