The Large-Scale Environment of the Poleward-Flowing Leeuwin Current, Western Australia: Longshore Steric Height Gradients, Wind Stresses and Geostrophic Flow

Abstract

Steric heights have been estimated using T?S relationships for all available XBTs within 10?40°S, 105?130°E to 1980, and merged with steric heights from hydrology data. These data were subdivided into bins following the Western Australian coast, and a two-harmonic best fit to the seasonal cycle obtained in each bin. Seasonal departures from the annual mean of surface steric height are combined with similar departures of pressure-corrected coastal tide gage data to obtain maps of seasonal departure in geostrophic flow, both on and off the continental shelf. These are added to annual mean patterns to give total seasonal flows, for comparisons with earlier work. Wind stress vectors, wind stress curls and longshore wind stress components are also obtained, using data from a marine climate altas. The results give an improved picture of the broad-scale environment surrounding the Leeuwin Current (a narrow, rapid poleward current along the Western Australian continental shelf edge) and of the wind stresses that may partly or wholly drive the Current. On annual average, the Leeuwin Current accelerates into the wind from 22.5 to 32.5°S; its greatest speeds are at the surface. Below 300 m there is a slow deep equatorward undercurrent with a mass transport comparable in magnitude to the nearsurface polewards flow. These and other results support Thompson's contention that the Leeuwin Current is primarily a convection current, driven by longshore steric height gradients at the continental shelf edge. Sverdrup balance may be violated near the shelf edge. It is suggested that throughflow from the Pacific to the Indian Ocean may create the large steric height gradient along the Western Australian continental shelf, which in turn results in the uniquely large poleward nearsurface flow in the Leeuwin Current. In the depth-integrated longshore momentum equation at the continental shelf edge, the seasonal cycles of the pressure gradient and wind stress terms reinforce one another, resulting in a strong net southward force in May, when the Leeuwin Current is strongest; this behavior is specific to the shelf edge, and does not occur at the coast. This seasonal variation seems to be part of a wavelike pattern; the ?wave? originates north of Australia during the Northwest Monsoon and progress rather slowly anticlockwise around Western Australia.