On the Leeuwin Current System and Its Linkage to Zonal Flows in the South Indian Ocean as Inferred from a Gridded Hydrography

The Leeuwin Current System (LCS) along the coast of Western Australia consists of the poleward-flowing Leeuwin Current (LC), the equatorward-flowing Leeuwin Undercurrent (LUC), and neighboring flows in the south Indian Ocean (SIO). Using geostrophic currents obtained from a highly resolved (1/8 degrees) hydrographic climatology [CSIRO Atlas of Regional Seas (CARS)], this study describes the spatial structure and annual variability of the LC, LUC, and SIO zonal currents, estimates their transports, and identifies linkages among them. InCARS, the LCis supplied partly by water fromthe tropics (an annual mean of 0.3 Sv; 1 Sv 10(6) m(3) s(-1)) but mostly by shallow (less than or similar to 200m) eastward flows in the SIO(4.7 Sv), and it loses water by downwelling across the bottom of this layer (3.4 Sv). The downwelling is so strong that, despite the large SIO inflow, the horizontal transport of the LC does not much increase to the south (from 0.3 Sv at 22 degrees S to 1.5 Sv at 348S). This LC transport is significantly smaller than previously reported. The LUC is supplied by water from south of Australia (0.2 Sv), by eastward inflow from the SIO south of 28 degrees S (1.6 Sv), and by the downwelling from the LC (1.6 Sv) and in response strengthens northward, reaching a maximumnear 28 degrees S (3.4 Sv). North of 288S it loses water by outflow into subsurface westward flow (23.6 Sv between 28 degrees and 22 degrees S) and despite an additional downwelling from the LC (1.9 Sv), it decreases to the north (1.7 Sv at 22 degrees S). The seasonality of the LUC is described for the first time.