The source of the Leeuwin Current seasonality

The seasonal circulation around the southwestern boundary of Australia is documented using sea level anomalies from satellite altimetry. Results extrapolated to the coast agree closely with tide gauge observations indicating that seasonal altimeter fields are realistic. Monthly sea level maps identify an annual propagating wave along a waveguide extending along the shelf edge, from the Gulf of Carpentaria to southern Tasmania. The annual sea level pulse does not originate from the Pacific Ocean, as annual Pacific sea level variations are completely out of phase with signals south of the Indonesian archipelago. The presence of a phase discontinuity is demonstrated in annual sea level, temperature, and salinity observations. The origin of the Leeuwin Current seasonality is in the Gulf of Carpentaria where monsoonal winds drive a massive buildup of sea level from November to December. During December–February, a sea level “pulse” emerges from the region, and rapidly propagates poleward along the western and southern Australian boundary. In the broad shelf region centered at 19°S, an independent process forms a high sea level feature when a positive heat flux anomaly induces an annual increase in sea surface temperature which is rapidly mixed through the water column by the strong regional tides. In March, the winds relax and switch to a downwelling favorable alongshore component. In this period, the sea level pulse is essentially in a quasi-static equilibrium with the annual propagating wind systems. The change in cross-shelf sea level gradient along the 8000 km path length at the western and southern boundaries, drives the seasonal changes in the Leeuwin Current flow.

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