A description of the currents off the eastern and southern boundaries of Australia from fifteen years of Altimetry
Ken Ridgway(1) and Jeff Dunn(1)
CSIRO Marine & Atmospheric Research,
Castray Esplanade Hobart Tasmania,
GPO Box 1538 Hobart,
We use an integrated dataset of satellite (both altimetry and SST) and in situ observations to characterize the main current systems off the eastern and southern boundaries of Australia. A high-resolution in situ climatology provides the mean flow and the time-variability is determined from the satellite time series, coastal tide gauges and repeated XBT sections. The Australian continent divides the south Pacific and Indian Ocean subtropical gyre circulations. Off the eastern side the gyre inflow bifurcates to form a northward flow and a southern component which provides the source waters of the East Australian Current (EAC), the main poleward flow of the Pacific gyre. The EAC has a relatively weak mean flow but is highly variable, being dominated by mesoscale eddies. Altimeter time series show that these eddies are influenced by both remotely forced Rossby waves from the east and the complex topography in the southwest Pacific. The EAC system follows a complicated seasonal pattern which is well represented in both the altimeter and SST observations. The poleward boundary flow is strongest in summer associated with an intensification of the cross-shelf pressure gradient. A summer pulse of warm, salty water penetrates as far south as Tasmania while a limited number of eddies reach the Indian Ocean from the Tasman Sea around southern Tasmania. The variability of SLA is dominated across southern Australia by seasonally reversing alongshore winds which produce Ekman fluxes which alternately increase and decrease the coastal sealevel. An eastward shelf-edge winter flow, representing the continuation of the Leeuwin Current, reaches the west coast of Tasmania. Instabilities in the coastal build-up of sea level separate and propagate westward. Finally the altimeter, SST and XBT time series show the distinctive interannual and multi-year variations of these current systems.