11.2
Oceanic connection between the Pacific and Indian Oceans south of Australia
Ken Ridgway, CSIRO, Hobart, Tasmania, Australia; and J. Dunn
The large-scale mean and seasonal circulation of the southeast Indian Ocean south of Australia is documented using gridded fields from historical in-situ data and satellite altimetry observations. The region is bounded in the north by the 2000-km extent of the zonally oriented southern shelf of Australia while the south the Antarctic Circumpolar Current (ACC) provides a southern boundary. The open boundaries at the eastern and western endpoints allow a free exchange of properties between the major basins. The southern shelf has a maximum width of 200-km in the central Great Australian Bight which narrows to about 20-km at each endpoint. The South Australian Basin, a major abyssal plain (depth 5500-m) is located offshore, adjacent to the shelf region. The zonally oriented coastline allows wind systems to propagate eastward, and readily promotes meridional exchange between continental and oceanic air masses. Within the limitations of the data coverage we present evidence of the Flinders Current, a westward flowing boundary flow off southern Australia between Tasmania in the east and Cape Leeuwin in the southwest corner of the continent. This current arises from the equatorward Sverdrup transport which is driven by the positive band of year around wind stress curl typical of the basin. A further input is provided by the Tasman Outflow which enters the region off the southern tip of Tasmania. The current provides the connection between the East Australian Current (EAC) extension, the poleward flowing remnant of the main EAC flow following its separation from the coast, and the South Indian Gyre. A portion of the outflow feeds into the Leeuwin Current undercurrent, a northward flow from Cape Leeuwin to Northwest Cape. Up to now the eventual fate of the EAC and the source of the undercurrent have remained elusive features of the Australian circulation. The results have been obtained from a new gridded atlas of oceanographic properties in the Australian region (CSIRO Atlas of Regional Seas, CARS).
The seasonal cycle of surface height is extracted from the 10-year satellite altimetry record. Seasonal reversing winds along the coast lead to alternate periods of onshore and offshore Ekman flux. Monthly surface height anomaly maps show that several eddies develop at those boundary regions with meridional orientation. These propagate westward across the basin with typical Rossby wave phase speeds. The full surface height field is reconstructed with the addition of the in-situ steric height mean and the time evolution of the surface current field is documented. We are therefore able to show the existence of a continuous Leeuwin Current, from Northwest Cape to the southern Tasmania from May to August. With the reversal of the coastal winds in summer the flow field is reversed and westward surface currents are observed at the boundary through summer. This augments the deeper westward transport of the Flinders Current
Session 11, The Southern Hemisphere oceans and air-sea interactions II
Wednesday, 26 March 2003, 10:30 AM-1:30 PM
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