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CSIRO Marine and Atmospheric Research
Past Seminars

Seminar Abstract

Throughflow from the Pacific to the Indian Oceans – the Other Connection

The mean and time-varying nature of the flow between the Pacific and Indian Oceans to the south of Australia is documented using a suite of observational tools. These include gridded fields from historical in-situ data (CSIRO Atlas of Regional Seas, CARS), composite drifter data, and satellite altimetry and SST observations. The region is bounded in the north by the 2000-km extent of the zonally oriented southern shelf of Australia and in the south by the Antarctic Circumpolar Current (ACC). The open boundaries at the eastern and western endpoints allow a free exchange of properties between the Pacific and Indian Ocean basins. The zonally oriented coastline allows wind systems to propagate eastward, and readily promotes meridional exchange between continental and oceanic air masses. In general, the circulation in this region has been poorly understood with quite sparse data coverage in both space and time.

It is shown that this southern circulation links the subtropical gyres of the Pacific and Indian Oceans. In fact, the three Southern Hemisphere gyres are nested within a ‘supergyre’ with connections south of Tasmania and South Africa. The northern arm of the gyre is composed of the Tasman Outflow jet, which arises from the residue of East Australian Current (EAC) derived transport entering east of Tasmania. This feeds the Flinders Current, a westward flowing boundary flow off southern Australia between Tasmania and Cape Leeuwin. 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 portion of the outflow provides source water for the Leeuwin Current undercurrent, a northward flow from Cape Leeuwin to Northwest Cape.

The seasonal cycle of surface height is extracted from both the satellite altimetry and SST time-series. 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. A continuous shelf-edge current is identified, from Northwest Cape to southern Tasmania during 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.

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CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
Peter Oke, (Oceanographic seminars) CSIRO Marine Research (03) 6232 5387
Keith Hayes, (Biological seminars) CSIRO Marine Research (03) 6232 5298
Katrina Nitschke, Antarctic Climate and Ecosystems CRC (03) 6226 2265 & IASOS, University of Tasmania (03) 6226 2509