SRFME

At 5,500 kilometres, the Leeuwin is our longest ocean current

Released: 22 March 2004 (by CSIRO Marine and Atmospheric Research)

A computer model showing how the Leeuwin Current contributes to dispersal of material across the Great Australian Bight (from a source indicated by the green highlighted box

One of Australia’s most influential natural features, the Leeuwin Current has been confirmed as the longest continuous coastal current system in the world.

Marine scientists have linked what was originally thought of as three separate coastal currents, with the same oceanographic signature from Western Australia’s North West Shelf to South Cape in southern Tasmania.

"Over the last 150 years the system has acquired three different names but we now know for sure it is one long ocean current that shapes marine and coastal biodiversity in western and southern Australia, and the climate of Western Australia," says CSIRO’s Mr Ken Ridgway.

"The intriguing aspect is that the current remains connected at all because very different factors must act together with perfect timing.


The Leeuwin current impacts greatly on WA's marine, coastal and climate characteristics. CSIRO scientists, Ken Ridgway (left) and Scott Condie.

"The large-scale circulation in the Indian Ocean sets up the flow off WA and delivers it to the south coast just as the seasonal winds change direction and push it further to the east", Mr Ridgway said.

The Leeuwin is one of four currents influencing life in the Australian region by regulating rainfall and temperature, fostering coastal recreation and distributing marine species.

Mr Ridgway and fellow CSIRO oceanographer, Dr Scott Condie, documented the complete, 5,500 kilometre path of the Leeuwin.

Their work, to be published in the Journal of Geophysical Research, was funded through the West Australian Strategic Research Fund for the Marine Environment (SRFME), a joint research initiative between CSIRO and the Western Australian Government.

He said scientists are still assessing the broader influence of the Leeuwin on the marine environment. However, they have established that the current and associated ocean eddies which spin from it govern nutrient distribution to the food chain and distribution of larvae from seafood species such as salmon and the western and south-eastern rock lobsters.

Seasonal comparisons showing the Leeuwin Current in January and June.

Although segments of the Current were identified in the Bight as early as 1853, it was not until 1980 that the Leeuwin Current was formally named by CSIRO scientists George Cresswell and Terry Golding.

The Leeuwin originates near North West Cape in Western Australia and flows southward towards Cape Leeuwin before turning eastwards into the Great Australian Bight. From south-eastern South Australia it was known as the South Australian Current and flowed to north western Tasmania. It then turned south again down the Tasmanian west coast to become the Zeehan Current, reaching as far as South Cape — and in winter flows north as far as the Freycinet Peninsula.

For researchers, the lack of oceanographic and satellite data across the Great Australian Bight had prevented researchers from linking the flow of water across the Bight with the Zeehan Current because of a lack of observations in the eastern Bight.

Mr Ridgway said the full length of the current was identified using European and US satellites, drifting instruments and measurements in recent years from scientific expeditions and commercial shipping companies which support ocean research.

He said the Current had been shown in earlier research to be vital to a range of ecological mechanisms; it provides a free ride for many marine species and assists their migration and subsequent distribution from the tropical north to the temperate southern waters.

The research will contribute to development of the new National ocean forecasting project, BlueLink.

How Australian ocean currents were viewed in 1921.

Background

Australia is influenced by four major ocean currents:

The Leeuwin Current
The East Australian Current — southward flowing from near Fraser Island to Tasmania
The Indonesian Throughflow — a system of westward flowing currents from the Pacific to the Indian Ocean
The Antarctic Circumpolar Current — the world’s largest ocean current and considered the powerhouse for global climate.

The Leeuwin Current takes an opposite course to geographically-similar currents which flow northward up the African (Benguela) and South American (Humboldt) continents.

Partly formed from the system of currents draining the Pacific Ocean into the Indian Ocean through Indonesia, its characteristics are detectable throughout the course of its long winter journey

The journey is governed by seasonal conditions and prevailing winds, with the Current generally following the edge of the continental shelf.

Click on image for larger view of Australian and Antarctic EEZ currents (52kb)

The southward flow of the Leeuwin is weakest from November to March when the winds tend to blow strongly northwards and reaches greatest flow is in the autumn and winter when the opposing winds are weakest.

In the Indian Ocean, typical current speeds in the Leeuwin Current and its eddies measured are about 1 knot although speeds of 2 knots are common, and the highest speed ever recorded by a drifting satellite-tracked buoy was 3.5 knots. The Leeuwin Current is about 300 m deep (quite shallow for a major current system, by global standards), and beneath it is a northwards countercurrent called the Leeuwin Undercurrent.

Web links - More Information

Ocean forecasting — BlueLink (website): http://www.marine.csiro.au/bluelink/
A map of global ocean currents (website): http://www.oceansonline.com/images/currents.gif

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