Leeuwin Current – Life of the West

factsthe CSIRO MARINE RESEARCH – UNDERSTANDING OUR OCEANS The Leeuwin Current – life of the West In the mid-1990’s, Australian oceanographers participating in the World Ocean Circulation Experiment laid a trail of sophisticated ocean measuring instruments out from the shallows of the North West Shelf to depths of more than 4,000 metres. To be retrieved a year later by the and beneath it is a northwards CSIRO research vessel, Franklin, countercurrent called the the instruments would contribute Leeuwin Undercurrent. to a decade-long snapshot of the world’s oceans but in particular The Leeuwin Current is quite add to the profile of one of the different from the cool, northward region’s most dominant natural flowing currents found along the features – the Leeuwin Current. southwest African Coast (the Benguela Current) and the long Considerable research in the past Chile-Peru Coast (the Humboldt four decades has clearly shown the Current), where upwelling of cool contribution made by the Leeuwin nutrient-rich waters from below Current to the life of Western the surface results in some of the Australia – from regional climate to most productive fisheries. fisheries and coastal development. Because of the Leeuwin Current, The Current the continental shelf waters of Western Australia are warmer in Satellite images of water temperature The Leeuwin Current is a warm winter than the corresponding ocean current that flows strongly for the southwestern coast reveal the regions off southern Africa and main ocean currents in the vicinity of southwards along the Western Chile in summer. The Leeuwin Geographe Bay (courtesy of the Australian coast, before turning Current is also responsible for Western Australian Satellite Technology eastwards at Cape Leeuwin and the presence of true corals at the and Applications Consortium, WASTAC). continuing into the Great Australian Abrolhos Islands and the transport The southward-flowing Leeuwin Current Bight where its influence extends of tropical marine species down (warmest water shown in red/orange, as far as Tasmania. the west coast and across into with red arrows) and the cool Capes the Great Australian Bight. Current (shown in blue, with blue It’s strength varies through the arrows) represent the summer year, with the weakest southwards The ‘core’ of the Leeuwin Current situation. Close inshore, the water is flow occurring from November to can generally be detected as a peak warmed (red) by atmospheric heating. March when the winds tend to blow in the surface temperature with a strongly northwards. The time of strong temperature decrease further greatest flow is in the autumn and offshore. The surface temperature History winter when the opposing winds difference across the Current is are weakest. about 1°C at North West Cape, 2° A century ago during a study of to 3° at Fremantle and can be over marine life of the Abrolhos Islands Typical current speeds in the 4° off Albany in the Great Australian (29ºS), a naturalist, William Saville- Leeuwin Current and its eddies are Bight. The current frequently breaks Kent, suspected that there was about 1 knot (50 cm/s), although out to sea, forming both clockwise probably a warm southward-flowing speeds of 2 knots (1 m/s) are and anti-clockwise eddies. current off Western Australia and not common, and the highest speed ever the expected cool northward current recorded by a drifting satellite- The Leeuwin Current is also known as those near southern Africa and tracked buoy was 3.5 knots. The to be influenced by El Nino South America. He based his view on Leeuwin Current is about 300 m conditions, with slightly lower sea observations of warm waters and deep (quite shallow for a major levels along the Western Australian tropical marine flora and fauna current system, by global standards), coast and a weaker Leeuwin Current. around the Abrolhos Islands. Subsequent reports by fishers along the edge of the continental Cape Naturaliste and scientists noted the presence shelf at about the 200 metre depth to Cape Leeuwin of southerly currents. It was only line. However, it periodically with the advent of satellite meanders offshore in the form of When the Leeuwin Current is flowing technology in the 1970’s that large loops and eddies which can strongly during the winter months, the existence of the southward carry the warm tropical waters over it tends to move onto the continental current was finally confirmed. 200 kilometres away from the coast. shelf as it approaches Cape Three or four of these ‘waves’ can Naturaliste. It general flows close During the 1970’s, drifting buoys be present at any time between inshore down to Cape Leeuwin and and satellite images conclusively Exmouth and Cape Leeuwin. then eastwards towards the Great showed a warm tropical current Australian Bight. In late spring, flowing down the West Australian Where the continental shelf however, it moves a little offshore coast and around Cape Leeuwin. narrows along the Ningaloo Coast, to be replaced by a cool northwards It was named the Leeuwin Current the Current can be as little as counter-current, recently named by researchers George Cresswell 10 km off the coast. Current the Capes Current. This in turn dies and Terry Golding. The name was speeds here average between away in about March/April as the sourced from the Dutch merchant 0.5 and 1 knot. strengthening Leeuwin Current ship Leeuwin, meaning Lioness, moves inshore again. Currents From Shark Bay to Fremantle which explored the south west coast measured across the continental of Western Australia in 1622 and was The Houtman Abrolhos Islands, shelf off Cape Mentelle (34°S) one of numerous Dutch vessels situated near the edge of the show the highest speeds are almost which explored coastal waters and continental shelf off Geraldton, two knots near the 100-fathom line, islands of the West Coast. are the southern-most true reef with weaker and more variable currents nearer the coast. Scientists working with building corals in the Indian Ocean, environmental and fisheries partly because of the influence agencies, WA universities and of the tropical waters of the CSIRO have an on-going monitoring Leeuwin Current. program using satellite sea surface Because the Leeuwin Current tends temperature and sea surface to flow along the 100-fathom line, height instruments, drifting buoys, it is generally close to the chain of weekly shipboard sampling and islands. The strongest southward moored measurement instruments, currents of up to 1.5 knots are, in including tide gauges. Additional fact, encountered just beyond the measurements are being obtained 100-fathom line between February by remote sensing and shipboard and August, and the flow is weaker sampling of phytoplankton that and more variable in direction provides the base of the marine between September and January. food chain. Large current meanders and eddies Profile of a Current feature prominently in this area. As meanders develop and carry the The source of the Leeuwin Current warm water away from the coast, is north of North West Cape, cooler offshore water can be drawn extending from where a system in against the shelf resulting in of ocean currents drain the Pacific northward currents off the islands. Ocean to the Indian Ocean through Below about 250 metre depth there the Indonesian Archipelago. is often a northwards undercurrent with speeds of up to 1/3 knot. Data from two satellite instruments Measurements across the broad is used in constructing this image of continental shelf at North West Throughout the year there can the Leeuwin Current. Sea surface Cape confirm the low-salinity be current reversals resulting temperature information comes from Leeuwin flows along the outer North largely from changes in the winds. the US NOAA 14 satellite while the West Shelf most strongly between The Leeuwin Current rarely surface current velocity is derived February and June. Strong winds to flows around the eastern side of from sea level measurements made by the satellite-borne altimeter. Sea the northeast along the shelf can Rottnest, but it frequently bathes level data from the US/French retard or even reverse the currents. the western and southwestern Topex/Poseidon and European ERS sides, influencing the flora and From North West Cape south, the altimeters are combined with coastal fauna there. Sea temperatures tide gauge data to make this velocity Leeuwin Current is identifiable in the in those regions in winter are map. To see many more images like satellite images as a jet-like stream several degrees higher than this one, go to www.marine.csiro.au/ of warm water flowing southwards against the mainland coast. ~griffin/WACD/ Cape Leeuwin into the Great transported offshore by surface kilometres. Although small, these Australian Bight wind-driven currents. Ocean ‘lumps’ indicate the presence of a sampling from research vessels in warm-water current such as the After rounding Cape Leeuwin, the the 1970’s showed that the larvae Leeuwin. This data is simulated Leeuwin Current generally flows spend the next 8 – 10 months drifting by scientists to trace the movement along the outer continental shelf in in the open ocean, as far as 1,000 of the larvae. its passage eastwards, at least as kilometres from the coast. By late far as Cape Pasley near 124°E). winter and early spring the puerulus Early results of this lobster larvae From about this longitude, it tends are carried by ocean currents back simulation show that larger anti- to move offshore again because of to the continental shelf and inshore clockwise eddies help larvae remain the distinct northwards kink in the coastal regions. off south western Australia, rather coastline. As on the west coast, than being swept away to the south, large offshoots or meanders can Earlier studies have shown that often by being temporarily trapped carry the warm water over 100 settlement of lobster puerulus in the cool clockwise eddies.

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