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. kilometres offshore. (small but recognisable rock lobsters) is closely linked with Other species Frequent reversals to westward variations in the strength of the WA Fisheries marine biologists have flow occur during the summer Leeuwin Current, which in turn is made a clear association between months. There can be eastward associated with El Nino – Southern the strength of the Leeuwin Current currents exceeding 3 knots in the Oscillation (ENSO) events as and the transport of pilchards core of the Leeuwin Current just well as westerly winds. (sardines) early life-history stages on off the continental shelf. the south coast region near Albany. During ENSO years, when the Influence on marine life Leeuwin Current tends to be Data on the fishery shows a weak, puerulus settlement is poor. Because of the tropical Leeuwin significant negative relationship In so-called La Nina years, on the Current, the coastal waters are between the abundance of two-year other hand, the Current tends to relatively nutrient-poor and the fish with the strength of the Leeuwin flow more strongly and settlement fisheries are correspondingly Current, measured two years is much greater. different. The Leeuwin Current previously during the fishes’ spawning period. therefore has an important influence Satellite sensors can measure on both the climate and the marine undulations in the ocean surface, Whitebait also appears to be affected ecosystem off Western Australia, of as little as 30 cm over tens of and is responsible for the presence by variations in the Leeuwin Current. of tropical marine organisms off the west and south coasts much further south than would otherwise be expected. It plays a major role in the life histories of (for example) the southern bluefin tuna, the western rock lobster and a number of our State’s coastal commercial fisheries (including salmon). Rock Lobsters

Collaborative studies between CSIRO and Fisheries Western Australia during the past 30 years have confirmed the link between ocean circulation and lobster recruitment. Partially-funded by the Fisheries Research and Development Corporation and providing an aid to management, scientists have modelled surface circulation patterns to explain the variability from year to year of lobster recruitment. September: In winter, the Capes Current is not flowing, and the strong Leeuwin Following hatching near the edge Current floods across the continental shelf to the coast. Note the large eddies of the continental shelf in summer, and offshoots extending westwards, as well as the tongues of warm water millions of early-stage lobster larvae pushing onto the continental shelf towards the coast. The shallow water in rise to the ocean surface and are Geographe Bay is cool (blue) because of heat loss to the atmosphere. A strong relationship exists between Inshore Currents summer countercurrent (the the annual catch variations and Ningaloo Current) has recently been the strength of the Leeuwin Current Along the inner continental shelf identified along the Ningaloo Reef, during the previous year when (and inshore of the Leeuwin and similar countercurrents are spawning occurred. Thus, the Current), the currents tend to be known to exist inshore of the stronger the Leeuwin Current, more influenced by the wind and Abrolhos Islands. Few current speed the greater the relative catch of so change direction seasonally. measurements have been made in whitebait in the following year. Because of the generally northwards these areas, although measurements winds which blow along the west near Perth and the Abrolhos Islands The abundance of juvenile Australian coast during the summer months, suggest mean currents of 10 to salmon in South Australia is linked there appears to be a “chain” of 20 cm/s and peak speeds of about to the Leeuwin Current. Fluctuations northwards countercurrents between 50 cm/s (1 knot), and there are in recruitment/catches of the west say October and about March, frequent changes between northerly coast salmon fishery may also be while in winter when the Leeuwin and southerly flows in sympathy associated with variations in the Current is flowing strongly the with changing weather patterns. strengths of the Capes Current and coastal currents tend to flow the Leeuwin Current – when the southwards. While the summer Future Research Leeuwin Current is flowing strongly countercurrents may be fairly Continued ocean observations and and warmer waters penetrate onto continuous right along the coast, monitoring will allow scientists to the continental shelf, it is thought they are probably still generated build on one of the best-established that the fish may simply migrate by local winds and so have been foundations linking physical and offshore into deeper cooler waters given different local names. biological conditions in the ocean rather than continue to migrate with benefits for the management around the coast. The Capes Current commences near Cape Leeuwin and flows of sustainable fisheries and an northwards past Cape Naturaliste improved understanding of the and on beyond Rottnest Island; there marine ecosystem. is often an upwelling region in the lee of Rottnest Island. Similarly, a

Further information: www.marine.csiro.au Bryony Bennett (communication group) • phone (03) 6232 5261 • email: [email protected] www.wa.gov.au/westfish/comm/broc/elnino/index.html • www.per.marine.csiro.au/public/oceanography (North West Shelf Study – www.marine.csiro.au/nwsjems/index.html

Other information sources: “Lumps in the Leeuwin Current”. Article by Alan Pearce, David Griffin (CSIRO), John Wilkin (NIWA), Nick Caputi, Chris Chubb (Fisheries WA) – Western Fisheries (Winter 2000).

“The Leeuwin Current – Yachtsman’s Friend and Foe?” by Alan Pearce and George Cresswell, a chapter in Western Australian Cruising – A Yachting Guide, by Ross Brown (1997)

“Our fickle salmon fisheries” By Alan Pearce (CSIRO Marine Research) and Gabrielle Nowara, Suzy Ayvazian & Rod Lenanton (Fisheries WA) Article for the Busselton-Dunsborough Mail, March 2001

Acknowledgments CSIRO Marine Research (Mr Alan Pearce, Dr George Cresswell, Dr David Griffin). Fisheries WA (Drs Nick Caputi, Chris Chubb and Rod Lenanton). WA Department of Land Administration (DOLA). Centre for Water Research, University of Western Australia (Dr Chari Pattiaratchi and Guy Gersbach). Fisheries Research and Development Corporation (FRDC). Western Australian Satellite Technology and Applications Consortium (WASTAC). Curtin University School of Physical Sciences (Assoc Prof Mervyn Lynch). National Tidal Facility (NTF).

Castray Esplanade, Hobart, Tasmania 7000 ...... Phone (03) 6232 5222 ...... Fax (03) 6232 5000 233 Middle Street, Cleveland, Queensland 4163 ...... Phone (07) 3826 7200 ...... Fax (07) 3826 7222 Leach Street, Marmion, Western Australia 6020...... Phone (08) 9422 8200 ...... Fax (08) 9422 8222

Fact sheet No. 44 August 2001