Leeuwin Current

CURRENTS THAT FOLLOW a coastline are known as boundary currents, and, at 5500km, the Leeuwin Current is the world’s longest. Within its warm and relatively nutrientpoor embrace, it carries larvae of fish, invertebrates and corals that travel as plankton until they find a suitable place to settle, grow and breed, influencing the species composition of Western Australia’s network of temperate latitude reefs.

The strength of the Leeuwin varies throughout the year. When it’s flowing strongly during the winter months, it tends to move onto the continental shelf as it approaches Cape Naturaliste. It generally flows close inshore down to Cape Leeuwin and then eastwards towards the Great Australian Bight. In late spring, however, it moves a little offshore to be replaced by a cool north-flowing counter-current, recently named the Capes Current. This, in turn, dies away during March or April as the strengthening Leeuwin moves inshore again.

The Leeuwin’s weakest southward flow occurs from November to March when the current needs to work against strong southerly winds. The time of greatest flow is in the autumn and winter when the pressure gradient is greater against weaker winds.

The Leeuwin differs from cool-boundary currents flowing along other west coasts, such as the Benguela Current that courses northward along the south-west African Coast, and the Humboldt that follows the Chile–Peru coastline, both of which involve coastal upwellings of cool, nutrient-rich waters.

Because of the Leeuwin the continental shelf waters of WA are warmer in winter than the corresponding regions off southern Africa and Chile in summer.

1 Near a site aptly known as the shark cave, off Rottnest Island, a scuba diver enjoys an encounter with a docile grey nurse shark. 2 Three western rock lobsters hide away in a crevice on a coral reef. 3 Shaw’s cowfish, like this female beneath Busselton Jetty, usually feed on the ocean floor by blowing away sand with a jet of water to expose small invertebrate prey. 4 A flock of silver gulls off Hamelin Bay feeds on prey chased by underwater predators such as this smooth stingray.