The Leeuwin current

The Leeuwin Current (LC) is a warm, poleward flowing ocean boundary current off the west and south coasts of Australia, driven by large-scale meridional (north-south) pressure gradient. On the interannual time scale, the strength of the LC is influenced by ENSO-related thermocline anomalies, and transmitted from the equatorial western Pacific into the southeast Indian Ocean through the Indonesian Archipelago. The LC and its interannual variability have profound impacts on marine ecosystems off the west and south coasts of Australia. For example, high recruitment of the western rock lobster (Panulirus cygnus) fishery of Western Australia is influenced by a stronger LC and the associated warmer water temperatures. Over the period from mid-1970s to mid-1990s, a trend of shallowing thermocline (subsurface cooling) in the equatorial western Pacific, which is coupled with a weakening trend of the trade winds in the Pacific, has transmitted into the southeast Indian Ocean and the LC region and caused a multi-decadal weakening trend of the LC. Comparing climate models and forced ocean circulation models suggests that the weakening LC is likely due to a combined effect of both global warming and natural variability in the climate system. There have been persistent warming trends observed in the LC and on the shelf in waters off the west coast during the past five decades. Over the same time period, more frequent Indian Ocean Dipole events and an upward trend of the Southern Annual Mode may have reduced the strength of the westerly winds and storm activity off the southwest coast, which may have adjusted the air-sea heat flux in the LC region and overcome the reduction of the LC heat transport to cause the warming trend. Both the changes in the LC and the air-sea freshwater flux may have also caused the observed increase in surface salinity off the coast. The surface warming and subsurface cooling, in combination with the reduction of storm activity, may have increased the vertical stratification in the water column and reduced vertical mixing in the LC region. Climate model simulations suggest that reductions of trade winds in the tropical Pacific, increase in the frequency of Indian Ocean Dipole events, and the upward trend of Southern Annual Mode in recent decades are mostly due to the effect of the increased Feng et al. 2009 www.oceanclimatechange.org.au 2 greenhouse gases in the atmosphere. Climate model projections suggest these climate trends will likely continue in the future, so that the LC could continue to weaken slowly