The East Australian current

The East Australian Current (EAC) is a complex and highly energetic western boundary system in the south-western Pacific off eastern Australia. The EAC provides both the western boundary of the South Pacific Gyre and the linking element between the Pacific and Indian Ocean gyres. The EAC is weaker than other western boundary currents and is dominated by a series of mesoscale eddies that produce highly variable patterns of current strength and direction. Seasonal, interannual and strong decadal changes are observed in the current, which tend to mask the underlying long-term trends related to greenhouse gas forcing. Observations from a long-term coastal station show that the EAC has strengthened and extended further southward over the past 60 years. The south Tasman Sea region has become both warmer and saltier, with mean trends of 2.28°C/century and 0.34 psu/century over the 1944-2002 period, which corresponds to a poleward advance of the EAC Extension of about 350 km. The observed intensification of the EAC flow past Tasmania is driven by a spin-up and southward shift of the Southern Hemisphere subtropical ocean circulation. Changes in the gyre strength are, in turn, linked to changes in wind stress curl over a broad region of the South Pacific. Oceanic changes are forced by an intensification of the wind stress curl arising from a poleward shift in the circumpolar westerly winds due to the trend in the Southern Annular Mode. Observational and modelling studies indicate that these changes in the wind patterns are at least in part attributable to ozone depletion over the past decades. However, at least some of the trend is likely to be forced by increases in atmospheric CO2. Climate models under observed CO2 increases, also produce an upward trend of the SAM and a consequent intensification of the Southern Hemisphere gyre system. There is strong consensus in climate model simulations that trends observed over the past 50 years will continue and accelerate over the next 100 years.