Indian Ocean Modeling Shows Australia's Worst-Case Drought Outlook May Have Just Gotten Worse

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IT has been labelled the cousin of El Nino, the Indian Ocean's equivalent of the climatic engine in the Pacific that drives the cycle of droughts and floods in Australia's southeast. But when CSIRO scientist Wenju Cai factored the Indian Ocean Dipole into his climate model, he found that this little cousin could contribute to droughts. It could depress spring rainfall by up to 30 per cent in Australia's southeast, a region encompassing the nation's food bowl, the southern Murray-Darling Basin. And with global warming set to increase the frequency of dipole events, Australia was likely to get even bigger climatic shocks than previously thought, the computer simulations suggested.

Much of Australia's climatological research has focused so far on the Pacific Ocean's El Nino-Southern Oscillation pattern. Droughts hit when the eastern Pacific warms, weakening the eastern trade winds that bear rain to southeastern Australia. La Nina episodes reverse the pattern, bringing floods to Australia. The outlook for ENSO is bad: the past 30 years have seen the most El Nino episodes since instrumental weather records began in 1880 and the number of droughts is projected to increase as the planet warms.

However, attention has shifted in recent years to poorly understood climatic drivers operating in the Indian Ocean, and the latest results suggest that even the gloomiest predictions on drought may have been optimistic. Cai, a climatologist with CSIRO's Wealth from Oceans national research flagship, led a research team that in 2003 uncovered the first climate modelling evidence that global warming was contributing to some of Australia's droughts. The scientists wanted to find out if climate change was affecting the winter westerlies, which deliver rain to southern Australia, including the Murray-Darling Basin.

They used a sophisticated global climate model and a supercomputer to gauge the effect of global warming on the Southern Annular Mode, the southern Indian Ocean climatic system that drives the westerlies. When they compared climate simulations based on modern and pre-industrial greenhouse gas levels, they found that a 35-year drying trend in the southwest of Western Australia was difficult to explain by natural climatic variability alone. Global warming was probably responsible for 5 per cent to 10 per cent of the drying trend, during which average annual rainfall plummeted by 15 per cent and up to 30 per cent in some regions. And the greenhouse contribution was likely to increase.

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http://www.theaustralian.news.com.au/story/0,25197,25468050-30417,00.html