All dry on the western front
The Australian, May 13, 2009
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.
While warming the lower atmosphere, the greenhouse effect cools the stratosphere above it, especially near Antarctica. This sets up a north-south temperature difference and therefore a pressure gradient that pushes the moisture-bearing westerlies southward, at times causing them to miss the Australian landmass.
Worryingly, the modelling suggested that the system would take 500 to 600 years to recover, even if greenhouse gas levels were stabilised.
The results came when the world was still in greenhouse denial, before the appearance of ominous signs, such as the acceleration of the Greenland glaciers and the melting of the Arctic sea ice and permafrost, which left no room for doubt that the planet was under pressure from human activity.
And they came before the Intergovernmental Panel on Climate Change handed down its fourth assessment report in 2007 that stated humans were very likely to have contributed to climate change.
However, even the worrying outlook for Australia in that report could be optimistic, Cai says. The report preceded new findings on the impact of the Indian Ocean Dipole on droughts.
A positive Indian Ocean Dipole index sounds the drought alarm bells. It occurs when the western Indian Ocean is warmer than the eastern basin. The dipole builds up in late winter and continues into September, October and November. The frequency of positive readings has increased three-fold during the past 100 years, with positive dipoles developing mainly in consecutive years.
"Each time the dipole index goes positive in consecutive years the intensity of drought is higher," Cai says. "It has contributed to an unprecedented intensity of drought. The question is whether this is the impact of global climate change."
Using global climate models, Cai's team compared scenarios to see how the Indian Ocean Dipole would respond to global warming.
The team, co-funded by the Australian Climate Change Science Program, investigated 1000 virtual years of the late 20th century in 20 virtual worlds with varying degrees of global warming, and presented its results at a recent meeting of the American Geophysical Union.
"The climate model is projecting that as we go into the future, with global warming continuing, the Indian Ocean Dipole will perhaps occur more often," Cai says. "In a warming world, the number of positive Indian Ocean Dipoles will increase by around 20 per cent."
An increase in the frequency of positive dipole events would exacerbate droughts kicked off in autumn - the critical crop-planting season, when rainfall plummets in El Nino years - and extended through winter by an awry Southern Annular Mode. It would drag the droughts into spring while increasing their severity.
Repeated droughts would dry out the Murray-Darling catchment, lessening the inflow to the ailing rivers when rain did come.
"If there's good autumn rain, the basin is soaked. If there isn't, the winter rain first has to wet the soil, and there isn't much inflow to the rivers," says Cai.
There are worrying signs that the Indian Ocean Dipole already could be out of kilter. The index went into the critical zone in the three years from 2006.
"In September 2008, southeastern Australia had the lowest rainfall on record," Cai says.