AM - Thursday, 5 February , 2009 08:12:00 Reporter: Shane McLeod TONY EASTLEY: Australia's south-east is still in the grip of drought but scientists believe they might have a better idea of what's causing the big dry. In recent years, the so-called El Nino effect has figured in the thinking based on rises and falls in sea temperatures in the Pacific Ocean.



But new research suggests it might actually be the waters on the other side of Australia that are to blame.



Environment reporter Shane McLeod has the story.



SHANE MCLEOD: Australians and especially Australian farmers have known since the 1980s that ocean temperature cycles like the so-called El Nino effect, or southern oscillation, have a big impact on rainfall.



But tracking the cycles in the Pacific Ocean hasn't brought much joy to farmers in south-eastern Australia who've watched La Nina phases come and go without delivering much hoped for rain.



Now climate scientists at the University of New South Wales in Sydney believe they know why that's happened.



Dr Caroline Ummenhofer is the author of a new paper on the role of the Indian Ocean in Australia's climate.



CAROLINE UMMENHOFER: Whenever we've looked at these drought periods we've seen a conspicuous absence of one phase of the Indian Ocean dipole, the negative phase, that normally brings wet conditions to south-eastern Australia. And this has been conspicuously absent during all these periods.



SHANE MCLEOD: The Indian Ocean dipole is a cousin of the southern oscillation but obviously tracking temperatures in the waters to Australia's west.



It moves through two distinct phases. In what's called the negative phase, warm water in the central Indian drives strong moist winds across Indonesia and down through north-western Australia, bringing good seasonal rains into Victoria and Tasmania.



In the positive phase, the reverse occurs with weaker drier winds drawing little rain across the Australian continent.



The researchers have found that since the early 1990s the dipole hasn't headed into negative territory, coinciding with what's become known as the big dry. Going back through the records, they've found the dipole's behaviour correlates with major droughts stretching back a century.



Professor Matthew England heads the university's Climate Change Research Centre.



MATTHEW ENGLAND: When it cycles between cold and warm phases it dictates how much moist air comes down to southern Australia. You can see it on the weather maps on the nightly news. You see these bursts of cloud, north-west cloud bands tracking down the continent from the north-west shelf region. And it's that driver of rainfall that we've identified as controlling this drought cycle to the south-east of Australia.



Really it's the Victorian region is kind of, at the moment seems to be slave to what's happening in the Indian Ocean.



SHANE MCLEOD: The question now is whether the phenomenon can be accurately predicted and whether global warming might cause it to change its behaviour.



Professor England again:



MATTHEW ENGLAND: It's alarming to see the frequency of them drop off over the last 20 or 30 years. We'd like a system where the rain relief comes every two or three years through the cycle and that's what happened previously. But certainly the fact that in the last 20 or 30 years we've seen a decline of these events is alarming. The link to climate change is not yet made but there's certainly a trend there we'd like to know more about.



TONY EASTLEY: Professor Matthew England from the Climate Change Research Centre at the University of New South Wales. The reporter, Shane McLeod.