The causes of south-eastern Australia's longest, most severe droughts have been discovered, with the surprise finding that they originate far away in the Indian Ocean.

A team of Australian scientists, led by Dr Caroline Ummenhofer and Professor Matthew England of the UNSW Climate Change Research Centre, has detailed for the first time how a phenomenon known as the Indian Ocean Dipole (IOD) - a variable and irregular cycle of warming and cooling of ocean water - dictates whether moisture-bearing winds are carried across the southern half of Australia.

The landmark new study explains the current record-breaking drought in south-eastern Australia and solves the mystery of why a string of La Nina events in the Pacific Ocean - which usually bring rain - has failed to break it.

It also reveals the causes of other iconic extreme droughts in recorded history, notably the World War II Drought from 1937 to 1945 and the Federation Drought from 1895 to 1902, and challenges the accepted understanding of the key drivers of Australia's climate.

The findings will be published in the journal Geophysical Review Letters. The team included researchers from CSIRO Centre for Australian Weather and Climate Research and the University of Tasmania.

"We have shown that the state of the Indian Ocean is highly important for rainfall and droughts in south-east Australia. More than the variability associated with the El Nino/La Nina cycle in the Pacific Ocean, the Indian Ocean Dipole is the key factor for driving major south-east Australian droughts over the past 120 years," says Dr Ummenhofer

"During this latest drought - the so-called 'Big Dry' - recent higher air temperatures across south-eastern Australia have exacerbated the problem.

"Our findings will help to improve seasonal rainfall forecasts and therefore directly benefit water and agricultural management."

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