Indian Ocean causes Big Dry: drought mystery solved

Dr Caroline Ummenhofer
Thursday, 5 February, 2009
Bob Beale

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

A team of Australian scientists 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.

When the IOD is in its negative phase, a pattern occurs with cool Indian Ocean water west of Australia and warm Timor Sea water to the north.  This generates winds that pick up moisture from the ocean and then sweep down towards southern Australia to deliver wet conditions. In its positive phase, the pattern of ocean temperatures is reversed, weakening the winds and reducing the amount of moisture picked up and transported across Australia.  So the south-east misses out on its usual quota of rain.

The study notes that the IOD has been in its positive or neutral phase since 1992, the longest period of its kind since records began in the late 19th Century.

How the Indian Ocean drives Australia's worst droughts. Click image to enlarge.

To make matters worse, this period has coincided with a trend towards higher average air temperatures over the land, which may be linked to human-induced climate change.

The team, led by Dr Caroline Ummenhofer and Professor Matthew England of the UNSW Climate Change Research Centre, details its findings in a paper accepted for publication in the journal Geophysical Review Letters. The team included researchers from CSIRO Centre for Australian Weather and Climate Research and the University of Tasmania.

"The ramifications of drought for this region are dire, with acute water shortages for rural and metropolitan areas, record agricultural losses, the drying-out of two of Australia's major river systems and far-reaching ecosystem damage," says Dr Ummenhofer.

"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.


"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."

Media contact:

UNSW Faculty of Science media liaison - Bob Beale 0411 705 435