February 4, 2009

Indian Ocean Believed Responsible For Australian Droughts

A new study by researchers at the University of New South Wales (UNSW) finds that warming and cooling cycles in the Indian Ocean may be responsible for Australia's major droughts.

El Nino events in the Pacific Ocean have were previously thought responsible for the droughts, which over the past 120 years include the Federation drought (1895-1902), the World War Two drought (1937-1945), and the present drought (post-1995) -- the worst in 100 years. 

However, the new research shows the droughts have all coincided with changes in ocean temperature known as the Indian Ocean Dipole (IOD).

The researchers say their work explains why a spate of La Nina weather events, which typically bring Pacific rains to Australia, have failed to quell the current drought.

When the IOD is in a negative phase, it creates warm Timor Sea water north of Australia and cool Indian Ocean water to the west, generating winds that collect moisture from the ocean and delivers wet conditions across southern Australia.

In a positive phase, the pattern is reversed, weakening winds and reducing the amount of moisture picked up and transported across the nation, wrote the researchers in a report about their study.

"What we have found is that there has not been a single wet event, not a single negative event in the Indian Ocean Dipole since 1992," said Caroline Ummenhofer from the UNSW Climate Change Research Center, who led the study.

"That means all you are left with in southeast Australia is dry events. The cause of the "Big Dry," the current drought, is actually due to a lack of negative Indian Ocean Dipole events that remove the wet years from southeast Australia," she told Reuters.

Dipole events typically last about six months, and it is not yet clear whether another positive event will take place this year. There have been positive IODs during the past three years.

"In a few months' time it will be more certain if there is an Indian Ocean Dipole event is occurring (in 2009)," Ummenhofer said.

Scientists had previously linked El Nino events in the Pacific Ocean with the Australian droughts.  El Nino occurs when the eastern Pacific Ocean warms up, and the warmer, moist weather moves east, leaving drier weather in Australia and the western Pacific.

La Nina, on the other hand, occurs when waters in the eastern Pacific Ocean cool.  This leaves the western Pacific warmer, increasing the chances of wetter conditions over Australia.

The researchers compared the IOD, La Nina and El Nino events with droughts between 1889 and 2006, and found positive IODs correlated with major droughts.

"We have shown that the state of the Indian Ocean is highly important for rainfall and droughts in southeast Australia, more than the variability associated with the El Nino/La Nina cycle in the Pacific Ocean," Ummenhofer said during a briefing on the study.

"The Indian Ocean Dipole is the key factor for driving major southeast Australian droughts over the past 120 years."

The researchers said additional research is needed to identify the underlying cause of IODs.  Half are believed linked to La Nina events in the Pacific, while the other half are though dependent solely on the Indian Ocean.

"The Indian Ocean is the least studied and therefore it's a lot less clear...what sets off these events," Ummenhofer said.

However, the IOD, like El Nino and La Nina was predictable three to six months out, the researchers said, with Indian Ocean Dipole events typically appearing in May or June and peaking between September and November.

"There are some indications that positive Indian Ocean Dipole are becoming more frequent and negative events less frequent. However, this needs to further investigation."

The study is due to be published in Geophysical Research Letters.


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