Indian Ocean driving Australia's big dry: study

By Science Online's Anna Salleh

ABC News ONline, Posted 5 Feb 2009

• http://www.abc.net.au/news/stories/2009/02/05/2482667.htm
• Related Story: Drought fear: 'Walk across the Murray'

Australia's severe drought is being driven by temperature fluctuations in the Indian Ocean, scientists have found.

The findings will help give farmers more reliable long-range weather forecasts, the Australian team, whose research is due to be published in the journal Geophysical Research, said.

"We have found the Indian Ocean plays a profound role in driving [the southern Australian] drought," Dr Caroline Ummenhofer of the Climate Change Research Centre at the University of New South Wales said.

"We really hope this will improve forecasting of rainfall in that area."

Traditionally, scientists have linked drought in Australia with El Nino - a climate pattern resulting from temperature fluctuations in the Pacific Ocean.

The reverse of El Nino, or La Nina, is thought to be responsible for bringing drought-breaking rains to Australia.

But, despite numerous La Nina events over the past 15 years, southern Australia has been virtually starved of rainfall, raising questions over the role of the Pacific Ocean climate pattern.

"El Nino and La Nina cycles cannot explain the cause," Dr Ummenhofer said.

Indian Dipole

Dr Ummenofer and colleagues investigated the role of a climate pattern called the Indian Ocean Dipole (IOD).

In its negative phase, the IOD is characterised by cool water to the west of Australia and warm water to the north, leading to winds that bring warm moist, rain-bearing air to the continent.

In the positive phase, water temperatures are reversed and less moisture travels to Australia.

Looking back over the last 100 years of data, Dr Ummenhofer and colleagues found that all of Australia's long-lasting droughts, including the Federation drought (1885-1902) and the World War II drought (1937-1945), were linked to a low number of negative IOD phases.

They said that the most recent big dry has seen no negative phases at all.

Predictions

Dr Ummenhofer says for the past 15 years the IOD has been either neutral or positive and in the last few years there were three consecutive positive phases.

"This is something new that in the historical record has never happened before," she said.

She says there are some indications that positive IODs are becoming more frequent, although this needs to be investigated further.

Dr Ummenhofer also says IODs can be predicted 3 to 6 months in advance but it is too early to predict what will happen this year.

Her colleague, Dr Matthew England, says current trends could have serious consequences.

"If these IOD events do follow the trend that we're seeing of more positive events and less negative ones, this is a terrible piece of info for the Murray Darling Basin," he said.

Heatwaves and climate change

The researchers have yet to determine whether the IOD trends are linked to climate change.

But they say the severity of the most recent drought is partly due to higher temperatures.

Dr England says the record-breaking heatwave experienced in recent weeks in south-east Australia is not in itself a sign of climate change, but due to a blocking high pressure system over the Tasman Sea, which is a natural meteorological event.

"But obviously with the planet already about a degree [Celsius] warmer than its background state then any heat wave you get is then going to be that much worse in a warmer world," he said.