05 February 2009

South-Eastern Australian Drought Explained

The Age carried this article  on 5 Feb:

THE cause of the record-breaking drought in south-eastern Australia has been discovered far off in the Indian Ocean, according to the surprise findings of a study that overturns decades of weather research.
While drought in Australia has traditionally been linked to El Nino events in the Pacific Ocean, researchers from the universities of NSW and Tasmania and the CSIRO have found that it is the Indian Ocean's cycle of warming and cooling that is to blame.
The water cycles of the Indian Ocean, which is experiencing unprecedented warming 2000 kilometres away, dictates the strength of the moisture-bearing winds that travel to Australia.
UNSW's own article  on this, which has some helpful diagrams, says
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.
This Tech Herald article says
"There hasn't been a single negative phase since 1992, and this is the longest no negative phase event we have on record" said Ummenhofer, who is lead author of the study. "All you're left with is dry events."
Not even neutral phases have been recorded in the past three years, a worrying sign.
"If the Indian Ocean Dipole events do follow the trend of seeing more positive events and less negative ones, this is a terrible piece of information for the Murray Darling Basin," said co-author Matthew England. "This Basin is under a lot of stress at the moment with drought, and it needs replenishment with more negative Indian Ocean Dipole events."
It is too early to predict any forward forecasts for the current drought said Ummenhofer, though she added researchers would have a better idea of predicting rain using the Indian Ocean Dipole in a few months.
"The ramifications of drought for [south east Australia] 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," Ummenhofer said.
"But, the Indian Ocean Dipole starts appearing around May and peaks during the September to November season, so it's a bit too early to be able to predict anything yet," she said. "In a few months it'll be more certain. We'll be able to predict movements closer to the event."

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