Feeling hot: Southeast Australia is in the grip of a record breaking drought, and now a heatwave too.
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SYDNEY: An Indian Ocean weather system may be causing, and prolonging, severe droughts in southeast Australia, according to a new study.
Scientists from the University of New South Wales (UNSW), in Sydney, have found evidence that El Niño, which is associated with bringing warm and dry conditions, and La Niña, with bringing cooler, wetter conditions, actually have little influence on the ongoing drought.
The current drought, which began in 1992, is the longest and more severe known since records began in the 19th century. The same study has linked the World War II drought (1937-1945) and the Federation Drought (1895-1902) to the weather system, known as the Indian Ocean Dipole.
Dry winds
That system determines if moisture-bearing winds or dry winds are carried across the southern half of Australia, and a continuous period of dry winds could be the cause of our current drought, say the scientists in a study to be published in the journal Geophysical Research Letters.
The Indian Ocean Dipole (IOD), is an oscillating weather system where warm and cool surface waters at either side of the Indian Ocean periodically swap places (see, Coral growth rings point to bad weather ahead).
This cycle involves a negative phase, a positive phase and a neutral position. In the negative phase, the Indian Ocean waters are cool, with warmer waters to the north in the Timor Sea. The westerly winds pick up moisture from the cool water and carry it across the land, so southeast Australia experiences moist, warm air and rainfall.
In the positive phase, the patterns are reversed. Less moisture is picked up from the warmer waters, which brings weak, dry winds and much less rainfall.
Losing streak
The new study reports that the IOD has only seen positive and neutral phases now for over 15 years.
"There hasn't been a single negative phase since 1992, and this is the longest no negative phase event we have on record" said Caroline Ummenhofer, a climate change scientist and lead author of the study. "All you're left with is dry events."
The last three years haven't seen a single neutral phase either, with a record breaking three positive phases in a row.
"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."
"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.
To help mitigate the problem, Ummenhofer suggested the IOD movements could be predicted for the next three to six months, which could help improve seasonal rainfall forecasts and benefit the management of water resources.
"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."
Greg Skilbeck, an earth scientist at the University of Technology, Sydney, says the findings are not totally unexpected but the attempt to predict the IOD movements are a little far fetched.
"The 'six month' or so predictability is not really predictability at all," he said. "This is just a time lag between the measurement of the relevant indices and the time it takes to manifest in Australia."
"So are we any closer to understanding and therefore really predicting when extended droughts occur? Maybe, but how much closer is not clear."
