Southeastern Australia has experienced almost continuous drought conditions, with some farming regions enduring a “big dry” of six or more years. Researchers at Australia’s Climate Change Research Centre at the University of New South Wales say the Indian Ocean Dipole (IOD), a variable and irregular cycle of warming and cooling of ocean water on the western side of the continent, dictates whether moisture-bearing winds are carried across the southern half of Australia.
The landmark study, co-authored by Dr Caroline Ummenhofer and Professor Matthew England, explains why La Nina events in the Pacific Ocean, which usually bring rain, have failed in recent years to break the drought. The study has been accepted for publication in the journal Geophysical Review Letters.
Dr England, Co-Director of the Climate Change Research Centre (CCRC), said the IOD was similar to the El Nino/La Nina cycle of the South Pacific, and both cycles need to be considered to understand the effects of oceanic warming and cooling on Australia’s weather patterns.
He described the oceans as “a big engine” forming weather patterns in a presentation on the CCRC website.
However, while the El Nino cycle was operating much as usual, the IOD seemed to be stuck in the positive phase.
Indian Ocean Dipole Causing Drought
When the IOD is in its negative phase, there is cool Indian Ocean water west of Australia and warm Timor Sea water to the north, Dr Ummerhofer said. This generates winds that pick up moisture from the ocean and then sweep down towards southern Australia to deliver wet conditions,
But 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 far less rain falls across the south-east.
The IOD has been in its positive phase since 1992, the longest period since records began in the late 19th century, she said. At the same time, parts of southern Australia have had series of deep droughts, with the current ‘big dry” considered the worst in a century.
“When it cycles between cold and warm phases the IOD dictates how much moist air comes down to southern Australia,” Dr England said.
“You can see it on the weather maps on the nightly news. You see these bursts of cloud, north-west cloud bands tracking down the continent from the north-west shelf region. And it's that driver of rainfall that we've identified as controlling this drought cycle to the southeast of Australia.”
Link Between IOD and Global Climate Change?
Dr England said it was too soon to conclusively state a link between positive IOD phases and global climate change, but it was clear that positive IOD caused drought.
Coincidentally, over the same period that the IOD has been “stuck”, Australia has experienced higher than usual temperatures over the land, which have exacerbated the drought and “may be linked to human-induced climate change,” he said.
Better Forecasting from Understanding IOD
Understanding climate patterns will be increasingly important over the next few decades, as higher temperatures and less rain affect crops and farmers’ livelihoods.
“More than the El Nino, the Indian Ocean Dipole is the key factor for driving major southeast Australian droughts over the past 120 years,” Dr Ummerhofer said.
"Our findings will help to improve seasonal rainfall forecasts and therefore directly benefit water and agricultural management."
See also: Gyres Affect Sea Life, Climate
Sue Cartledge - I'm a science, health, nutrition and lifestyle journalist, fascinated by the way the physical world operates in all its forms, and how ...
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