A weather pattern centred on the Indian Ocean may provide an early warning system for major bushfires in southern Australia, climate experts say. Dr Wenju Cai and Tim Cowan, of CSIRO's marine and atmospheric research centre, have uncovered a link between the Indian Ocean Dipole (IOD) and Victoria's killer bushfires. Dr Cai will tell the Greenhouse 2009 conference today that 11 of 16 major bushfires in Victoria since 1950 have been preceded by what is known as a positive IOD event. He says an unprecedented three consecutive positive IOD events preceded February's devastating Black Saturday bushfires. The IOD refers to temperature fluctuations in the east and western Indian Ocean.
The picture of the left demonstrates change in the sea surface temperature between the western and eastern Indian Oceans (between month 0 and 12) which is apparently a very strong predictor for rainfall and potential bushfire threat patterns in Australia.
In its negative phase, the IOD brings cool water to the ocean west of Australia and warm water to the north, leading to winds that bring rain-bearing air over the continent. In the positive phase, water temperatures are reversed and less rainfall travels to Australia, particularly to Victoria where the negative IOD provides winter and spring rains. As part of their research, Dr Cai and Mr Cowan recorded changes in the IOD using Argo floats; robotic devices that measure the sub-surface ocean temperature.
They found the IOD was in an "unprecedented" positive state for three consecutive years leading up to 2009. They say this preconditioned the environment to the extent that it was almost inevitable the bushfires, which claimed more than 200 lives, would occur. "If you look at the accumulative soil moisture in Victoria, it's unprecedented, it's never been so dry," Dr Cai said.
The researchers also found an IOD link to the Ash Wednesday bushfires of February 1983, with a positive event reducing rainfall during the winter of 1982. Mr Cowan says of the 11 bushfires preceded by a positive IOD, six were coupled with an El Nino event. But, there was only one occasion where an El Nino alone preceded a bushfire, compared to the five times when only an IOD impacted on the rainfall. This shows the influence of the IOD was enough to precondition the environment to high bushfire risk, says Mr Cowan.
Dr Cai and Mr Cowan say climate change projections show the frequency of positive IOD events will increase in the future. "Almost all climate models say under climate change we are going to have an Indian Ocean warming pattern," Dr Cai said. "That means it has to be manifested in either more frequent positive IOD events or higher intensity positive IODs."
According to Dr Cai, the effects of climate change can already be seen. Between 1900 and 1930 there were four positive IOD events, he says. But in the past 30 years there have been 12 positive IODs, a 400 per cent increase. For Victorian residents living in bushfire-prone areas that is bad news. Dr Cai says the continued suppression of rainfall in Victoria will only make conditions more fire friendly. "The implication [of the research] is if we have a positive IOD in one year then the following season you have a higher bushfire risk," he said. According to Dr Cai this knowledge could provide an early warning system. "It gives us four to five months' lead time [to prepare for bushfires]," he said.
He says modelling shows that climate change will also lead to a 30 per cent increase in the number of consecutive events, while the odds of three consecutive IODs occurring increases by 300 per cent. "In 1,000 virtual years without climate change we get two occurrences [of three consecutive positive IOD events]. With climate change factored into the modelling this becomes eight."
The research is due to be published in a series of papers in the Geophysical Research Letters.
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