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Climate change to almost triple risk of extreme Indian Ocean weather events

A change in surface water temperatures on the Indian Ocean due to global warming could produce extreme weather events in Australia.

A change in surface water temperatures on the Indian Ocean due to global warming could produce extreme weather events in Australia. Photo: Greg Wood

Shifting climate patterns in the Indian Ocean driven by global warming are likely to increase the frequency of “devastating” weather events for much of Australia, Indonesia and eastern Africa, a study led by Australian researchers has found.

While attention has focused on the prospect of an El Nino forming in the Pacific, a similar phenomenon may be under way in the Indian Ocean that could exacerbate dry and hot conditions for large areas of Australia.

Tropical sea-surface temperatures in the eastern Indian Ocean are becoming cool relative to those in the west.

Known as a positive-Indian Ocean Dipole (IOD), such conditions typically mean less convection off north-western Australian and reduced rainfall in winter and spring for south-eastern and central Australia. Indonesia also tends to endure drought and bushfires while east Africa gets hit by floods.

The Bureau of Meteorology’s latest outlook says global climate models are showing “a slight trend” towards a positive-IOD event developing in the spring.

The bureau also continues to rate the chance of an El Nino – when waters in the eastern equatorial Pacific warm relative to the ocean’s west, typically shifting rainfall eastwards away from Australia – at about 70 per cent for this year.

Positive-IODs coincide with El Ninos 70 per cent of the time, and in combination tend to result in dry, mild winters extending into dry, hot summers for most of NSW, Victoria and elsewhere in south-eastern Australia.

“If you get less rainfall, there’s less moisture in the soil, and when there’s less moisture in the soil, when summer comes, it’s easier to generate heatwaves,” said Agus Santoso, a senior research associate of the University of NSW’s ARC Centre of Excellence for Climate System Science.

Agricultural output may also be affected. In its latest quarterly report released on Wednesday, the Australian Bureau of Agricultural and Resource Economics and Sciences predicts a 12 per cent drop in this year’s winter crop from near-record levels to about 38.8 million tonnes.

While southern regions had good autumn falls, much of northern NSW and Queensland had only low levels of soil moisture and the winter rain outlook was for below-average falls, ABARES said: “Yields are likely to be lower than currently assumed if sufficient and timely rainfall is not received."

While climate models will need another month or two to be certain about an El Nino and a positive-IOD, the longer-term outlook is for the frequency of extremes of both phenomena to increase.

A new study led by Wenju Cai from the CSIRO, UNSW’s Dr Santoso and others has found the frequency of extreme positive-IODs will almost triple from one every 17.3 years over the 20th century to one every 6.3 years this century.

“The extreme (positive IODs) will be more extreme,” Dr Cai said. “What we get as a moderate event now will become more extreme.” 

The research, to be published in the journal Nature on Thursday, used 23 climate models and assumed greenhouse gas emissions would continue on their current trajectory. Lower emissions would likely reduce the frequency of extremes, Dr Cai said.

“Extreme IODs are bad for Australia," he said. "In fact any IOD is bad for Australia because they are very bad for rain.”

Earlier this year, research led by Dr Cai identified that global warming would double the incidence of extreme El Ninos, such as 1982-83 and 1997-98, from one in 20 years to one per decade on average.

“Indian Ocean climate change is more emphatic than in the Pacific,” Dr Cai said.

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