Global warming triples odds of Southern Australian droughts

A key climate event that brings significantly lower rainfall and drought to southern and central Australia may occur three times more often this century compared to the 20th century as a result of global warming.

New research in the journal Nature involving researchers at the Climate Change Research Centre at UNSW, CSIRO,  and institutions in India and Japan has shown for the first time that extreme positive Indian Ocean Dipole (IOD) events will increase from an average of one event every 17.3 years to one every 6.3 years during the 21st-century if greenhouse gas emissions continue to rise at the current rate.

"An extreme positive IOD alters atmospheric circulation in such a way that it brings droughts and bushfires to East Asia and Australia; floods in parts of the Indian subcontinent and East Africa; coral reef death around Western Sumatra and increased malarial outbreaks in East Africa," said CCRC researcher Dr Agus Santoso, an author of the paper.

"This increased frequency of extreme IODs, coupled with the fact that positive IOD’s coincide with El Niños 70% of the time, will significantly amplify drought and rainfall impacts associated with these events in Australia and abroad."

The IOD is a measure of the difference in ocean temperature between the Western Indian Ocean and the Eastern Indian Ocean. This difference usually peaks during the Australian spring.

A positive IOD occurs when waters in the eastern Indian Ocean are cooler than normal and in the western Indian Ocean warmer than normal. A negative IOD is the opposite of this.

In general, a negative dipole generally brings wetter conditions to the southern parts of Australia while a positive dipole generally results in significantly lower spring rainfall and can enhance drought.

An "extreme" positive IOD actually causes currents and winds along the equator in the Indian Ocean to reverse their normal direction, amplifying even further the impacts in countries that surround it.

This is the first time that researchers have revealed a global warming impact on the formation of extreme IOD events.

"Global warming creates conditions that make it far easier for easterly equatorial winds and westward flowing ocean currents to form in the Indian Ocean," said CSIRO lead author Dr Wenju Cai.

"This markedly increases the difference in sea surface temperatures between the Eastern and Western Indian Ocean, which our modelling has shown leads to a vastly increased chance of extreme positive IODs forming.

“The research adds to the weight of evidence suggesting that as global warming takes hold, the southern parts of Australia are likely to receive less rainfall and become drier over time.”

Nature paper: Increased frequency of extreme Indian Ocean Dipole events due to greenhouse warming. DOI: 10.1038/nature13327 (pdf)