Australasian Science: Australia's authority on science since 1938

Indian Ocean Dipole Causes Winter Droughts

By Stephen Luntz

The Indian Ocean Dipole has dried southern Australia out, and we can expect more of the same according to Dr Wenju Cai of CSIRO Wealth from Oceans.

The good news is that understanding of the phenomenon may enable us to predict bushfire seasons.

The Indian Ocean Dipole is based on the relative temperatures of the two ends of the Indian Ocean. When the eastern end is relatively cold and the western end is warm, compared with their long term averages, it is said to be positive while the reverse conditions are described as negative. The weather of surrounding regions is affected just like much of the planet changes in response to El Niño and La Niña events.

However, while El Niños and La Niñas are becoming more intense (AS, Jan/Feb 2014, p.10) there is no evidence of a trend one way or another. The same is not true for the Dipole.

“Over the past 50 years, the Dipole has been trending upwards, increasing the number of positive events, occurring an unprecedented 11 times over the past 30 years,” says Cai. “For example, there were three consecutive positive Dipole events between 2006 and 2008, which preconditioned the catastrophic Black Saturday bushfires in Victoria.”

Cai explains that the Eastern Indian Ocean is historically warmer than its counterpart to the west. This creates a huge convection tower that brings winds in from the west.

However, global warming has seen the troposphere warm faster than the ground, “making the atmosphere more stable” and reducing the inflowing winds. The warm rising air once produced rain over southern Australia in winter and spring, but this is now less likely.

“The convection tower is still strong in summer,” says Cai. “It’s very hard to change that.” However, the resulting rain falls on northern Australia or Asia.

“This warming pattern will continue in the decades to come, according to the state-of-the-art global climate models used in the study,” says Cai, who published this result in Nature Geoscience. He says there is no equivalent trend in El Niños because the difference in average temperatures between the two sides of the Pacific Ocean is 6°C, while in the Indian Ocean it is just 0.5°C, making it far more easily overturned by warming trends.

Although the Dipole has much more effect on southern Australia than El Niño, it has only recently gained attention (AS, May 2009, p.8) since the global effects are much smaller. Cai says that the strength of the Dipole dried Tasmania out during 2012, leading to the 2013 fires. Future events may be predicted 6 months in advance.