The last two major bushfires in Victoria, Australia, were perhaps linked to lower than normal sea-surface temperatures in the eastern Indian Ocean.

The Ash Wednesday bushfires in February 1983 and the Black Saturday bushfires in February of this year were preceded by months of very dry conditions. Those dry conditions were partly caused by cooler ocean sea-surface temperatures in the eastern Indian Ocean, which contributed to a substantial reduction in spring-time rainfall over the south-east of Australia, say researchers with the Wealth from Oceans National Research Flagship, an arm of the Commonwealth Scientific and Industrial Research Organisation(CSIRO).

The see-sawing nature of sea-surface temperatures in the east and western Indian Ocean is commonly referred to as the Indian Ocean Dipole. When the dipole is in a positive phase sea-water off the Sumatra-Java coast, northwest of Australia, tends to be cooler than normal, leading to a reduction in the rain-bearing systems that normally extend to Victoria during spring.

According to CSIRO’s Dr Wenju Cai, the recent bushfires in Victoria occurred during a protracted drought made worse by an unprecedented three consecutive positive Indian Ocean Dipole events from 2006 to 2008.

“The sequence of these dipole events were captured by Argo measurements, which use robotic floats that spend most of their life drifting below the ocean surface,” Dr Cai says.

“Another study examining temperature records of the past 100 years shows that the frequency of positive Indian Ocean Dipoles in the past three decades is much higher than over the previous 70 years. This trend is consistent with climate change experiments from the IPCC Fourth Assessment Report, published in 2007, which projects a mean warming pattern across the Indian Ocean reminiscent of a positive dipole pattern.”