The last two times the Indian Ocean Dipole (IOD) index recorded measurements similar to the readings being taken now, Australia was hit with record levels of spring rainfall.
Australia’s last negative IOD occurred from July to December in 2021, a period that included the country’s wettest November on record.
Prior to this, the most recent fully-fledged negative IOD happened in 2016, triggering Australia’s wettest September on record.
That year also saw South Australia notch its fourth rainiest year in history.
“Negative phases of the IOD are akin to La Niña in the Pacific Ocean because they enhance the flow of moisture-laden air towards Australia and promote rainfall,” Weatherzone meteorologist Ben Domensino said.
Negative IOD events typically cause above average rainfall over large areas of the country, particularly the south and south-east.
It also brings cooler-than-average days across the south-east mainland.
A negative event is usually declared when the index sits below -0.4 for at least eight consecutive weeks.
That scenario appears almost certain to unfold in the coming weeks, Domensino confirmed, explaining how cooler-than-average water has been pooling near the Horn of Africa and unusually warm water off the north-west of Australia.
“Most forecast models suggest that the IOD will remain in a negative state during the rest of the southern hemisphere’s winter and into spring, most likely gaining more strength in the coming weeks.”
The index has sat near or below the crucial threshold for the past eight weeks.
“(With) more negative values on the horizon, it is only a matter of time before the Bureau of Meteorology declares a negative IOD officially underway,” Domensino added.
“It may even happen later this week.”
Experts say triple dips are so rare it’s difficult to predict if a third La Niña would be weaker or stronger than in the past two years, where parts of the country have been battered by heavy rain and extreme weather.
La Niña event sees warmer water in the Pacific Ocean push towards Australia and Asia, resulting in more rain and higher chances of cyclones in summer.