Extreme positive Indian Ocean Dipole events have been strongly linked to drought in Australia. (ABC News)
One of the big drivers of drought in Australia, the Indian Ocean Dipole, is trending towards a more drought-causing positive state due to climate change, according to new research.
- One scientist says changes in the Indian Ocean Dipole’s behaviour is increasing the risk of more droughts for Australia
- Indian Ocean Dipole events have become stronger and more frequent since the 1960s
- One of the main reasons this has happened, according to another scientist, is because the Indian Ocean off Africa was warming faster than the Indian Ocean off Australia
Professor Nerilie Abram from the ANU Research School of Earth Sciences said the changes were occurring in the Indian Ocean Dipole’s (IOD) behaviour.
Professor Nerilie Abram says Australia is at a heightened risk of experiencing more hot, dry years. (Supplied: Australian National University)
“That is going to increase the risk that we will have these very dry and hot years and those are the years where we precondition our landscape to burn,” she said.
The 2019 drought occurred at a time where the Indian Ocean Dipole was at a record positive level.
Extreme positive IOD events have been strongly linked to drought in Australia.
Increases since 1960s unusual
Professor Andy Pitman, director of the ARC Centre of Excellence for Climate Extremes said quantifying how likely potential changes to the IOD could be and understanding the consequences for Australia were urgent national priorities.
“The discoveries recently reported by Abram and Cai are exciting and if correct could have profound consequences for Australia’s future climate,” Professor Pitman said
Professor Abram said positive IOD events had become stronger and more frequent, particularly since the 1960s.
“Paleoclimate data confirms that, yes, that recent increase that we’ve seen since the 1960s is unusual”, she said.
“And then if we look at climate models, they produce this increasing frequency and intensity of positive Indian Ocean Dipole events and project that, as the climate continues to warm, we will see those trends continuing.
“When we put all those three forms of evidence together, we do start to get a much clearer picture as to how the Indian Ocean [Dipole] is changing because of climate change.”
A positive IOD is caused by cooler than normal water in the Indian Ocean and is associated with a reduced chance of rain in Australia. (Supplied: Bureau of Meteorology)
Warmer sea temperatures
Professor Wenju Cai says the rain was moving towards the warmer Indian Ocean off Africa. (Supplied: CSIRO)
Professor Wenju Cai, the director of the Centre for Southern Hemisphere Oceans Research at the CSIRO, said one of the main reasons this was happening was because the western Indian Ocean — off the coast of Africa — was warming faster than the eastern Indian Ocean, off the coast of Australia.
“The rain bands are moving away to where the [sea] temperature is maximum,” Professor Cai said.
“Rain bands and convection always move to regions where the ocean temperature is the maximum.”
At the height of last year’s drought in Australia, the east coast of Africa was hit by terrible floods, driven by warmer sea temperatures. And now the African deluge has led to locust plagues there of biblical proportions.
Professor Cai has used computer models to project these changes into the future. He forecasts Australia will experience twice as many drought-causing extreme positive IOD events if temperatures warm by 1.5 degrees Celsius.
“In the future, we will be seeing quite a lot of these kinds of extremes that we are seeing in 2019,” Professor Cai said.
BOM yet to use research
Wenju Cai’s research shows that drought-causing extreme positive IOD events will be more frequent in a warmer world. (Supplied: Wenju Cai)
The Bureau of Meteorology (BOM) has not yet incorporated Wenju Cai and Nerilie Abram’s IOD research into its official climate change position.
“There has been a decline in cool season rainfall over southern Australia in recent decades, with the strongest signals over the south-west and south-east of the country,” BOM said in a statement.
“These declines are associated with changes in atmospheric circulation, consistent with global warming.
“There is a high degree of confidence that southern Australia will spend more time in drought in future years, consistent with projected declines in rainfall.
“There is a medium or low level of confidence that other regions around the country will spend more time in drought.”
The statement reflects the broad scientific agreement that southern Australia is getting less winter rain, and that this is driven by climate change.
There is also a broad scientific agreement that Australia is getting hotter due to climate change.
This excerpt from the State of the Climate Report 2018 shows winter rainfall over southern Australia has been declining over the past 20 years. (Supplied: CSIRO and Bureau of Meteorology)
More work needs to be done
Dr Scott Power from the Bureau of Meteorology says climate predictors, such as the IOD, can be used with the highest level of confidence. (Supplied: Bureau of Meteorology)
But the Bureau of Meteorology’s senior principal research scientist, Scott Power, said there was still work to be done refining the way climate models represent the IOD before long-term predictions could be taken with the highest level of confidence.
“Climate models are fantastic tools. They’re one of the great achievements of humankind based on centuries of advances in maths, science and technology. But they’re not perfect,” Dr Power said.
He said there was higher confidence when it came to understanding sea level rise, warming, and lower rainfall over southern Australia during winter and spring.
“Whereas when it comes to how might the positive phase of the IOD change, I think the confidence wouldn’t be as high,” he said.
Professor Abram said the IOD was only formally described about 20 years ago.
“So we are still building up our understanding as to what it actually is,” she said.
“For a long time, the scientific community has been grappling with, ‘Well, is the Indian Ocean Dipole its own climate phenomenon or is it just how we see El Nino events manifest in the Indian Ocean?'”
She said the 2019 weather event that had been observed was going to go a long way in terms of advancing these discussions.
“Because we have seen this very large event happen in the Indian Ocean and at the same time the Pacific Ocean was in a neutral state, then not doing very much at all,” Professor Abram said.
“There is still research to do. It will be really interesting to see how the science develops following this very extreme event that we saw in 2019.”