Indian Ocean surface temperatures that helped drive hot and dry conditions in eastern Australia last year were more clearly influenced by climate change than previously thought and are likely to worsen in future, researchers have found.
Scientists studying a phenomenon known as the Indian Ocean Dipole say their observations suggest Australia could experience future conditions even more extreme than those that elevated the bushfire risk during the 2019-20 fire season.
The work, led by the Australian National University and the ARC Centre of Excellence for Climate Extremes, used coral records from the eastern equatorial Indian Ocean to examine the occurrence of “extreme positive” Indian Ocean Dipole events over the past millennium.
The Indian Ocean Dipole (IOD) is the difference in temperature between the western and eastern Indian Ocean. When the IOD is in positive mode, warmer waters develop off the Horn of Africa and cooler waters develop off Indonesia. This leads to hotter and drier weather in Australia.
An “extreme positive” event is when these conditions are particularly severe. Last year, an extreme positive IOD played a role in bushfires in Australia and floods in Africa.
The scientists used fossil and modern coral cores to examine how frequently these events occurred back to 1240. Analysing data from 500 of those years, they found an extreme positive Indian Ocean Dipole like the one Australia experienced in 2019 was rare, occurring just 10 times.
Lead author Nerilie Abram, a climate scientist at the Australian National University , said four of the extreme positives were in the past 60 years.
“We’ve seen these events becoming more frequent and our climate models suggest that’s a response to human-caused climate change,” she said.
The strongest event on the instrumental record occurred in 1997. But using the coral records the researchers were able to find another more extreme case in 1675.
Abram said this was one of the most concerning aspects of their findings.
“We expect these events to continue becoming more frequent in future,” she said.
“But one of the worrying aspects of our findings was that the largest events in our observations over recent decades weren’t as extreme as these events can get.
“So it’s possible to have even more extreme events than what we’ve witnessed in recent decades.”
The findings, published in Nature, are the largest record developed for the Indian Ocean Dipole. Other research has also suggested that Indian Ocean Dipole events have become more frequent in recent decades.
“When we have these events we have hot and dry conditions in Australia,” Abram said.
“In 2019 that elevated the bushfire risk in southeastern Australia and then we obviously saw the devastating effects that had over summer.”
She said the new research showed countries needed to be aware that “recent decades don’t give us the full range of the risk these events pose”. But stabilising global warming by reducing greenhouse gas emissions would also stabilise how often we experience these events in future.
A co-author of the paper, Matthew England, said the research also found a strong coupling between variability in the Indian Ocean Dipole and the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean.
While IOD and El Niño events can occur independently of each other, periods of large year-to-year swings in Indian Ocean variability “also had heightened ENSO variability in the Pacific”.
“Looking at the tropical oceans in this interconnected way improves our understanding of seasonal to decadal climate variations in regions that profoundly impact Australia,” England said.
He said this could help Australia “to be better prepared for future climate risks caused by the Indian Ocean Dipole”.