11/03/2020

(AU) More Drought In Australia's Future As Weather Patterns Change

Sydney Morning Herald - Peter Hannam

The Indian Ocean weather pattern that contributed to eastern Australia's intense drought and bushfires is becoming more common because of climate change - and can reach worse extremes.
Using coral cores drawn from Indonesian islands, an international research team led by Australians reconstructed climatic conditions in the Indian Ocean over more than 500 years. Of particular interest was a phenomenon known as the positive phase of the Indian Ocean Dipole (IOD).
Sheep on the parched lake floor of the Burrendong Dam. The reservoir, one of the largest in the Murray Darling Basin, was down to about 1 per cent full during the worst of the recent drought. Credit: Nick Moir
During the positive phase of the IOD, waters off Indonesia are cool relative to the western Indian Ocean. That pattern typically means spring rainfall shifts westwards, increasing the odds of heavy rain in eastern Africa but also unusually dry and hot weather in south-eastern Australia.
While 1997 and 2019 recorded extreme positive IODs, both were dwarfed by one in 1675 that was as much as 42 per cent stronger than these, according to results published in Nature journal on Tuesday.
Source: NOAA Climate


Source: Nature
"Historically, strong events like the one we saw in 2019 have been very rare," said Nerilie Abram, an Australian National University professor and lead author of the paper. "Over the reconstruction beginning in the year 1240, we see only 10 of these events, but four of those have occurred in just the last 60 years."
Professor Abram said human-caused climate change was upping the odds of these extreme events because the western part of the Indian Ocean was warming faster than the east. "That puts the Indian Ocean in a state where it's possible to generate more of these positive [IOD] events," she told the Herald and The Age.
While it is not clear what impact the 1675 pattern had on Australian rainfall, other impacts typical of a big event show up in historical records, such as drought in Indonesia and the failure of rice crops in Thailand and India, Professor Abram said.
Cai Wenju, a senior CSIRO researcher who has published extensively on the Indian Ocean climate, said the new research "gives us a very rich historical perspective of how IODs have been changing with time".
Dr Cai said the 1675 event was "humungous" and showed what nature was capable of generating even before man-made global heating began raising background temperatures.
"In the future, climate change will be producing more frequent and huge positive IODs," he said. "It spells big impacts" on places such as Australia.
The paper, which included researchers from the US, Indonesia, Taiwan and China, also showed how the Indian Ocean often moved in tandem with the Pacific Ocean. The positive phase of the IOD, for instance, was often coupled with El Ninos in the Pacific.
“Our research indicates that while Indian Ocean Dipole and El Nino events can occur independently, periods of large year-to-year swings in Indian Ocean variability also had heightened [El Nino Southern Oscillation] variability in the Pacific,” said Matt England from the Climate Change Research Centre at the University of NSW.
With climate change, extreme positive IODs and El Ninos "will both go up together" in intensity and frequency, Dr Cai said.
The 1997-98 combination of an extreme positive IOD followed by extreme El Nino and La Nina events in the Pacific is about a one-in-200 year event, he said.
The combination will become more like a one-in-45 year event before 2100 if greenhouse gas emissions continue to rise at the current pace.

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