A negative Indian Ocean dipole event has been declared, increasing the chances of above-average rainfall over much of Australia during the rest of winter and spring.
It is believed to be the first time since reliable records began in 1960 that negative Indian Ocean dipole events have occurred two years in a row.
The Indian Ocean dipole is determined by sea surface temperature differences between the eastern and western Indian Ocean. The climate phenomenon affects rainfall and usually begins in late autumn or winter.
BREAKING: A negative Indian Ocean Dipole (IOD) has just been declared, increasing the likelihood of above-average rain over large areas of Australia in the coming months. This is the first time we have seen back-to-back negative IOD years since reliable records began in 1960. pic.twitter.com/vCCsPosRss
— Ben Domensino (@Ben_Domensino) August 2, 2022
In a negative dipole event, warmer water concentrates in the eastern Indian Ocean and moisture-rich air flows towards Australia.
The last negative Indian Ocean dipole began in July 2021, the first in five years.
Ben Domensino, a meteorologist with Weatherzone, said the event increased the likelihood of above-average rain over parts of Australia for the next three to four months and has prompted concerns after so much rain already in the country’s east this year.
“When we have a negative Indian Ocean dipole in place, typically southern and south-eastern inland Australia have a high chance of seeing above-average rain, and most other areas of central, eastern and southern Australia are also more likely to be wet,” Domensino said.
“One thing that a lot of people are concerned about with this IOD declared is that Sydney just had a record wet year to date.”
He said there was still an expectation of above average rain in eastern New South Wales but the influence from the negative dipole is more pronounced to the west of the Great Dividing Range.
“So this will more influence rainfall for the Murray-Darling basin rather than the eastern seaboard,” he said.
“The IOD doesn’t have much of an influence on rainfall in south-western WA, or Tasmania and far north Queensland.”
He said the declaration on Tuesday is “confirmation that we’ve been in a negative Indian Ocean dipole pattern for the last eight weeks”.
The Bureau of Meteorology said in a statement that “all five international climate models surveyed … anticipate a negative IOD is likely to persist through to November” and that “it also increases the chances of warmer days and nights for northern Australia”.
Domensino said the BoM had reliable records of these events dating back to 1960. “In that time, including this year’s, there have been 13 negative Indian Ocean dipole events,” he said.
In addition to the two negative Indian Ocean dipoles, there have been two consecutive La Niña events, in 2020-21 and 2021-22. “This is very rare, to have these consecutive years of wet phase climate drivers,” Domensino said.
Some models suggest that another La Niña event may occur later this year.
