The wet weather that has triggered record floods and filled dams across much of eastern Australia looks set to extend well into winter and beyond, global models indicate.
While autumn is typically the most difficult period to make season-ahead predictions, conditions favouring increasing cloud formation off north-western Australia look likely, even as rain-inducing weather from the Pacific recedes.
According to the Bureau of Meteorology’s latest climate driver update, the dominant La Niña pattern in the Pacific is continuing to break down. In its stead, though, are signs that a negative phase of the Indian Ocean Dipole – a gauge of relative sea-surface temperatures on the west and east of that ocean’s basin – will again take hold.
The negative phase increases the odds of above-average winter-spring rainfall for much of Australia. For northern Australia, it also increases the chances of warmer days and nights.
“While model outlooks have low accuracy at this time of year and hence some caution should be taken with IOD outlooks beyond May, there is consistency across the forecasts from all international models,” the bureau said.
By next month, all five of the major models used by @BOM_au to assess convection conditions in the Indian Ocean point to the negative - or wet for us - phase starting. Damper than usual winter and beyond likely for much of Australia. pic.twitter.com/P8uFqYqi1O
— Peter Hannam (@p_hannam) April 28, 2022
“Four of five international climate models surveyed by the bureau indicate the possibility of a negative IOD development in May, with all five reaching negative IOD thresholds by June,” it said.
Ben Domensino, a senior meteorologist at Weatherzone, said that another negative IOD phase would make it back-to-back events, something unseen in six decades.
“It looks like we’ll have two officially declared negative Indian Ocean Dipoles, which hasn’t happened at least in [bureau] records that go back to 1960,” he said.
After two La Niñas in two years in the Pacific – a pattern that sees strengthening easterly winds pushing rainfall towards Australia – the landscape “is a bit more primed for flooding” than a year ago.
Sydneysiders, are among those who won’t need much convincing of the flood risk. April will end on Saturday with the city’s rain gauge receiving more than 250mm – double the monthly average.
The harbour city remains on track for its wettest year in more than a century-and-a-half of records, tracking more than 100mm above the record wet year of 1950, Domensino said.
The prolonged wet comes as NSW dams are still at an average of 93% full, including about 110% at one of the biggest, Burrendong. Sydney’s dams too remain at close to 99% capacity, so it won’t take much rain to spill them again.
The prospect of a negative IOD also shows up in the bureau’s seasonal outlook.
Domensino said the typical north-west cloudbands bring extra rainfall to central Australia and dump most of their moisture on the western side of the Great Divide, partly sparing coastal regions such as around Sydney.
Near-term outlook is for a bit of rain in the east (good for western Tasmania). Outlook for winter is for above-average rainfall for pretty much the whole continent. @BOM_au pic.twitter.com/BOJcIh3LnR
— Peter Hannam (@p_hannam) April 27, 2022
Whether the additional precipitation falls as rain or snow on the alpine regions during winter may hinge on the strength of another of Australia’s climate drivers, the Southern Annular Mode.
A negative phase of that Southern Ocean gauge sees the typical west-to-east storm tracks shift farther north, bringing bursts of cold weather to southern Australia. During winter, that set-up usually means more snow in the alps.
“The ingredients are there for a good [snow] season but it will just come down to the individual events,” Domensino said.
While conditions in the Indian Ocean are tilted more strongly towards a negative IOD, the return of another La Niña event for next summer can’t be ruled out either.
A triple of three La Niñas in as many years has happened three times since 1950, according to the bureau. Prediction skill remains low at this time of year.
“Most of the seven international climate models surveyed by the bureau anticipate the La Niña will ease in strength over the coming months, with a return to neutral ENSO conditions (neither El Niño nor La Niña) most likely in late autumn or early winter,” the bureau said this week.
“By June three models exceed the [La Niña] threshold and by July only one does.”
The Pacific patterns have widespread effects, such as worsening drought during La Niña periods in the western regions of North and South America.
Commodities analysts at Rabobank said this week that US drought had worsened, resulting in the worst wheat crop conditions in 16 years.
