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BOM declares negative Indian Ocean Dipole, tipping the odds towards more wet weather

Wednesday July 21, 2021 - 02:25 EST
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A negative phase of the Indian Ocean Dipole could help tip the odds towards a wet end to winter. - ABC

The climate phenomenon that brought Australia one of its wettest winters has been officially declared for this year.

Bureau of Meteorology experts have announced that the Indian Ocean Dipole (IOD) is in a negative phase for the first time in five years.

Every event is different, but a negative phase usually brings wet conditions to regions from Western Australia's Gascoyne down and across to Tasmania.

The last time a negative dipole event was declared was in 2016, between June and September.

That winter was the second wettest on record, with more than half of Australia receiving rainfall totals in the highest 10 per cent of historical records for the June to August period.

May to September was also the wettest on record.

Bureau of Meteorology senior climatologist Felicity Gamble said this year was not expected to be as strong as the last event.

"So while the strength of this years event is expected to be weaker than our last negative IOD in 2016, significant events can still occur and it's important to keep up to date with our latest climate updates and forecasts," she said.

What could it mean for farmers?

North-west of Wagga in the Riverina, New South Wales agronomist and farmer Rohan Brill is happy with how the season is shaping up so far.

"We are in the middle of July and crops have a good bank of soil moisture underneath them, which is what we always hope for, really ? so that's a good thing," he said.

[embed winter rainfall datawrapper]But 100 kilometres north, in the state's lower Central West, conditions are already quite wet.

"If it came in with a fairly wet period from now on I think some of that area would suffer a bit of damage," Mr Brill said.

Wet years can lead to extra fungal issues, nitrogen depletion and smothered crops, but for the most part they are welcome.

"A year like 2016, some people suffered, but overall they're still usually big years," Mr Brill said.

'Amazing, really'

Negative dipole years typically bring wetter than average conditions to the Gascoyne, Pilbara and Goldfields regions of WA.

But BOM climatologist Simon Grainger said this years event may not deliver the usual "above average" rain for WA.

"What really seems to enhance negative IOD events for WA in winter and spring is if the waters are exceptionally warm all around Australia," he said.

"That's what we saw in 2016.

"But this time around we're not seeing as strong of a negative IOD event."

Nevertheless, any additional rain over the next four months will be seen as a bonus for farmers in those areas and could lead to a record wet year.

Caroline Thomas, who runs Marron Station near Carnavon with her husband Simon, said their average annual rainfall had already doubled.

"We haven't even finished the year yet ? it's amazing, really," she said.

"As much as you can it's a good time to be building up number and trying to make the most of the feed, really.

"It's very green, it looks like farmland ? and the animals, they're pretty relaxed.

"They go out to feed until about morning tea and then lay around all afternoon because they are full."

Will negative events get more common?

Research has showed extreme, positive dipole phases, like that of 2019, could be more frequent in future.

But Australian National University climate scientist Nerilie Abrams said the future frequency of negative events were more difficult to predict.

"There's an interplay between the overall warming of the ocean and what's happening with the variability of the Indian Ocean Dipole," she said.

"Climate models would tell us we expect the negative events to become less common.

"But at the same time we know that the ocean is warming because of global warming, so there's a balancing of those two different effects that will ultimately determine what the impact on the Australian landscape is."

IOD events occur in the cooler months of the year and generally break down as summer weather patterns establish over the Indian Ocean.


© ABC 2021

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