BOM declares negative Indian Ocean Dipole, likely to add to unrelenting rain in Australia’s east – Michmutters

BOM declares negative Indian Ocean Dipole, likely to add to unrelenting rain in Australia’s east

The Bureau of Meteorology (BOM) has officially declared yet another climatic phenomenon likely to add to unrelenting rain on Australia’s east coast.

Known as the Indian Ocean Dipole (IOD), the climate driver affects rainfall patterns across Australia.

Persistently warm seas to the north-west of Western Australia have swung the IOD into a “negative” phase for the second year in a row.

That typically means wetter than normal weather for most of the country, particularly the south-eastern states.

A map indicating regions which typically receive more rainfall during negative IOD
While every year is different, the areas shaded in green usually receive wetter than normal weather during a negative Indian Ocean Dipole — particularly the south-east of Australia.(ABC: Shakira Wilson)

Following a La Nina phase during the last two summers, it means rain-bearing climate drivers have now been in play for eastern states for two years straight.

Bureau of Meteorology head of long-range forecasting Andrew Watkins said there was also the chance La Niña could re-form for a third time during spring.

“Certainly, we are in an unusual time to have so many climate drivers pushing Australia’s climate toward wetter conditions for the past two to three years,” he said.

Increased flood risk for already sodden land

While rain is usually welcomed during winter, an already sodden landscape means the risk of flooding is a lot higher for the next few months.

“At the moment we have full dams, full rivers and we’ve also got high soil moisture, and with the wet outlook the flood risk is elevated in eastern Australia,” he said.

Since February, Australia’s east coast has endured four intense weather systems, leading to record rains and flooding.

The events, which have occurred in southern Queensland, northern New South Wales and Sydney, have been devastating, and at times even deadly.

Purple hues on clouds with lightning
Warm waters to the north-west of WA encourage the development of cloud bands full of tropical moisture that then sweep across WA’s Gascoyne, central Australia and into eastern parts of the country.(Supplied: Marcus Scott)

Dr Watkins said this year’s negative IOD was shaping up to be stronger than last year.

“This year is looking a little more on the weak to moderate scale, at least at this stage anyway,” he said.

But he said it would not change their winter outlook, which was already projecting a particularly soggy season for most of Australia.

Dr Watkins said this was because they had already anticipated it would occur and factored it into the outlook, along with a range of other drivers.

Impacts already at play

A negative IOD encourages the development of north-west cloud bands filled with moisture, which carry rain from the Gascoyne region of WA, across the central desert and into the eastern states.

This acts as a source of tropical moisture that typical winter systems, such as cold-fronts and lows, can tap into.

A satellite image of a north-west cloud band
A negative IOD encourages the development of cloud bands, full of tropical moisture, that stretch from the ocean to the north-west of Western Australia into the eastern seaboard.(Supplied: BOM)

Dr Watkins said it is likely to have already played a role in some of the big rain events of the last two months.

“We have a seen a few north-west cloud bands already this year,” he said.

“So to some degree, it is having an influence already, alongside all of those other climate drivers that influence our climate.”

It is potentially playing a role in the severe weather currently being experienced across southern Australia, including damaging winds, heavy rain and dangerous seas.

The severe weather has been driven by a series of strong cold fronts moving across the country.

However, tropical moisture from a north-west cloud band has also fed into the system, adding to rainfall totals.

Not wet for everyone

Even though the IOD develops off the coast of Western Australia, its impacts are typically minimal to the state’s most populated area, the south-west.

In June, large parts of south-west Western Australia, including Perth, finished the month with less than half their average rainfall – a trend becoming more frequent with climate change.

July rainfall was closer to average for most.

The negative IOD is expected to last until November or December.

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