Australia is likely to receive more heavy rain in late winter and into spring after the Bureau of Meteorology declared a negative Indian Ocean Dipole was under way in the nation’s north-west.

The Indian Ocean Dipole (IOD) measures the difference in sea surface temperature between the eastern and western sides of the tropical ocean.

When the Indian Ocean is cooler than average on the western side, and warmer than average on the eastern side, it causes more moisture-laden air to flow to Australia and fuels the north-west cloud bands that develop over north-western Australia, extending to the south-west of the continent.

The wet weather is likely to linger over the country after a negative Indian Ocean Dipole was declared on Tuesday.Credit:Twitter/@andrewmiskelly

It also brings a high chance of above-average rainfall over large areas of Australia in late winter and spring.

“We are seeing one of those developing this week, thanks to a strong cold front linking up with tropical moisture,” said Weatherzone meteorologist Ben Domensino.

This is the second negative IOD in a row, with the same weather pattern occurring last year. This is only the second time a negative IOD has occurred back-to-back since reliable records began in 1960.

“This has coincided with back-to-back La Niña events as well,” Domensino said. “Australia, at the moment, is stuck in this prolonged period of wet phase climate drivers – the broadscale patterns that influence our weather.”

The outlook for eastern NSW and eastern Queensland, which have been inundated with recent rains, is for above-average rain in the coming months, both because of the IOD and the La Niña weather pattern.

As the north-west cloud bands drift across the continent, most of the rain will fall on the western side of the Great Dividing Range, a different weather pattern to that which has caused flooding in Sydney and other regions over the past eight months.

It may also reach some areas of southern NSW and northern Victoria that have had below-average rainfall this year.

There are not enough long-term trustworthy records to be able to say if climate change is a factor in this back-to-back negative IOD, says University of Melbourne climate scientist Andrew King.

There is some evidence we may be seeing more extreme IOD conditions as the planet warms, he said.
Earth’s background climate is warming because of greenhouse gas emissions, but there is also climatic variability that is influenced by the Indian and Pacific oceans, he said.

“When we look back in time, we see we sometimes have these conditions – warmer sea temperatures in the east Indian Ocean … and wetter conditions over south-east Australia,” King said.

It comes as the BOM has issued a severe weather warning and minor to major flood warning over much of NSW. The flood warning, driven by a series of troughs and a cold front, covers the central west and south-west of the state from Wednesday to Friday.

NSW SES spokesman Greg Nash said the worst of the wet weather was likely to hit on Wednesday and into Thursday, with some areas to receive between 20 and 50 millimetres of rain.

“The forecast that has been provided to us is that there is going to be substantial rains coming up around the western slopes and around the ACT,” he said. “The SES is preparing by moving additional resources into those areas.”

The BOM previously warned that saturated soil from recent rainfall events, as well as full water systems – including most dams around NSW above 70 per cent capacity – would exacerbate flooding risks. It will be several more months until the agency can declare a third successive La Niña event, but there is a 50 per cent chance one could occur.

Sydney recorded its wettest July on record after 404 millimetres of rain fell in the CBD. The previous wettest July was 1950 with 336 millimetres. The usual average rainfall in July for Sydney is 96 millimetres. During August, the city records an average of 80 millimetres.

Meanwhile, a severe weather warning is in place in Victoria with large parts of the state to see damaging winds of up to 100km/h occurring overnight and during Wednesday.

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