Two separate climate systems are conspiring to bring a wet and warm end to winter, the Bureau of Meteorology has forecast.
After a calm few months, the Indian Ocean Dipole and La Nina are brewing up which could see a dramatic change in the weather over the coming months.
La Nina’s last appearance earlier this year led to massive floods across the country’s east. Widespread floods in the southeast are now a real possibility again but there is no indication yet of how serious they might be should they occur.
The Bureau’s climate outlook for August to October, released in recent days, also included a look back on the current conditions.
Last month there was record breaking rain in southwest Western Australia. Perth is expected to have had its wettest July for more than two decades.
The north of Australia was warmer than average during the first two months of winter with the southeast of the country cooler. Adelaide saw its coldest day on record on July 22 with the mercury not managing to reach even 10C.
But those chilly temperatures will soon be long gone with warmer-than-average days and particularly nights for the coming months.
Climate drivers cranking up
The Indian Ocean Dipole (IOD) and El-Nino Southern Oscillation (ENSO) climate drivers are the causes.
Both are a measure of sea surface temperatures aided by powerful trade winds. The IOD, as the name suggests, is centred on the Indian Ocean and ENSO in the Pacific.
How these temperatures rise and fall can have a huge effect on rainfall in not just Australia but Africa and the Americas too. Generally, if it’s wetter than usual in Australia it will be drier in either or both of the other two continents.
“In the Indian Ocean, a negative IOD has developed which brings warm ocean waters to the north and northwest of Australia and typically enhances winter and spring rainfall over much of southern and eastern Australia,” said Dr Paul Feikema, a senior hydrologist with the BOM.
That’s combining with a lowering of water temperatures in a patch of the Pacific Ocean which is crucial in the formation of positive or negative ENSO conditions – the latter of which is known as a La Nina.
A La Nina occurs when trade winds increase, hauling cooler waters from the depths of the ocean up to the surface in the eastern Pacific.
These stronger winds from east to west also push warmer seas closer to Australia. That aids in the creation of more clouds and so moisture and windier conditions for the continent.
“Some models indicate this cooling could be sufficient to reach La Nina levels during spring,” Dr Feikema said.
“However, even if La Nina levels are not reached (and ENSO remains in neutral) this cooling may strengthen the wet outlook for much of Australia. As a result, the August to October outlook suggests above average rainfall is very likely across much of the country”
Flooding a ‘possibility’
Already sodden catchments means that widespread flooding is a real possibility in the coming months across south eastern parts of the country.
However the increased rainfall is less likely to occur in south west WA, bringing a reprieve from the incessant rain in Perth. Southern Victoria and much of Tasmania may also miss out.
As for temperatures, days are likely to be warmer than average across Tasmania, Victoria, coastal New South Wales and southern parts of South Australia as well as the nation’s north.
But nights are likely to warmer across almost the entirety of the country with the exception of southwest Western Australia.
The Bureau added that Australia’s climate has warmed by around 1.44C for the 1910–2019 period, while southern Australia had seen a reduction of 10–20 per cent in cool season rainfall in recent decades.
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