Forecasters have zeroed in on three climate events behind Australia’s current incessant rain – and it’s bringing even more soggy conditions.
There are now predictions this November may turn out to be one of the wettest on record – and that’s before the fierce La Nina climate driver, bubbling up in the Pacific, has even been declared.
This weekend, an intensifying rain event is set to bring widespread falls of 50mm or more in eastern and southern Australia swelling already full waterways.
So where is all this incessant rain coming from? In the east at least – isolated rain dumps aside, the west has been mostly dry.
Forecasters have said Australia is being whacked by a triple threat of weather events.
“It seems like the three major oceans surrounding Australia are all helping to produce extra rainfall at the moment,” Sky News Weather meteorologist Rob Sharpe told news.com.au.
Systems in the Pacific, Indian and Southern oceans are all currently ganging up on Australia, conspiring to send as much moisture as possible our way.
La Nina and its opposite number El Nino (which together form the El Nino Southern Oscillation), are a measure of sea surface temperatures and winds in the Pacific Ocean. During a La Nina event, trade winds amp up dragging cooler waters from the depths of the ocean to the surface in the central and eastern equatorial Pacific.
These winds, travelling from east to west, in turn push warmer seas closer to Australia which helps in cloud formation and brings more rainfall.
The US’ National Oceanic and Atmospheric Administration has already deemed La Nina to be in play. But the Bureau of Meteorology, which has a slightly different La Nina threshold, has yet to do so. All the signs are, however, that we’re in the cusp of it being announced.
Three climate drivers fuelling moisture to Australia
“So, we’ve got the developing La Nina event in the Pacific Ocean,” said Sky’s Mr Sharpe.
“In the Southern Ocean we’re stuck in a positive phase of the Southern Annular mode, which increases rainfall chances in south eastern Australia at this time of year.
“While in the Indian Ocean we’ve just come out of a negative Indian Ocean Diploe event, meaning we are now returning to neutral, but the remnants of that event seems to still be delivering some extra moisture to us from the west.”
The Southern Annular Mode, also known as SAM, and Indian Ocean Dipole are both climate drivers. And just like La Nina, depending on what mood they’re in, they can help dry out Australia or make it a whole lot soggier.
The SAM describes the actions of strong westerly winds that rotate around Antarctica. In a positive phase during summer – not far off what’s happening now – those winds move further away from Australia and that allows more rain to reach the east.
The Indian Ocean Diploe, like La Nina and El Nino, is a measure of sea surface temperatures but to the west of Australia. The current negative phase means it is funnelling moisture across the continent but particularly towards the south and east.
With all three climate drivers running amok, the upshot is a rinse and repeat pattern of rain, followed by more rain.
‘A lot of potential for rainfall’
A rainband will careen through south eastern parts on Thursday but it’s nothing much to get concerned about.
But a much larger rain event is heading for Adelaide, Sydney, Melbourne, and everywhere in between for the weekend.
“A low will form over Western Australia and track across the Nullarbor. At this stage it’s likely to intensify and focus its energy on Victoria and southern NSW on the weekend with the potential for heavy rainfall,” said Mr Sharpe.
“There is uncertainty about how intense it will be and which regions will see the heaviest falls, but it is very likely we’ll see a rain event in some form. Therefore there is potential for renewed river rises.”
That’s bad news for place like Forbes which are in the midst of floods following last week’s rain. Water levels there are now peaking but it won’t take much rain to see more floods.
Melbourne could see 20mm of rain this weekend, Adelaide 25mm and the gauge might top out at 50mm in both Sydney and Canberra.
“Looking even further ahead there are some early indications of another rainband due mid next week,” said Mr Sharpe.
“That one is a long way off, but it is an indicator that there is a lot of potential for rainfall in the atmosphere at the moment and it’s important for people to be ready for flash flooding and river flooding through the next six months.
“Most rivers in Australia are now primed for flood – so we’re just watching and waiting for the big rain event to come along and cause serious problems.”
Originally published as Trio of climate drivers behind prolonged rain spell
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