A rare pair of atmospheric rivers have formed off Australia this week, channelling tropical moisture to both sides of the nation.
Atmospheric rivers, sometimes referred to as cloud bands, are ribbons of water vapour stretching thousands of kilometres from the tropics to southern states, transporting vast quantities of water.
"It's pretty rare to have two of these big cloud bands and atmospheric rivers across the country at the same time," the Bureau of Meteorology's Dean Narramore said.
In Western Australia, a particular kind of atmospheric river called a northwest cloud band has formed, stretching from deep in the tropical Indian Ocean all the way to the Nullarbor coast.
"We've had a big low pressure system in WA that's connected with tropical moisture over Christmas Island, well off in the Indian Ocean," Mr Narramore said.
"That's tapped into that moisture and brought widespread rain across western and southern WA over the last 24 hours, which is awesome."
At the same time, a second atmospheric river has formed off Australia's east coast, feeding warm moisture into the storm that hammered New South Wales over the past few days.
"That cloud band extends all the way back to Papua New Guinea," Mr Narramore said.
'More than two Amazon rivers worth of water'
Kimberley Reid is a scientist from the University of Melbourne and expert in atmospheric rivers.
"They look like giant rivers in the sky," she said.
Ms Reid estimated the northwest cloud band off WA is transporting more than two Amazon rivers worth of water over the state.
"That's about 660 Murray Rivers," she said.
Ms Reid has studied more than 30 years of records of northwest cloud bands, and discovered they are occurring more frequently than in the past.
"Global studies have shown that atmospheric rivers are likely to increase in both frequency and intensity with climate change," she said.
"We found that since about the mid-1980s, there's been an increase in northwest cloud bands of about one per year, over about 30 years."
Ms Reid said atmospheric rivers are much better known overseas.
"There's actually an atmospheric river in the United States that has its own name — 'The Pineapple Express' — that transports water from Hawaii all the way to California," she said.
Here in Australia, atmospheric rivers are less documented, but Ms Reid suspected the phenomenon may be behind some of Australia's biggest floods.
"The Queensland flooding event on Boxing Day 2010 is known as one of the wettest days in Queensland," she said.
"There was a massive atmospheric river that day running all along Cape York."
Northwest cloud bands can bring drought-breaking rain
Northwest cloud bands occur more frequently in years when the waters off WA are warmer than average, which climatologists call a negative Indian Ocean Dipole or negative IOD.
Those years are associated with higher than average rain in central and southern Australia.
"For example, 2010, 2011 and 2016 were examples of when there was a negative Indian Ocean Dipole and got lots of cloud bands bringing rain to inland Australia," Ms Reid said
"Last year we had a positive Indian Ocean Dipole, which means we've had cool waters near Northwest Australia, meaning that it's just much harder for these cloud bands to form.
"So we haven't had as many in the past year or so."
What can we expect to happen this week?
Northwest cloud bands can stretch all the way from Broome to Melbourne, but Dean Narramore from the Bureau of Meteorology does not think this one, presently over WA, will make it that far.
"It's moving into to eastern WA and western South Australia now," he said.
"With just a little connection to the tropics, there's some moisture still to come, but we're only going to see one to five millimetres there. And then it's going to weaken completely on Tuesday."
Mr Narramore said the atmospheric river off Australia's east coast will weaken over the next few days as well.
"It'll bring widespread rain to Norfolk Island and Vanuatu and maybe even northern New Zealand today into tomorrow, before moving out further into the Pacific," he said.ABC