Two
© Kimberley Reid / University of Melbourne
Two "atmospheric rivers" formed over Australia's east and west coasts over the past few days, carrying huge amounts of rain from tropical locations.

A rare weather phenomenon has occurred above Australia, with two almost symmetrical cloud "rivers" forming at opposite ends of the country.

An "atmospheric river" is caused when a stream of very moist air moves in from the tropics and causes very heavy rain.

The Bureau of Meteorology calls these streams "atmospheric rivers".

They are cloud bands that can stretch for thousands of kilometres, carrying large quantities of water.

"It occurs when any strong weather system is able to tap into moisture in tropical locations," a BOM spokesperson told nine.com.au.

"This one has tapped into moisture around Papua New Guinea and brought rain from there.

"The two that are forming above Australia's north-east and south-west coasts are responsible for the heavy rainfall experienced in NSW, Victoria, South Australia and WA in recent days.

While an atmospheric river is not uncommon, having two form in such close proximity to each other is rare.

Bureau officials say the river on the west coast is due to a large low-pressure system connecting with tropical moisture from Christmas Island, in the Indian Ocean, stretching all the way to the Nullabor Coast.

At the same time, the river forming off Australia's east coast is feeding warm moisture into the storm that wreaked havoc on the NSW coast over the past few days.

"These cloud bands can occur at anytime of the year but tend to be more common around autumn and winter in Australia," the Bureau said.

According to studies by the University of Melbourne, atmospheric rivers could be becoming more frequent as a result of climate change.


Comment: Indeed - just not 'man-made climate change'. See also: Study reveals atmospheric rivers to double in size.


They say at the end of 2010, there was a large atmospheric river moving along Cape York and this may have been the cause of the devastating Boxing Day floods in Queensland that year.

While atmospheric rivers can cause intense, damaging downpours, climatologists say they can also help drought affected communities.

Cloud bands that form in the north-west of Australia occur more frequently when waters off WA are warmer than average, an occurrence they call negative Indian Ocean Dipole (negative IOD).

Other years where negative IODs have occurred were in 2010, 2011 and 2016, according to the BoM, and this often increases rainfall in central and southern parts of the country.

While the current atmospheric river combo looks impressive, it will likely weaken over the next few days and move out further into the Pacific Ocean according to meteorologists.