Weather News

Two looming cyclones off WA set to create rare weather phenomenon known as Fujiwhara Effect

By Tyne Logan, Wednesday April 7, 2021 - 10:52 AEST
ABC image
The Fujiwhara Effect played out in the South China Sea in 2009, when Super Typhoon Parma interacted with Super Typhoon Melor. - ABC

A rare weather phenomenon is developing off the West Australian coast as two brewing cyclones look set to interact this week.


Tropical Cyclone Seroja, currently sitting several hundred kilometres north-west of Broome, was named earlier this week and is tracking south-west over open waters.


Meanwhile, a second tropical low has now formed nearby, expected to strengthen to cyclone intensity as early as tomorrow.†


With the two systems on path to interact with each other, a phenomenon known as the Fujiwhara Effect will likely occur, causing them to rotate around each other.


Bureau of Meteorology (BOM) tropical cyclone forecaster Craig Earl-Spurr described the complex interaction as similar to a "dance" between the two systems,†rarely seen in Australia.


"Because both are trying to throw each other around their own wind fields, as each one moves the wind field moves with it," he said.


"So depending on how that then pushes the other [system]†around, little changes can become big changes very quickly."


What will the impacts be for WA?


Holiday-makers and residents on the west coast are being urged to stay alert and prepare for severe weather from Saturday to Monday, anywhere between Onslow and Perth.


The BOM's forecast map from Wednesday afternoon predicts TC Seroja will hit the WA coast as†a fast-paced Category One on Sunday or Monday anywhere between Coral Bay and Perth, but most likely between Carnarvon and Jurien Bay.


Meanwhile the tropical low could hit the west Pilbara coast on Saturday, but may take a sharper turn and avoid the coast altogether.


The speed of TC Seroja when it hits the WA coast is expected to be travelling at least twice as fast as a†normal cyclone would, at 35?40km an hour, which could bring†intensive rainfall over a short period, high winds and high seas off the coast.


Mr Earl-Spurr†said the "tug of war" between the two systems made predicting cyclone tracks difficult, and it could change very quickly.


"They can take a full 90-degree turn when they first start to interact with each other," he said.


Will they merge to form a mega-cyclone?†


The good news is their interaction means it is†more likely their track will be impacted rather than their intensity.


"It's unlikely that one of them would become significantly stronger through the interaction," Mr Earl-Spurr said.†


"They could intensify afterwards, once they're kind of no longer interacting with each other."


There is also a third tropical low currently to the far west over the Indian Ocean, but Mr Earl-Spurr said this was unlikely to add to the interaction at this stage.


The weather phenomenon was named after Japanese meteorologist Sakuhei Fujiwhara, who originally described it in 1921.







- ABC

© ABC 2021

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