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The rare weather phenomenon which sent Cyclone Seroja down south

Tuesday April 13, 2021 - 21:23 EST
ABC image
Ex-Tropical Cyclone Seroja maintained its intensity due to higher than normal sea temperatures. - ABC

People in the small tourist towns of Kalbarri and Northampton in WA woke on Monday morning to utter destruction left in the path of ex-tropical cyclone Seroja.

This kind of weather system is rarely seen below Carnarvon, some 315 kilometres north up the WA coast.

So much so, that this level of devastation is in part because buildings are not built to the same code as those in "cyclone alley" in the northern Pilbara and Kimberley regions of the state.

So what led to ex-Tropical Cyclone Seroja taking this unusual path through WA?

Drawn further south by TC Odette

Very early forecasts had tipped TC Seroja to cross the Pilbara coast.

But a rarely seen weather phenomenon known as the Fujiwhara Effect changed that.

According to Bureau of Meteorology senior forecaster James Ashley, instead of curving inland and crossing the North-West, Seroja started to interact with a second system, now ex-Tropical Cyclone Odette.

"Because of that complex interaction with the other tropical low, that became Tropical Cyclone Odette, it kicked Seroja out further to the west," he said.

"So when it did recur it missed the Pilbara coast and came in along the Central West coast."

In Kalbarri wind gusts of up to 170 kilometres an hour were recorded just after 7pm on Sunday, while various other places throughout the Central West recorded 139 kilometre an hour winds.

Rainfall was relatively short-lived but intense.

Mr Ashley said Kalbarri had falls of 167 millimetres, Binnu had 75 millimetres and Carnarvon 60mm.

The weather pheonomenon also heavily influenced the path of ex-tropical cyclone Odette, but for the better.

As it orbitted around TC Seroja, Odette took a tighter turn and swerved past, but did not hit, the Gascoyne coast near Exmouth bringing a brief but strong period of wind to the North-West cape as it passed offshore.

Warm seas strengthened system

TC Seroja also maintained its intensity, due largely to warm sea surface temperatures up to 2-3 degrees higher than normal.

"It had access to more energy because the warmer temperatures allow the system to maintain that intensity and it was able to reach a category three intensity," he said.

University of WA oceanography professor Charitha Pattiaratchi said the warm seas were created by environmental conditions influenced by the La Nina year.

"The winds have weakened during La Nina and are more easterly," he said.

"So in combination you have less mixing and less upwelling."

Did climate change influence its path south?

While it was unusual, Mr Ashely said it was not possible to say if climate change played a part based on a single event. 

"It's a significant system and, while unusual, is a one off event and hard to attribute to climate change," he said.

He said it was difficult to say whether or not it would occur as far south in the near future.

"This hasn't happened in the Geraldton area in over 50 years so hopefully it's not going to be a common occurrence," he said.

"Cyclones are such an individual weather event that that's why we name them and systems in the future will behave in a manner of their own."


© ABC 2021

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