Jamie Priest
Jamie Priest is a science journalist at Cosmos. She has a Bachelor of Science in Marine Biology from the University of Adelaide.
South Australia is the driest state on the world’s driest inhabited continent. With an arid landscape dominated by shrublands and dusty red soils, the twin spectres of drought and bushfire loom large in our collective psyche, an inevitable consequence of living in such a parched climate.
But as the first month of 2022 draws to a close, towns across the state are mopping up the damage caused by record-breaking rains and surging outback floodwaters. Entire sections of highway have been swept away in the deluge. The towns of Winter Springs and Kimba each recorded daily rainfall in excess of 160mm, more than some parts of the state receive in an entire year. More than 300 calls for assistance were made to the State Emergency Service (SES), many from people trapped by rising floodwaters.
In a state unaccustomed to flooding, the widespread downpour has been met with a degree of bewilderment. What lies behind this out-of-character weather event?
The answer is ex-tropical cyclone Tiffany, which formed over the Coral Sea before veering across the Cape York Peninsula. The system briefly weakened before regaining intensity over the Gulf of Carpentaria, ramping up ahead of its push into the Northern Territory.
The cyclone battered Australia’s tropics, causing floods and extensive property damage. While this meteorological drama was unfolding in the north, atmospheric conditions were aligning to sweep the storm system far into the south. Carried by the double drivers of a slow-moving upper low-pressure system and a surface trough, tropical rains found their way across the arid interior to drench South Australia.
“Even though the tropical cyclone and the circulation have broken apart, the tropical airmass associated with this system has continued to move across central parts of the country,” says Jonathon How of the Bureau of Meteorology.
“That’s why we’ve been seeing very, very heavy rainfall. It’s been pure tropical moisture brought down from the Coral Sea.”
Cyclonic havoc wrought in the tropical north doesn’t often unleash highway-destroying floodwaters in the south. Is it unusual for a cyclone to push moisture into these latitudes?
Not particularly, says How.
“Every season we do see these ex-tropical cyclones move across central parts of the country. But what is more unusual is that this cyclone originated across the northeast, across Queensland.”
South Australia often receives moderate rainfall in the wake of weakening ex-tropical cyclones that form off the coast of Western Australia, crossing the Pilbara and bringing welcome rains to the Eyre Peninsula. But ex-tropical cyclone Tiffany has been different, tracking inland from the east coast.
“It’s not the typical movement you’d expect from a tropical cyclone,” explains How. “You’d normally expect it to form in the Coral Sea and then hit the coast of Queensland, not travel too far towards the west. But this time the conditions were perfectly aligned to see it move quite backwards towards the west and into the south, and then from there towards the south-east – it’s done a bit of a loop.”
Why did it take this uncommon path? How explains that while it’s not a standard trajectory for cyclones in the region, it’s not unheard of – it just requires the right conditions.
“It’s due to the upper-level atmospheric conditions outlining the case,” he says.
An upper-level ridge sitting across the east of the country funnelled the weather system along its unexpected track, and it simply continued to collect moisture from the humid airmasses it met along the way.
Could the event be linked to climate change? It’s too soon to tell, says How. Linking major weather events to the broader picture of the state of the climate requires careful analysis, and is difficult to assess while the event is still unfolding.
What we do know is that events like this will become increasingly likely as climate change marches on. Even if the current floods can be attributed to nothing more than a chance alignment of atmospheric conditions, they can potentially offer us a window into the future.
“As oceans warm there is more moisture to tap into to bring these type of heavy rainfall events. We know that with global warming ,strong cyclones and widespread flooding events will become more commonplace.”
In the meantime, South Australia could face a longer clean-up than the eastern states that have also experienced the effects of the dissipating weather system.
“In terms of flooding impacts, because the land is so flat in South Australia the water is very slow moving,” says How.
“It’s going to take days or weeks to make it to inland lakes, whereas along the east coast the waters are faster moving down the mountains and down the ranges.”
The BoM has warned that while the most severe weather has passed, further rain and storms could be on their way over coming days. The air remains uncharacteristically humid, and an unstable airmass means isolated thunderstorms are likely to spark across the state, particularly in the far north.
But for now, South Australia can only keep those hatches battened, knowing that the worst of the wild weather is behind us.