After a heatwave, we crave relief, not more extreme weather. But increasingly, we have to contend with a succession of extremes - ricocheting from extreme heat to intense storms to flooding waterways.
We saw this in the Victorian Otways region last summer, when extreme heat, fires and floods all occurred in the space of two weeks .
We studied this sudden "weather whiplash", where a heatwave is followed by heavy rainfall, in more detail to understand which parts of Australia are affected, why this occurs and how this phenomenon is changing as the world warms.
The results were sobering. Not only are weather whiplashes a repeating feature of Australia's climate, but they are becoming more frequent and affecting larger areas of the country than a century ago.
A land of extremes
Australians know extreme weather, and many of us have experienced its repercussions firsthand. Heatwaves are a " silent killer ", responsible for more deaths than all other natural hazards combined, while extreme rainfall can bring either cooling relief or destruction through flash flooding. Climate change is increasing their frequency , and they do not always occur in isolation.
It's common to think of weather extremes as separate events: a heatwave is a heatwave, and a storm is a storm. But we found heatwaves and heavy rainfall often come within days of each other, creating "weather whiplashes". This means communities, landscapes, ecosystems and infrastructure are already under stress when the next extreme weather event arrives.
Our new paper identified hundreds of whiplash events where extreme rainfall immediately followed heatwaves, using Australian climate data starting in 1910. Of most interest were whiplashes that happened in the warm season, from October to April, because it's the time of year when you would expect to see swings from heat to extreme rain.
It showed south-eastern Australia is a whiplash hotspot, where cold fronts often replace heatwaves with heavy rain, affecting large areas at once. Southern Queensland recorded the highest number of whiplash events - being on the east coast leaves it vulnerable to heavy rain pushed inland by winds travelling across the sea. In the tropical north, thunderstorms flush out heat.
More frequent, and more widespread
Next, we wanted to understand whether these whiplash events had changed over the past 110 years. Many people feel as though the weather is becoming more rapidly changeable, but is this supported by the data?
Overall, yes. Weather whiplash events are more frequent and affect larger areas of the country than they did a century ago. More Australians are now exposed to whiplash weather.
We zoomed into big cities with big populations. The north of Australia, including Cairns and Darwin, has experienced the biggest increases, mainly driven by more frequent and intense heatwaves, combined with a wetter climate. And temperatures don't drop as much after heatwaves as they once did.
Melbourne's four seasons in a day is here to stay too - while cities in the south may have weaker whiplash trends, these cover much larger areas.
The expanding area affected by whiplash weather means more locations are facing simultaneous hazards, potentially straining the capacity of local government areas and states to support one another.
Other studies in Europe, and over shorter time periods in Australia , have also shown weather is becoming more volatile.
These events are linked, not random
Extreme rainfall follows heatwaves too often to be coincidence, suggesting a physical connection between the events.
In parts of southeastern Australia, more than 20% of heatwaves resulted in a whiplash. We think this is because the termination of heatwaves is associated with an unstable atmosphere. The hot conditions also allow the atmosphere to hold more moisture, so when it rains, it pours.
Over southern Australia, the high-pressure systems that allow heatwaves to form are replaced by low-pressure systems. These can sweep in like a bulldozer, pushing surrounding air upwards and forcing out moisture that falls as heavy rain in big cold fronts.
Heat and fire can also pre-condition the land, making it dry, barren and more prone to flooding when rain hits directly afterwards.
Colliding weather
No two weather whiplashes will be the same. Each brings unique impacts and there are many other types of weather that can collide in different combinations, on different timescales.
We need to be better prepared for this whiplash as we head further into an ever-warming world.
And one thing is clear: it's not enough to understand individual extremes, we have to understand how they interact with each other.
Juliana Neild receives funding from Australian Research Council.
Andrew King receives funding from receives funding from the Australian Research Council (Centre of Excellence for the Weather of the 21st Century and a Future Fellowship) and the Australian government's National Environmental Science Program.
Linden Ashcroft has received funding from the Australian Research Council (ARC) and is affiliated with the ARC Centre of Excellence for 21st Century Weather. Linden is a Fellow of the Climate Council.
Alexander Borowiak does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
