On 5 December a system of storms and low pressure converged around Solomon Islands to form the first tropical cyclone of the season for Australia, making landfall just north of Port Douglas on 13 December.
Cyclone Jasper hit the coast as a category two cyclone but it took the system almost five days to move west across Queensland. It left havoc in its wake, with many areas recording more than a metre of rain.
Experts say it was not the intensity of the cyclone that has made Jasper stand out but its slow speed and the incredible amount of rain it generated.
So why did it produce so much rain, and might a rapidly warming planet have made things worse?
What were the earliest signs?
Prof Michael Reeder, a meteorologist and cyclone expert at Monash University, says that in late November a European climate model predicted that there was potential for a cyclone to form in the Coral Sea.
“All of the models were saying it would make landfall at about the time that it did,” he said.
Why so much rain?
The direction and speed of a cyclone is dictated by what is known as the “steering winds” high in the atmosphere, between 1.5km and 10km above the surface.
Major weather systems, like an ex-tropical cyclone or low pressure systems that move slowly, can dump more rain in a concentrated area than a system where the steering winds blow strongly.
Dr Andrew Dowdy, an associate professor and expert on tropical cyclones at the University of Melbourne, said: “A slow-moving tropical cyclone can cause a lot more rain in a particular region, as compared to if a cyclone was moving faster with that rain spread over a wider region.
“Slow-moving tropical cyclones therefore have the potential to cause more intense impacts in some cases, such as from severe flooding from extreme rainfall over a relatively long duration in a particular region.
Reeder added: “What’s interesting and unusual is just how slow [Jasper] has been and it has been producing what I think will be record rainfall.”
Some parts of the region around Port Douglas, north of Cairns, received more than a metre of rain in just a few days.
The meteorologist Ben Domensino reported ex-Cyclone Jasper had dumped 2 metres of rain into one gauge, with two other locations seeing more than 0.8m in the 24 hours to 9am on Monday.
Several experts told Guardian Australia that another reason for the system bringing so much rain is the location where it made landfall.
Dr Hamish Ramsay, a climate and cyclone expert at CSIRO, pointed to a phenomenon called “topographic enhancement” that can intensify rainfall. The hills around the Cairns region help to lift more air and moisture into the cyclone as it passes over.
What happens to ex-Cyclone Jasper now?
On Monday there were four major flood warnings in place for the north Queensland rivers Daintree, Barron, Murray and Herbert. The Bureau of Meteorology said the Daintree River had peaked at 15 metres – more than 2 metres above the previous record set in 2019.
The bureau’s forecast suggests the ex-tropical cyclone system could head north-west into the Gulf of Carpentaria then head back over the far northern tip of Queensland, with a low chance of reforming into a tropical cyclone.
What about El Niño and climate change?
Australia usually experiences fewer tropical cyclones in El Niño periods like the one the continent is now experiencing, but cyclones do still occur.
All weather systems, including tropical cyclones and El Niño climate patterns, now exist in an atmosphere that contains almost 50% more carbon dioxide than before the world started burning fossil fuels.
Dr Jaci Brown, head of CSIRO’s Climate Science Centre, says: “We do expect a reduction in tropical cyclones in El Niño years. But we are in a new climate regime that we haven’t seen before.
“We have an El Niño with drought, but also with intense rainfall. Scientists are trying to understand how much of this is associated with climate change. Is this the new normal?”
Ramsay says projections from climate models are inconclusive about any influence from global heating on the speed that cyclones may travel.
“I think this event will motivate some research to try and understand previous trends in the Australian region,” he said.
But he said the extreme rainfall was “in keeping” with projections from climate models that show the amount of rainfall from tropical cyclones would increase as the world keeps heating up.
Dr Andrew King, a climate scientist at the University of Melbourne, said: “I would expect climate change has played a small role in this event because we know sea surface temperatures have been abnormally high to the north-east of Australia [as the cyclone was forming].”
Generally, scientists know that for each degree of global heating the atmosphere can hold about 7% more moisture that’s available to fall as rain.
Dowdy said: “We have already had over one degree of human-caused global warming, so there is already potential for extreme rainfall to be more intense.”
But he said when water vapour condenses it releases heat that can add more energy to storms to suck up more moisture.
“This means that the increase in extreme rainfall due to climate change can be more than this 6-7% increase per degree of global warming in some cases.
“For example, about double to triple this rate of 6-7% per degree of warming has already been observed through Australia.”
He said some scientists were now advising that for Australia, planners should consider that each degree of warming could increase rainfall by as much as 15%.
