An average one system tracks within 550km of New Zealand each November-to-April cyclone season, usually around February and March. Photo / JMA
An average one system tracks within 550km of New Zealand each November-to-April cyclone season, usually around February and March. Photo / JMA
They’re the ultimate storm system, bringing torrential rain and winds powerful enough to cause millions of dollars of damage.
Now, a Niwa-led study has strengthened scientists’ expectations that tomorrow’s tropical cyclones will be even more destructive, thanks to a warming climate.
As February’s Gabrielle dramatically demonstrated, these giantatmospheric heat engines remain one of New Zealand’s biggest weather hazards – even if our region receives only one visit over an average cyclone season.
As the planet heats, climatologists have been trying to understand whether that number is likely to increase over time – and whether future cyclones are likely to pack much more of a punch.
“One of the key things that we want to try to understand is how storms might change in the future,” Niwa climate scientist Dr Jonny Williams explained.
They included those that generally followed the east-west “storm track” in New Zealand’s latitudes, and tropically born systems like Gabrielle, which swung our way from the north.
In their new study, Williams and colleagues drew on sophisticated models of the ocean and atmosphere, including the recently-developed New Zealand Earth System Model, or NZESM.
They also turned to open-source cyclone-tracking software that could be used to study both past observations and model projections of a warmer future.
Waves crash over Paihia wharf’s marlin statue, amid massive swells generated by February's ex-tropical cyclone Gabrielle. Photo / Peter de Graaf
By the last two decades of this century, the modelling suggests that the storms’ damage potential could worsen by about 10 per cent in a middle-of-the-road emissions scenario – or by as much as 26 per cent in a worst-case, though highly unlikely, scenario.
“The biggest unknown is what the future emissions and concentrations of greenhouse gases will be.”
Similarly, there was also much uncertainty about their future frequency.
Rather than an increase, some scientists have suggested a slight drop, due to changes in the atmosphere resulting in fewer storms being needed to maintain the flow of heat from the tropics to the poles.
At the same time, Williams said, there was some tentative evidence from projections that worst-case warming scenarios could bring a small increase.
“The reason why the number of cyclones in the future is so uncertain is due to the highly complex and interacting factors – such as temperature, humidity and how winds change with height - which cause them to form in the first place,” he said.
“It’d be really interesting in further studies to home in more on these factors to assess specifically how the frequency may change in the future.
“That said, the strength of cyclones is set to increase, albeit by an uncertain amount, and this is because of warmer sea surface temperatures which provide the fuel for them once they’ve formed.”
Ultimately, he said, what the picture looked like by 2100 came down to what effort we made today.
“There’s a positive message here, in that there is always more we can do to mitigate how strong the cyclones will be in the future,” he said.
“This is because it is pretty unambiguous that the more warming we have, the stronger the cyclones will be, and so whatever we do at an individual, whānau, national or global level will make a difference.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
