Damage from Tropical Cyclone Ana in Fiji, in 2021. Scientists are warning that warming seas mean stronger tropical storms, which can hit New Zealand as ex-Tropical Cyclones. Photo: Fiji government
Increasingly strong cyclones may mean a new higher-wind-speed category needs to be created, researchers say, as global heating ups the forces at play.
Although New Zealand uses a different system to the one used by weather forecasters from the United States, most people know a 'Category 5' means a very bad storm.
But climate change is turbo-charging cyclones to the point some forecasters and climate researchers say Category 5 is no longer severe enough.
Researchers from the United States have published a study on what would happen if there was a hypothetical Category 6 cyclone, indicating wind speeds above 309 kilometres an hour.
Their work was presented in the influential journal Proceedings of the National Academy of Sciences, and has attracted interest here.
New Zealand's most damaging storms tend to start as tropical cyclones, so forecasters here say they are watching developments.
The US researchers found five storms since 1980 would have qualified for the new category, with all five occurring since 2013.
Niwa climate scientist Sam Dean says as the oceans heat from greenhouse gases, cyclones gain more energy from the warm ocean, and hotter air carries more water vapour, which fuels them further.
Right now the wind speed of a tropical cyclone is rated from one to five.
But Dean says as cyclones are getting stronger, the top rating, which covers anything above 252kph, is no longer enough to show the true severity of the strongest storms
MetService switched to using a colour-coded system of storm warnings five years ago, and has issued 14 red warnings since, more than it ever expected.
MetService head of weather communication Lisa Murray says the system was changed because people were getting 'warning fatigue'. But forecasters had originally expected only a few 'red' warnings a year.
Murray welcomes the research looking into adding a Category 6, and says MetService would take it on if it became part of the official scale.
Although New Zealand and neighbouring countries use a different classification system, ours, too is open-ended.
University of Otago senior lecturer Daniel Kingston says the majority of the country's most damaging storms start their lives as tropical cyclones, which was the case for both cyclones Gabrielle and Bola.
By the time they reach New Zealand, they are no longer tropical cyclones but ex-tropical cyclones, and the rain and storm surges tend to be more dangerous than the wind.
Kingston says scientists do not necessarily expect more frequent cyclones with climate change, but when they come, they are expected to be more intense.