Hurricane Florence landed in September 2018
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Rising sea temperatures mean that hurricanes stay stronger longer, researchers have found. The average hurricane in the North Atlantic now takes 33 hours to weaken, compared to 17 hours about 50 years ago.
“Hurricanes will wane more slowly as the oceans warm up further,” says Pinaki Chakraborty of the Okinawa Institute for Science and Technology in Onna, Japan.
Chakraborty and his colleagues examined data from hurricanes in the North Atlantic from 1967 to 2018. They found that 50 years ago, hurricanes would weaken about 75 percent on the first day after reaching the country. They only weaken by around 50 percent after landing, which means that hurricanes are twice as intense today and last twice as long.
“A longer decay time means a hurricane will stay stronger longer,” says Chakraborty. “And because the decay is exponential, the magnitude of this difference in intensity can be significant.”
While previous studies have indicated that wind speed is the single most important factor in sustaining a hurricane, the team argues that moisture has been overlooked. Hurricanes pull moisture from the ocean to propel their strong winds and rain, but once they reach land it is harder for them to replenish. “We show that moisture plays a dominant role in the dynamics of hurricanes after landing,” says Chakraborty.
Rising ocean temperatures provide hurricanes with more moisture to develop faster, keep them fueled on land without decay, and generate higher rainfall. This year’s hurricane season in the North Atlantic was the worst that has ever been recorded. Climate change will no doubt make storms more frequent, and slower hurricanes will do more damage. While looking specifically at North American hurricanes, the results likely apply elsewhere as well, says Chakraborty.
“The study opens up a rather unexplored research problem,” says Kerry Emanuel of the Massachusetts Institute of Technology, but is not convinced that moisture is key. “The results … do not agree with the existing theory of hurricane decay,” he says.
The Chakraborty team urges people living in offshore regions who may be unprepared to be aware that they may be exposed to more extreme hurricanes due to this slower decay time. With hurricanes, which typically travel 430 kilometers per day, a prolonged storm means that the destruction is pushing further inland. “I’m afraid that’s a grim message,” says Chakraborty.
Journal reference: Nature, DOI: 10.1038 / s41586-020-2867-7
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