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Warming can cause hurricanes to weaken more slowly after landing

By studying the effects of climate change on hurricanes, scientists focused on what happens above the water, when storms form and intensify, capturing heat and humidity as they turn in over the ocean.

But a new study is looking at what happens after hurricanes make landfall and make their way inland. Research suggests that climate change also affects storms during this phase of their life, causing them to weaken more slowly and remain destructive for longer.

The findings could have implications for how emergency management agencies prepare for storms after landing.

In the study, published Wednesday in the journal Nature, Lin Li and Pinaki Chakraborty from the Okinawa Institute of Science and Technology Graduate University in Japan analyzed data from North Atlantic hurricanes that made landfall from 1967 to 2018, by examining the decrease in intensity, or wind speed, of storms on the first day after hitting land.

They found that if 50 years ago a typical storm would have lost more than three-quarters of its intensity in the first 24 hours, when it could travel several hundred kilometers inland, it did not ‘would now only lose about half.

“Decomposition has slowed down considerably over the past 50 years,” Dr Chakraborty said in an interview. “There may very well be a climatic link.”

By comparing the decay data with changes in sea surface temperatures, then using simulations of hurricanes moving across land, the scientists found what they say is the connection: rising sea temperatures. oceans linked to global warming leads to slower weakening of storms, even after storms, move away from the source of moisture.

Scientists warned that there were caveats to their research, among which they used a relatively small data set – only 71 hurricanes made landfall in five decades.

A prominent hurricane researcher Kerry Emanuel of the Massachusetts Institute of Technology said he was skeptical of the results. In an email message, Dr Emanuel said he did not agree with the researchers’ theoretical argument and that the data and simulations, “while suggestive, do not definitively prove that the decomposition is slower. in warmer climates regardless of other factors, such as the size of the storm. . “

But other researchers said the study was compelling and opened up a whole new field of research into hurricanes, their behavior on earth. Even weakened, the winds from these storms can knock down trees and power lines, damage homes and cause further destruction inland.

Dan Chavas, an atmospheric scientist at Purdue University who wrote an accompanying article in Nature, said the work was “definitive to identify a topic that hardly anyone has thought of and could be very important. . “

Suzana Camargo, a hurricane researcher at the Lamont-Doherty Earth Observatory, part of Columbia University, said she and her colleagues published a study last year that showed stronger hurricanes produced more precipitation after making landfall.

In the new study, she says, “They say the humidity stays in the storm for a while, and that makes perfect sense with what we saw in our study.

A hurricane is essentially a heat engine, a rotating storm powered by moisture from the warm ocean. The prevailing theory of how hurricanes weaken after they arrive on earth is that once they lose this fuel source, friction with the earth slows their rotation.

Dr Chakraborty compared it to a swirling cup of tea. “Over the ocean, because the moisture supply is there for the heat engine, you are constantly stirring the tea,” he says. But when it hits the ground, the power is cut off, the agitation stops and the friction slows down the vortex

“Above all, thermodynamics play no role” in this slowing down process, according to the theory, he said.

What he and his co-author suggest, however, is that the humidity that remains in the storm plays a thermodynamic role, affecting how quickly the storm weakens. And in a warming climate, with warmer sea surface temperatures, more moisture remains in the storm.

“Once we understand that humidity plays a key role, the link with climate becomes obvious,” said Dr Chakraborty.

Their hurricane simulations allowed them to test the idea that humidity plays a role in creating “dry”, moisture-free hurricanes that decomposed much faster than normal hurricanes. The models also allowed them to determine that factors such as topography and inland weather conditions played less of a role in weakening the storms.

Dr Camargo said a potential weakness of the study was that the models used were, of necessity, rather simple.

Modeling hurricanes after landing is difficult, she said. “It’s a difficult problem. The models need to capture a lot of things that are happening – the interaction with the topography, for example. “

“I don’t know if what they did in the model is the best way to represent hurricanes that have made landfall,” added Dr Camargo. “But at least in this model, it seems to be in keeping with their idea.”

Dr Chakraborty said he was not surprised that there was some skepticism about the results. “Overall, our study challenges widely held ideas about hurricane disintegration,” he said. “I hope this will stimulate more research and shed new light on this important area which has long been considered well understood.”

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