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Hurricanes weakening more slowly

By Henry Fountain

© The New York Times Co.

In studying the effects of climate change on hurricanes, scientists have focused on what occurs over water, when storms are forming and strengthening, picking up heat and moisture as they churn over the ocean.

But a new study looks at what happens after hurricanes make landfall and work their way inland. The research suggests that climate change is affecting storms during this phase of their life as well, causing them to weaken more slowly and remain destructive longer.

The findings could have implications for how emergency management agencies prepare for storms post-landfall.

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

They found that although 50 years ago a typical storm would have lost more than three-quarters of its intensity in the first 24 hours, when it might travel several hundred miles inland, now it would lose only about half.

“The decay has slowed down tremendously over the last 50 years,” Chakraborty said in an interview.

Comparing the data on decay with changes in sea surface temperatures then using simulations of hurricanes moving onto land, the scientists discovered what they say is the link: Rising ocean temperatures linked to global warming are causing the storms to weaken more slowly, even after storms move away from the source of the moisture.

The scientists cautioned that there were caveats to their research, among them that they used a relatively small data set; only 71 hurricanes made landfall over five decades.

One prominent hurricane researcher, Kerry Emanuel of the Massachusetts Institute of Technology, said he was skeptical of the findings. In an email message, Emanuel said he disagreed with the researchers’ theoretical argument and that the data and simulations, “while suggestive, do not definitively prove the case that decay is slower in warmer climates independent of other factors, such as storm size.”

But other researchers said the study was compelling and opened up a new field of hurricane research — on their behavior over land. Even weakened, winds from these storms can topple trees and power lines, damage homes and cause other destruction well inland. Dan Chavas, an atmospheric scientist at Purdue University who wrote an article accompanying the paper in Nature, said the work was “definitive in identifying a topic almost no one has thought about and could be very important.”

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

In the new study, she said, “they are saying that moisture stays in the storm for a while, and that completely makes sense with what we saw in our study.”

A hurricane is essentially a heat engine, a rotating storm fueled by moisture from the warm ocean. The prevailing theory as to how hurricanes weaken after landfall is that once they lose that source of fuel, friction with land causes their spinning to slow down.

What he and his co-author suggest, however, is that the moisture remaining in the storm plays a thermodynamic role, affecting the rate at which the storm weakens.

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