Researchers reanalyzing historical data on tropical cyclones discovered that over the last four decades, one-quarter of landfalling cyclones over northeastern North America retained their tropical characteristics instead of weakening into a different storm type.

The study, published April 12 in JGR: Atmospheres, suggests that tropical cyclones, the scientific name for hurricanes and typhoons, survive over land more frequently than typically thought, which could result in a greater risk of damage from extreme hurricanes as these storms increase in severity due to climate change.

"There's a lot we don't know about storms, despite the threats they pose to communities and economies," said first author Alex Baker, a meteorology research scientist at the University of Reading in England. "We're trying to understand how climate change is affecting tropical storms. Will they be more [or] less frequent, more [or] less intense? Will they hit new areas?"

The destruction wrought by tropical storms affects millions of people all over the world. And while the overall number of storms each year has remained relatively constant, these storms are becoming more extreme, and more are striking outside the tropics. A typical year has 12 named storms in the Atlantic, including six hurricanes, and three major hurricanes. However, the 2020 hurricane season broke the record with 30 named storms.

For their study, Baker and his colleagues focused on midlatitude tropical cyclones, which occur between the tropics and the poles. Specifically, they looked at tropical cyclones affecting Europe and North America.

"It's really about the characteristics of storms we might see more of in [the] future and about potential risk," Baker said.

Because these areas are densely populated and contain economically significant cities such as New York and London, midlatitude tropical cyclones "are some of the costliest natural disasters on record," according to Baker.

He explained that as tropical storms move out of the tropics, they tend to weaken and turn into different storm types. However, some storms may retain their strength as they move north, becoming the hurricanes that impact North America and Europe. These storms tend to be more intense at landfall, with the potential to be more dangerous.

In 2002, Hurricane Sandy ravaged the East Coast, including New York, which suffered an estimated $30 million to $50 million in losses and damage to infrastructure. Across the Atlantic, in 2017, Hurricane Ophelia caused almost EUR 70 million worth of damage in Ireland, even after downgrading to a post-tropical cyclone as it traveled inland.

The unreliability of historical storm data often forces researchers to rely on reconstructions and meta-analyses to study trends in tropical cyclones. Because the record of hurricanes in the Northeast U.S. is patchy for storms that originate in the tropics and make landfall in the midlatitudes, the researchers used reanalysis datasets from 1979-2018, sourced from government agencies including NASA and the European Centre for Medium-Range Weather Forecasts.

They then used an objective storm-tracking algorithm to identify tropical cyclones that impacted North America and Europe, looking for trends in landfalling storms that remained strong enough to be considered a tropical cyclone.

They found that in Europe, one out of every 10 storms retains its tropical characteristics. In North America, it's one in four. According to Baker, these figures are higher than what he and the scientific community would have expected, particularly for Europe.

"The scientific community as well as weather centers like the Met Office have tended to think that these storms are rare events, and not such a regular part of our overall storm risk," he said. "But it's important to say that not every such storm is an Ophelia."

However, climate change may cause more of these landfalling storms to develop into destructive forces of nature like Ophelia or Sandy. The researchers are now studying the ways the risks associated with these storms may change in the coming years, using their current project to put future changes into a historical context.

Meanwhile, Baker hopes that realizing the frequency with which tropical storms can successfully make landfall will encourage weather services to stay vigilant to the dangers posed by these storms, particularly as they become more extreme due to climate change.

"Our study recommends that weather centers like the Met Office should pay greater attention to tropical storms and develop ways now to communicate risks to the public," Baker said, referring to the United Kingdom's national weather service. "Not all storms are equal, and we're prepared to deal with some storm types better than others."

The study, "Historical variability and lifecycles of North Atlantic midlatitude cyclones originating in the tropics," published April 12 in JGR: Atmospheres, was authored by Alexander J. Baker, Kevin I. Hodges, Reinhard K. H. Schiemann and Pier Luigi Vidale, University of Reading.