Author(s): Nathaniel Scharping

Atlantic hurricanes are intensifying faster

Source(s): Eos - AGU
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Hurricane Florence seen from Space in September 2018
NASA Johnson / flickr (CC BY-NC-ND 2.0)

When Hurricane Otis screamed onto shore near Acapulco, Mexico, last month, it was by far the strongest storm on record to hit the region, according to the National Hurricane Center. 

Just a day before, the storm’s fury would have been almost unthinkable.

Otis strengthened faster than nearly any other hurricane observed by scientists. The storm’s winds increased by 50 meters per second (115 miles per hour) in just 24 hours, and it grew from a category 1 hurricane to a category 5 in just hours, stunning forecasters.

That kind of “explosive intensification” aligns with a recent trend toward more rapid hurricane intensification as climate change warms waters around the world.

Otis was a Pacific storm, but the Atlantic Ocean is not immune to the trend: Peak intensification rates for Atlantic hurricanes were as much as 29% higher in the past 20 years compared with the 1970s and 1980s, according to a new study published in Scientific Reports.

“Storms that intensify especially quickly can often be hard to forecast,” said Andra Garner, a climate scientist at Rowan University and sole author of the study. “That creates challenges when you add to that the fact that they’re also hard to plan for.

Climate of Intensification

“You go to bed thinking that it’s going to be a category 1 hurricane, and then you wake up and it’s a category 5 hurricane.”

All hurricanes start as smaller storms, becoming stronger and more organized as they feed on warm waters that drive powerful updrafts. When a hurricane’s winds increase by at least 15 meters per second (30 knots; 35 miles per hour) in a 24-hour period, it’s said to be rapidly intensifying.

“You go to bed thinking that it’s going to be a category 1 hurricane, and then you wake up and it’s a category 5 hurricane,” said Karthik Balaguru, a climate scientist at the Pacific Northwest National Laboratory who researches hurricane intensification but wasn’t affiliated with the new study.

Climate change is the obvious culprit for why more storms are falling into that category, though the full story is likely more nuanced.

“We know that as humans have warmed the planet, about 90% of that excess energy and warmth has gone into our oceans,” Garner said. “We also know that hurricanes require warm water to intensify, and especially to intensify quickly.”

To find out how those simple facts figure into complex, large-scale weather events, Garner analyzed data from the National Hurricane Center’s HURDAT2 database, which includes information on every Atlantic hurricane since 1971. She compared the peak intensification rates for hurricanes among three periods: the historical era, from 1971 to 1990; the intermediate era, from 1986 to 2005; and the modern era, from 2001 to 2020.

Across 12-, 24-, and 36-hour windows, hurricanes now see mean peak intensification rates that are at least 26% higher than 5 decades ago.

“Seeing that rate more than double across a 24-hour or 36-hour window was something that definitely caught my attention.”

The data also show worrying increases in the rates at which relatively small storms become major hurricanes, defined as a category 3 storm or stronger. There’s now an 8% chance that a given storm will move from category 1 to category 3 or above within 24 hours, compared with 3% in the past.

“Seeing that rate more than double across a 24-hour or 36-hour window was something that definitely caught my attention,” Garner said.

The trend is even more pronounced among the storms that intensified the most. Between 1971 and 1990, just 13 storms grew from tropical storms to major hurricanes in 36 hours. In the past 20 years, 49 storms made that jump—something that’s statistically impossible without a corresponding increase in intensification rates, according to Garner’s research.

Garner found increases in the odds of a hurricane intensifying rapidly along the U.S. Atlantic coast, as well as in the southern Caribbean and the tropical eastern Atlantic, but not in the Gulf of Mexico. It’s also becoming more likely for storms to intensify when they’re near the coast, as opposed to far out over the ocean, she found. That could make forecasting and preparing for strong storms even more difficult.

Stronger Storms Surge

The paper lines up with previous studies showing increases in hurricane intensification in the Atlantic and beyond, including near coasts. But where other studies focused on specific regions and often looked only at hurricanes that intensified rapidly, the new study is much broader, Balaguru said. Garner showed that regardless of location or hurricane strength, the same trend holds.

“You kind of arrive at the same conclusion, no matter which way you look at it,” he said.

The research is also pointing to future directions for new research, said Gregory Foltz, an oceanographer with NOAA. “The patterns of where the intensification rates are increasing in the Atlantic basin. . .are pretty interesting,” he said.

For example, it’s not quite clear why hurricanes are intensifying more rapidly near coasts and why some regions are seeing more intensification than others. Although it’s clear that warmer ocean waters create stronger hurricanes, the dueling effects of other factors such as humidity, wind shear, and atmospheric rotation also need to be better studied, Foltz said.

For now, what’s abundantly clear is that hurricanes seem poised to grow stronger faster in coming decades. That’s certainly cause for alarm, but, Garner stressed, there are seeds of hope embedded in that trend as well.

“Because we’ve been the cause of warming ocean waters, we can also be the solution and change our behavior and limit additional warming,” she said.

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