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Warming oceans fuel earlier Atlantic hurricane seasons, study finds

The Washington Post logo The Washington Post 8/16/2022 Matthew Cappucci
Tropical Storm Bertha seen off the coast of South Carolina on May 27, 2020, with the bright red colors indicative of the highest, coldest cloud tops. White dots are lightning strikes. (Weathernerds.org) © Weathernerds.org Tropical Storm Bertha seen off the coast of South Carolina on May 27, 2020, with the bright red colors indicative of the highest, coldest cloud tops. White dots are lightning strikes. (Weathernerds.org)

For years, meteorologists have advertised June 1 as the historical start to Atlantic hurricane season — but a new paper finds tropical storms are more frequently forming before that date as the ocean warms.

The study published in Nature Communications found there’s both an earlier start date and earlier first U.S. landfall each year. It comes as the National Oceanic and Atmospheric Administration is considering moving the official start date of the season from June 1 to May 15, which would reflect the observation of early storms and would also match the start of the eastern Pacific hurricane season.

It’s been a slow start to hurricane season — but it’s still early

The start date is one of several attributes of hurricanes that appear to be changing in the face of warming oceans and human-caused climate change. Strong hurricanes are also wandering farther north, and there’s been an uptick in instances of rapid intensification — leading to stronger, wetter and more destructive storms.

“This was a timely topic,” said the paper’s lead author, Ryan Truchelut, a meteorologist who founded the consulting firm WeatherTiger. “There had not been a peer-reviewed paper on the onset of hurricane season for about eight years now.”

Since 2012, Truchelut said there have been at least seven instances of the National Hurricane Center issuing tropical storm watches and warnings before the official start date of the season. Atlantic hurricane season has historically peaked around mid-September on average, and activity ordinarily simmers down by the end of November. An average season contains about 14 named storms, with seven hurricanes and three major hurricanes.

Especially since the 2010s, Truchelut said the “the preponderance of unusual and preseason activity that the Atlantic saw … stands out to me.”

What the study found

  • Since 1979, the first named Atlantic storm has been occurring about five days earlier on average per decade.
  • The average first U.S. landfall of a named storm has been trending earlier by about two days per decade since 1900. That equates to roughly three weeks since the turn of the 20th century.

What scientists looked at

(Brian McNoldy/University of Miami) (Brian McNoldy/University of Miami)

The study focused on observational data — not forward simulations or model outputs. Using statistical methods, the group sifted through noise in the data to draw conclusions about when storms were forming and striking land.

It’s important to note that the satellite era — when a complete view of every nook and cranny of the Atlantic has been available — only dawned in 1970. Before that, knowledge about the position and intensity of tropical cyclones largely came from ship reports and buoy observations. In earlier years in particular, that often made for possible limitations, gaps or inconsistencies in the historical record.

It’s improbable that any landfalling U.S. storms were missed, however, given the dense population of the U.S. and Gulf coastlines. That makes the “two day per decade” figure pertaining to the shifting first landfall date more reliable.

What it means

The authors point to warming sea surface temperatures, which can be closely linked to human activity, as playing a role in the expanded season. In particular, they found ocean temperatures during the spring months contribute to jump-starting a season, ruling out other factors that affect storm formation.

“The developing environment is more favorable for [storm] genesis mainly because of sea surface temperature,” Truchelut said. “We’re not seeing big changes in shear, relative humidity or upper-level temperatures.”

Truchelut said the ocean warming may be tied to both human-caused warming and natural variability.

“We do not attempt to attribute what portion of the observed 0.75 degree Celsius warming we’ve seen in May since the late 70s could be climate change versus what’s driven by natural cycles or longer-term oscillations,” he noted. “But certainly there’s some in the mix, both with climate change and natural cycles.”

Study finds Atlantic hurricanes becoming more frequent, destructive

This trend is likely to continue, he said, something that’s reflected in modeling by the Intergovernmental Panel on Climate Change.

While 2022′s first named storm in the Atlantic (Alex) didn’t form until June 5, after the season’s official start date, the previous seven years featured early-season storms:

  • Ana (2021) — a subtropical storm that formed on May 22 and briefly swirled northeast of Bermuda.
  • Arthur (2020) — a 60 mph tropical storm that formed north of the northwest Bahamas on May 16 and traced a loop in the western Atlantic while avoiding land.
  • Bertha (2020) — a 50 mph tropical storm that formed northeast of Florida on May 27 before drifting into South Carolina and bringing flash flooding and a tornado.
  • Andrea (2019) — a subtropical storm that formed several hundred miles east of Florida on May 20. A broad, diffuse strip of low pressure congealed into an upper-level low, beneath which thunderstorms evolved into a subtropical storm.
  • Alberto (2018) — Alberto was a 65 mph tropical storm that formed east of the Yucatán Peninsula on May 25. It moved north, dropping 14.41 inches of rain in Heriberto Duquezne, Cuba, before making landfall near Laguna Beach, Fla., on the evening of May 28. A gust of 59 mph was recorded at the St. George Island Bridge.
  • Arlene (2017) — a nontropical storm was swirling through the open ocean east of Bermuda on April 16. By the 19th, its fronts dissipated as it pinched off from the surrounding environment, with enough thunderstorms forming around its center to classify it as a subtropical storm. It yielded estimated winds of 50 mph but never tracked close to land.
  • Bonnie (2016) — Bonnie developed north of Great Abaco, the Bahamas, on May 28, but gradually weakened into a tropical depression before making landfall near Isle of Palms, S.C., Rainfall of 6 to 10 inches occurred, and rip currents resulted in the drowning of a 20-year-old male on the beaches of Brevard County, Fla.
  • Ana (2015) — the earliest landfalling U.S.-named storm on record, Ana formed on May 8 east of Florida before making landfall in South Carolina two days later. Roads were washed out near Myrtle Beach, and Kinson, N.C., saw 6.7 inches of rain.

The study concluded that warming oceans in the future could continue to advance the start date of Atlantic storms by about a half-day to one day per year.

James Kossin, a hurricane researcher at the Climate Service, who was not involved in the study, wrote in an email that this study builds on previous work connecting Atlantic tropical-storm season length to ocean temperatures. But he also wrote that there is not strong evidence to support additional lengthening in the future.

“[T]here is no clear reason to expect the trend to continue,” he wrote. In a blog post at RealClimate last year, he and co-authors argued that even as the oceans warms, the atmosphere may respond in ways to counter early season storm formation in the decades ahead.

Jason Samenow contributed to this report.

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