Tropical storms and hurricanes require sea surface temperatures of at least 79 degrees to form and sustain themselves, and have expanded farther north than usual this summer.
Edouard, for example, formed amid sea surface temperatures near 80 degrees, about four to five degrees above average.
Dolly, which came before Edouard in late June, formed over the same plume of toasty water, just somewhat to the west. It flared up farther north than any tropical storm on record, said Sam Lillo, a postdoctoral researcher at the National Oceanic and Atmospheric Administration.
“Both Dolly and Edouard were named on the outer edge of the envelope for all storms forming before August,” Lillo tweeted.
As a measure of just how warm the water was in the northwest Atlantic when Dolly formed, a buoy at Georges Bank, 170 nautical miles east of Hyannis, Mass., displayed a sea surface temperature over 80 degrees on June 26. Data shows that water that warm is unprecedented during the month at that location dating back to at least 1984.
Waters are unusually warm not only in the northern reaches of the Atlantic but in many areas, including off the coast of the Mid-Atlantic and the southeastern United States.
At a dock on Virginia Key, a little island about two miles east of downtown Miami, the sea surface temperature shot up to 92.5 degrees last week, the warmest on record, said Brian McNoldy, a tropical-weather researcher at the University of Miami.
Marshall Shepherd, a professor of atmospheric sciences at the University of Georgia, called these anomalous waters “high octane” fuel for hurricanes in a column at Forbes.com.
Another weather disturbance being monitored by the National Hurricane Center over the Southeast will encounter abnormally warm waters when it moves off the coast of the Carolinas.
This disturbance has a chance to become Tropical Storm Fay later this week. On average, the sixth named storm doesn’t occur until Sept. 4 — almost two months from now, McNoldy said.
Well before Edouard formed, the hurricane season got off to an unusually early start when Tropical Storm Arthur was named on May 16, two weeks before the official June 1 start date. Arthur fit into a recent pattern of preseason storms, becoming the sixth to develop prior to June 1 in the past six years and the eighth in the past decade.
Scientists have linked the uptick in these storms, at least in part, to warming ocean waters from human-caused climate change.
The preseason storm activity “seems to be, certainly, very attached to sea surface temperature,” said Jim Kossin, an atmospheric scientist with NOAA. “There’s no question about that.”
Kossin attributed the warm ocean waters to both increasing greenhouse gases from human activities and naturally fluctuating weather patterns. He also said a reduction in sulfate aerosols, pollutant particles that block sunlight, are contributing to warming.
His research has found tropical storms and hurricanes lingering later in the season as well.
“If we have anomalously warm waters, probably caused by natural variability with other factors sitting on top of it that are more human-caused, you’re going to have a long season,” Kossin said.
In other words, the times of the year and locations that previously couldn’t support tropical storms now can, with a nudge from climate warming, McNoldy explained. “Places that used to be a little too cold [for storms] are now more favorable,” he said.
This year’s toasty waters are a significant reason that most seasonal hurricane forecasts are calling for above-normal to much-above-normal activity.
Colorado State University’s updated seasonal forecast issued Tuesday, led by researcher Phil Klotzbach, is calling for 20 named storms, nine hurricanes and four major hurricanes (these numbers include the five named storms that have already formed), compared with the averages of 12, six and three.
“The tropical Atlantic is somewhat warmer than normal, while the subtropical Atlantic is quite warm,” the forecast says. “Most of the eastern Atlantic is warmer than normal, and anomalously warm temperatures in this region in June have been typically associated with more active Atlantic hurricane seasons.”
In an email, Klotzbach was careful to point out that the flurry of storms we’ve seen so far isn’t by itself a guarantee of a busy season. Some years have started off fast but ended up with “pretty quiet seasons overall.”
He also noted that the storms that have formed this year have been relatively weak. The record 2005 hurricane season, which also saw a fast start, included Dennis and Emily, category 4 and 5 hurricanes, among its first five storms.
While the storms in 2020 so far haven’t matched the intensity of 2005′s tempests, the quantity of heat available to fuel future storms is “unsettling,” meteorologist Jeff Masters says.
“More heat energy is available in 2020 than was the case in 2005 in many regions, so there is a high likelihood of seeing very intense hurricanes in 2020,” Masters wrote in his blog at Yale Climate Connections.
Other factors support very busy season
In addition to the warm ocean waters, a number of weather patterns taking shape support forecasts for a busy season.
Already, plentiful rainfall has been observed across the tropics of Africa leading into July, with some locales approaching record territory. A correlation has been found between summer rainfall over the western Sahel and hurricane incidence in the western Atlantic and particularly the East Coast after Aug. 1.
Moreover, the Madden-Julian oscillation, a large-scale overturning circulation that accounts for the greatest variability in the tropical atmosphere, is forecast to evolve into a favorable configuration for tropical storm development. One side of the circulation favors broad rising motion that can be conducive to thunderstorms and tropical storms, while the other side, characterized by sinking air, squashes their prospects.
Initial data suggests that the monsoon contributing to rainfall over Africa is being enhanced by the upward branch of the Madden-Julian oscillation. While sinking air could reside over the Atlantic for much of the second half of July, models indicate renewed lift — favorable for hurricanes — arriving by early August.
Also tipping the scales toward an active season: Water temperatures in the eastern equatorial Pacific are cooling, hinting that a La Niña pattern is in the works. While the pattern may not become fully established this season, a fledgling La Niña helps weaken upper-level westerly winds in the tropical Atlantic. Weaker winds aloft make it easier for clusters of thunderstorms to tower, brewing greater chances of tropical storms and hurricanes.