The Saharan Air Layer was our summer friend, helping disrupt the conditions that develop Atlantic storms.
But now our pal SAL is leaving at the worst possible time: the peak of hurricane season.
This summer, in the midst of what could turn into a historic storm season, a historic Saharan phenomenon created a 24-day lull of tropical tranquility in the Atlantic Ocean. Only one storm formed from June 10 to July 4 but never approached land, disappearing in the North Atlantic.
Scientists attribute that lull to a massive plume of Saharan dust that floated from Africa to the U.S., sucking up moisture along the way and robbing potential storms of the “fuel” they need to form in the Atlantic.
The Saharan Air Layer’s June outbreak was the “dustiest” on record, said Weather Channel meteorologist Carl Parker in an email to the Tampa Bay Times. It covered an area larger than the 48 contiguous states and traveled 5,000 miles from Africa to the southwestern states.
That helped temporarily quell the tropics, said National Oceanic and Atmospheric Administration hurricane scientist Jason Dunion. But that calming effect is waning.
“The Saharan Air Layer ramps up in early summer and peaks from late June to mid-July, meaning outbreaks are larger and they’re reaching farther to the west and the United States,” Dunion said. “It starts to really ramp down when you get to about mid-August.”
The size and reach of plumes become smaller, he said, while the conditions that fuel Atlantic storms grow stronger.
In short, the dust is running out at the worst possible time.
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There are three ways a Saharan Air Layer can break up a storm.
First, it can decrease the amount of tropical moisture by half.
The second factor is a “jet” within the layer of winds that are anywhere from 20 to 60 mph that can break up a storm from within.
And the dust often soaks up energy from the sun, which stabilizes the atmosphere and prevents clouds from forming. That can “hobble the heat engine of developing tropical cyclones,” Parker said.
“It’s a trifecta that works against storm development in the Atlantic,” Dunion said.
The phenomenon could still affect storms forming in August and September, he said. But its ability to do so will decrease as the dust plumes shrink, limiting its reach across the Atlantic.
The receding dust is a factor in the latest hurricane season forecasts, which say this could become one of the most active storm seasons on record.
Last week, the National Oceanic and Atmospheric Administration’s Climate Prediction Center updated its forecast, saying warmer Atlantic waters and weaker wind shear have created the conditions for an “extremely active” season.
Colorado State University also updated its forecast, which now predicts 10 hurricanes could form in the next four months.
Philip Klotzbach, a research scientist at Colorado State, said Saharan dust has always influenced the formation of tropical storms and hurricanes.
“That’s very typical in June and July,” he said. “That’s why normally we don’t see a lot of storms in June and July, especially in the deep tropics.”
But we have seen already seen a lot of storm activity this year: A record nine named storms formed, including Hurricanes Hanna and Isaias. In a typical year, the ninth storm doesn’t form until Oct. 4.
This year’s historic dust plume had a calming effect during a very active period. How many more storms would have formed without that summer lull?
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There is also a geographic component to the storm-sapping power of Saharan dust. Most of the powerful storms form off the coast of Africa near the Cape Verde Islands. The Saharan Air Layer often leaves land just northeast of there.
“Saharan dust comes from just north of the Atlantic’s nursery for hurricanes,” said Dunion. That’s where more than half of named storms usually form, he said, including 80 percent of major hurricanes.
Dust plumes typically leave the African coast every three to five days during the early summer, Dunion said. The dust chokes up moisture when it enters a region, weakening tropical systems that have already formed. Once the dust is in the air, it can stop cyclones from forming altogether.
Not all systems form off the coast of Africa, and sometimes the dust doesn’t reach them until they’ve already become tropical storms or hurricanes. But it can still stunt those storms.
That is what scientists believe happened to Tropical Storm Gonzalo in July, which the National Hurricane Center had initially forecast to become the first hurricane of 2020.
A Saharan dust plume left Africa’s west coast soon after Gonzalo started gaining strength. The dust is believed to have weakened the system, which dissipated before it reached most of the Caribbean islands.
Something similar may have affected Isaias, which vacillated between a Category 1 hurricane and tropical storm as it wobbled toward Florida as July turned to August.
Dunion said he believes a mix of Saharan dust and wind shear battered the disorganized hurricane, preventing it from growing even stronger in the warm waters of the Florida Straits.
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The Saharan Dust Layer affects the ecosystem in many ways.
It replenishes phosphorous in the Amazon basin, replacing plant nutrients that are lost every year to flooding and rain. It can also fuel Red Tide toxic algae blooms. Florida’s west coast was hit hard by the 2018-19 Red Tide, which devastated sea life and tourism.
A historic dust plume also poses a significant problem during a historic pandemic.
The dust can exacerbate breathing issues for those with allergies, said Texas A&M professor Timothy Logan, who works in the atmospheric science department.
That is especially crucial in the time of COVID-19, because it makes it hard for those with allergies or respiratory issues to know whether it’s the dust or the coronavirus exacerbating their breathing — and one is far scarier than the other.
The way to tell the difference, he said, is how long it takes for those breathing issues to take hold. COVID-19 breathing issues take days or weeks to develop.
“If you do have allergies of some kind of health problems, the span of time you can get sick (from the dust) is immediate,” Logan said. “All of a sudden you can’t breathe, and there’s a dust layer — there’s a greater probability it’s because of the dust.”
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2020 Tampa Bay Times Hurricane Guide
HURRICANE SEASON IS HERE: Get ready and stay informed at tampabay.com/hurricane
PREPARE FOR COVID-19 AND THE STORM: The CDC's tips for this pandemic-hurricane season
PROTECT YOUR PETS: Your pets can’t get ready for a storm. That’s your job
NEED TO KNOW: Click here to find your evacuation zone and shelter