Saharan dust: It’s headed our way. Why that’s not a bad thing for storms
A belch of Saharan dust drifting through the Atlantic is the first significant plume of desert-infused air this year, marking a seasonal shift that can dim hurricane activity on the main tropical thoroughfare into August.
The billow of brown clearly seen on GOES-East satellite images could reach the Bahamas and South Florida next week, but whether it will have any effect on local weather is still unknown. The thickest clouds of dust can change the sky from blue to a gritty tan and sack thunderstorm activity with an oasis of warm, dry air aloft.
Or the traveling cloud could miss the Peninsula all together, riding the underbelly of the Bermuda High straight into the Gulf of Mexico and Texas.
“We’ve had a few outbreaks, but this one is significant based on size and how far west it is reaching,” said Jason Dunion, a University of Miami meteorologist and research scientist with NOAA’s Hurricane Research Division. “This one lifted a lot of dust off the Sahara.”
Saharan dust, also called the Saharan air layer (SAL), is a fleeting visitor to the tropics that makes an appearance during the months of June and July before fading in early August.
It is a product of the parched Saharan desert and African easterly jet — a river of wind similar to North America’s jet stream but that flows east to west because of the temperature differences between the hot Sahara and cooler Gulf of Guinea to its south.
The jet also is the machine that creates the tropical waves that tumble off West Africa about once every three days between June and November, said Todd Kimberlain, a senior meteorologist with the National Weather Service in Miami.
“When a wave moves off the coast you get a large plume of dust that precedes it,” Kimberlain said. “Early in the season it tends to be a detriment to tropical waves developing.”
Waters also are cooler early in the season in the main hurricane development region that stretches from the Cabo Verde Islands into the Caribbean. The Saharan dust wanes in early August as rains pick up in the Gulf of Guinea and “scavenge” the dust particles out of the air, Kimberlain said.
But the jet keeps chugging.
“We have about 60 tropical waves that come off between June 1 and November 30, but only about 10 percent develop,” Kimberlain said.
A Saharan dust layer is made up of sand and mineral particles that are swept up from 3.5 million square miles of desert and carried by air currents 4,000 miles west across the Atlantic.
The top of the dust layer is 16,500 feet above the Earth's surface, according to NASA.
Dunion said the Bermuda High acts as a conveyor belt shuttling the Saharan dust west.
“As you get into summer, the high starts to shift a little farther north so it makes a more direct route from Africa to South Florida,” he said. “We still haven’t perfected forecasting SAL activity. There are a lot puzzle pieces to look at.”
NOAA and NASA were planning a mission this summer to take a closer look at Saharan dust and tropical waves, Dunion said. The coronavirus pandemic forced researchers to pause the project until next year. The last major research was done in 2006, prior to advances in instrument and satellite technology.
There is still some mystery in the dust and why it tempers hurricane development.
Chris Davis, a senior scientist and associate director at the National Center for Atmospheric Research, said dry air and wind shear that come with the dust may be a bigger player than the dust itself.
“In my view, the effect of dust is secondary to these other effects in terms of limiting tropical cyclone formation,” Davis said.
This hurricane season is expected to be a busy one with the possibility of a La Niña forming late summer into early fall. Already, three named storms have formed — Arthur, Bertha and Cristobal — with Bertha and Cristobal both making U.S. landfalls.
The Saharan dust usually is greatly diminished by the peak of hurricane season, which runs from about mid-August through October.
“I’m not crazy about us already being through the ”c“-named storm,” Dunion said.
The next name on the 2020 tropical cyclone list is Dolly.
What is “Saharan dust”
∎ It’s also called the Saharan air layer (SAL)
∎ It is made up of sand and mineral particles swept up from 3.5 million square miles of desert. The top of the dust layer is 16,500 feet above the Earth's surface.
∎ It makes an appearance in the tropics during June and July before fading in early August.
∎ It is a product of the parched Saharan desert and African easterly jet (a river of wind similar to North America’s jet stream but that flows east to west)