It can be kryptonite for hurricanes.
The Saharan Air Layer (SAL)—a mass of dry, hot, and dust-laden air that forms over the Sahara Desert during the late spring, summer, and early fall, and moves over the tropical North Atlantic Ocean—can suppress tropical cyclone formation.
"It certainly has a great impact on a number of processes, particularly the development of tropical cyclones and hurricanes," said Joseph Prospero, an emeritus professor of atmospheric sciences at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science and the discoverer of the SAL.
With a plume of African dust now having reached South Florida and with another Atlantic hurricane season underway, interest in how the Saharan Air Layer affects tropical cyclone development is once again ramping up.
"These [dust] outbreaks are often associated with easterly waves, and easterly waves are the nucleating phenomenon that can initiate the process of tropical storms. So that has certainly proven to be a very significant feature of the Saharan air outbreaks compared to dust storms from other areas of the world," explained Prospero, who established a specialized laboratory on the east coast of Barbados that documents the transport of African dust to the Caribbean Basin, allowing scientists to study its impact on climate, clouds, and air quality.
"It's a trifecta of atmospheric processes," said Jason Dunion, a meteorologist with the University of Miami Cooperative Institute for Marine and Atmospheric Studies and the Hurricane Research Division of the National Oceanic and Atmospheric Administration.
First, the Saharan Air Layer's dry, dusty air has about 50 percent less moisture than the typical tropical atmosphere, according to Dunion. "It's super dry air," he said, noting that the outbreaks occupy a two- to two-and-a-half-mile thick layer of the atmosphere, with the base starting approximately a mile above the surface. "Any clouds that run into that kind of dry air tend to collapse. So, the outbreaks make it very difficult for thunderstorms that could eventually become hurricanes to develop."
The second factor in that trifecta is the presence of powerful winds in the Saharan Air Layer that can significantly increase vertical wind shear in and around storms. "Thunderstorms are like skateboarders. They don't want to be tipped over. They want to remain vertically aligned," Dunion said. "So, when you get that mid-level jet of strong winds, which is typically found in the Saharan Air Layer anywhere from 6,500 to 14,500 feet, it starts to misalign the thunderstorm, causing the tropical cyclone vortex to tilt."
Warm temperatures complete the trifecta. "The Saharan Air Layer is very warm, and all the dust in the layer absorbs solar energy," Dunion said. "It's that process that keeps the layer warm, and anytime you have warm air hiding in the upper atmosphere, it's almost like a lid that makes it hard for clouds to break through."
Dunion has studied this trifecta extensively, flying on and leading some of the first Hurricane Hunter flights that collected data on how tropical cyclones interact with Saharan dust. "Those missions have allowed us to get a much better sense of how these outbreaks affect the entire Atlantic," he said.
During the peak period, SAL outbreaks emerge from the coast of Africa every three to five days, reaching as far west as Florida, Central America, and even Texas, and covering vast areas of the Atlantic. "Sometimes, larger than the Lower 48 states," Dunion said.
But before storm-weary residents in Florida and other coastal cities in the U.S. get a false sense of security, the Saharan Air Layer's impact on hurricane activity comes with a caveat. While the dust outbreaks ramp-up in mid-June and peak from late June to mid-August, they begin to rapidly wane after mid-August.
"Once we get into August, those dust outbreaks start to become smaller, and they don't reach as far to the west," Dunion said. "And that is probably one of the ingredients that then allows hurricanes to develop. It's that switch point in the season that we must keep an eye on."
