Why Massive Saharan Dust Plumes Are Blowing Into the US

The pandemic is still raging , the Arctic is burning up , and microplastics are polluting every corner of the Earth , but do try to take a deep breath. Actually, belay that, especially if you live in the southern United States. A plume of dust thousands of miles long has blown from the Sahara across the Atlantic, suffocating Puerto Rico in a haze before continuing across the Gulf of Mexico. Yesterday, it arrived in Texas and Louisiana.It’s normal for Saharan dust to blow into the Americas—in fact, the phosphorus it carries is a reliable fertilizer of the Amazon rainforest. The dust makes the journey year after year, starting around mid-June and tapering off around mid-August. The good news is, the dust plumes can deflate newly forming hurricanes they might encounter on the way over. But the bad news is that dust is a respiratory irritant, and we could use fewer of those during the Covid-19 pandemic. Also, the current plume is particularly dense, and it’s not alone: The African desert is now releasing another that’s working its way across the Atlantic and will arrive in a few days. Still more could be on the way as the summer goes on.
A satellite captures a dust plume leaving Africa on June 19. Video: CSU/CIRA/NOAA
En route to the continental US, the plume struck Puerto Rico on Saturday, cutting visibility down to 3 miles. It’s the worst Saharan dust event the island has seen in 15, maybe 20 years, says Olga Mayol-Bracero, an atmospheric chemist at the University of Puerto Rico. Her air-analyzing instruments were working in real time, detecting the component elements of the desert dust. “We were quite surprised, seeing such high values for all these different parameters—we had never seen that,” Mayol-Bracero says. “So it was quite shocking.”
How does Saharan dust make it all the way across an ocean? It’s a lesson in atmospheric science.Because it’s a desert, the Sahara is loaded with particulate matter, from coarse sand down to the tiniest of dirt specks, none of which is very well anchored to the ground. By contrast, the lush rainforests to the south of the Sahara have trees that both block the wind and hold on to the soil with their roots, keeping all the muck from taking to the air. The conflict between these two atmospheric regions is what births the plumes that blow clear across the Atlantic.
The dust plume arrived in the Caribbean a few days after it left Africa. Video: CSU/CIRA/NOAA
The Sahara is notoriously dry and hot. But down south, around the Gulf of Guinea, it’s much cooler and wetter, on account of its proximity to the equator. “The setup between those two—the hot to the north and the cool, moist to the south—sets up a wind circulation that can become very strong, and it can actually scour the surface of the desert,” says Steven Miller, deputy director of the Cooperative Institute for Research in the Atmosphere at Colorado State University, which is monitoring the plumes. (You can watch the dust’s progress from a satellite with this neat tool. Look for the gray clouds on the map.)