Dust Plumes – July 1, 2020
In the last week, the skies have been noticeably hazier across the Caribbean and the Gulf and East Coasts of the US. It’s not due to wildfires or increasing pollution from traffic. In fact, it is due to dust coming from 5,000 miles away! Every year, a mass of dusty air forms over the Sahara Desert in Africa and then moves westward across the tropical Atlantic. What’s different this year is the dust layer’s thickness, its low altitude and its geographic reach. The thickness of dust particles in the atmosphere is the highest observed in 25 years of satellite measurements!
How does this dust plume occur? Unlike in vegetated or forested areas with well-anchored soils, the Sahara is a desert loaded with loose sand particles and tiny dirt specks which are easier to loft into the air. The Sahara is hot and dry, while further south, near the Gulf of Guinea, it’s cooler and wetter. The hot region to the north and the cool, moist region to the south sets up a wind circulation that can become very strong and scour the surface of the desert, lofting up particulates. What happens next depends on local weather conditions: when strong, high winds are present, they can carry the floating dust straight to the coast. There, the dust cloud encounters a system of westward-moving trade winds. The hot, dry air gets lifted up over the trade winds, developing a layer which, in the late 1960s (when it was discovered), was named the ‘Saharan air layer.’
The pros of this situation? The hot, dry Saharan air layer can soak up moist, hurricane-friendly air like a sponge, creating sinking air and changing winds that tear apart baby hurricanes before they get big. Although the dust layer itself is full of warm air, it can also reflect and block sunlight from getting through to the Earth’s surface which can cause sea surface temperatures to temporarily cool, making conditions less favorable for storm formation. Dust contains a variety of minerals and nutrients, such as iron and phosphorus. As dust sprinkles out of the air, these nutrients may help fuel local ecosystems. Saharan dust storms are thought to be one important source of phosphorus for plants in the Amazon rainforest! Other experts have suggested that dust plumes may help fertilize the ocean with iron, feeding micro-organisms in the water.
The cons of this situation? Some studies have linked dust clouds to toxic bacterial and algal blooms in coastal areas. As the dust cloud moves over landmasses in the Caribbean and the Americas, it can also significantly alter local air quality. This past week’s massive event, for instance, has sparked concerns that the extra pollution could be a threat to people recovering from COVID-19. As the weather cools in Africa starting in mid-August, the temperature differential between the desert and the forests to the south will weaken, stopping the Saharan Air Layer “conveyor belt” of dust clouds rolling across the Atlantic.
It’s unclear whether this particular event is a “meteorological anomaly” or whether it could be a glimpse into the effects of continued warming. If large dust storms became more common or more intense, for instance, it’s possible they could have a bigger effect on Atlantic hurricanes, or they might have a bigger influence on ocean ecosystems. Only time, and better models in the future, will tell.
photo: U. Horodyskyj