The Moderate Resolution Imaging Spectroradiometer (MODIS), aboard NASA’s Aqua satellite, captured this true-color image of Saharan Desert dust blowing southwestward off the coasts of Morocco and Mauritania in West Africa. The light brown plume can be seen wending its way over the Cape Verde Islands in the Eastern Atlantic Ocean.
The Saharan Air Layer (SAL) is an extremely hot, dry and sometimes dust-laden layer of the atmosphere that often overlies the cooler, more-humid surface air of the Atlantic Ocean. It carries upwards of 60 million tonnes of dust annually over the ocean and the Americas. This annual phenomena sometimes cools the ocean and suppresses Atlantic hurricane creation.
The SAL is a subject of ongoing study and research. Its existence was first postulated in 1972.
The dust cloud originates in Saharan Africa and extends from the surface upwards several kilometers. As the dust drives, or is driven out over the Atlantic ocean, it is lifted above the denser marine air. This atmospheric arrangement is an inversion where the temperature actually increases with height, as the boundary between the SAL and the marine layer suppresses or “caps” any convection originating in the marine layer. Since it is dry air, the lapse rate within the SAL itself is steep, that is, the temperature falls rapidly with height.
Disturbances such as large thunderstorm complexes over North Africa periodically result in vast dust and sand storms, some of which extend as high as 6,000 metres (20,000 ft). The layer is transported westwards cross the Atlantic by a series of broad anticyclonic eddies that are typically found 1,500–4,500 metres (4,900–14,800 ft) above sea level. An estimated 60-200 million tons of dust particles are carried from the Sahara Desert region of North Africa, where it originates, and moves westward annually
Credit: Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC