Saharan Dust Plume Streams Over the Atlantic
While the Pacific has kept seeing its share of intense tropical systems, the Atlantic has been quieter. This is consistent with what we expect with a strong El Niño, as the global scale impacts from that phenomenon tend to include increased wind shear in the Caribbean and western Atlantic which prevents storms in the tropics from intensifying and consolidating into cyclones. But we’re also seeing another major inhibitor of tropical cyclone development in large quantities of Saharan dust streaming off of north Africa into the skies over the eastern Atlantic. The air associated with these dust plumes is very dry, having originated over one of the largest and driest deserts on Earth. When this air is advected over the Atlantic, it mixes with the existing air and disrupts the development of any deep convection while also enhancing local wind shear. The radiative impact of the dust also stabilizes the atmosphere by absorbing solar energy above the surface while preventing warming at the surface, thus increasing the strength of the trade wind inversion and further limiting the vertical growth that is critical for hurricane formation and maintenance.
Since we’re interested in an African phenomenon, the geostationary satellites operated by EUMETSAT are going to be well-positioned to investigate this. The Flexible Combined Imager (FCI) on the geostationary Meteosat 12 plaform captured the following true color animation of the brown dust stretching hundreds of miles off of the shore of Africa and into the Atlantic air. FCI, like the Himawari Advanced Himawari Imager (AHI) has true red, green, and blue channels and is thus able to depict and event like this with chromatic fidelity. This animation was made on the EUMETSAT’s Eumetview site.
Of course, dust has different radiative properties than clear or cloudy air does, and with the numerous channels on the FCI, we can exploit those differences to create a Dust RGB product that highlights the position of the dust. Here’s the same period and time window as the true color animation above, but this time see how the dust can be made visually distinct from its surroundings through the application of infrared channel differences. The full recipe is given in the Dust RGB Quick Guide from EUMETSAT. Here, the purple and magenta colors represent the locations of the dust. Note how this product is even able to distinguish between a dust storm on the Algeria/Mali border (the bright magenta on the right side of the animation) from the surrounding desert; this is almost impossible to see in the true color loop above. The Dust RGB uses only infrared channels (including differences between different infrared channels) so it can be used 24 hours a day without issue.
Dust in the air, of course, means an increase in the optical depth. As this dust streams westward, it emerges into the field of view of the GOES-19 (GOES East) Advanced Baseline Imager (ABI). Here’s an animation from CSPP Geosphere that shows the Level 2 Aerosol Optical Depth product from the CSPP Geo software suite. This view shows data on the curved surface of the earth, with northern South America in the bottom left and western Africa curving away from the viewer in the middle. The dust can be seen as the bright colors at the center of the animation. As the day goes on, clouds in the dust region grow and thicken. Since the aerosol optical depth product is a clear sky and daytime only product, this results in increasing areas of the loop showing no data.
Finally, we can use VIIRS to take a very high resolution view of the dust. Unlike geostationary satellites which are fixed to specific locations, NOAA’s polar orbiting satellites roam the entire planet. Here’s a true color view from NOAA-21. The 375 m spatial resolution of the raw visible-wavelength channels enables ultra-detailed views of the dust, as can be seen in this image of the Atlantic between the Cap-Vert peninsula that is home to Dakar, Senegal on the right and the islands of Cabo Verde on the left.
These dust plumes aren’t uncommon this time of year, and have even been known to make their way all the way to the continental United States where they can affect everything from air quality to the beauty of the sunsets.