Wind-driven land surface features in the Sahara

By Martin Setvak (CHMI)

Most of us consider images from weather satellite instruments like SEVIRI, AVHRR or MODIS as tools which show instant weather — clouds, air-masses, derived wind vectors and other products, and in the case of geostationary satellites their evolution over time.

However, some of these images may show us quite different features, documenting long term weather patterns. Of course, not every place and surface is suitable to log and preserve such weather records, however there are places around the world, which seem to be just designed for this purpose. One of these places is the Sahara Desert.

Even when looking at the early low-quality satellite images showing the central-eastern Sahara, it was impossible not to notice a strange dark spot north of the Tibesti Mountains, in the southeast part of Libya, with its dark plume oriented to the southwest from the spot.

After one's initial uncertainty about the origin of this strange feature (resembling the smoke plume of a large oil fire) and after inspecting more images of the area captured at different times, it becomes obvious that the dark spot and its plume are not a transient phenomenon, they are always there. Therefore, these must be something related to the surface, an object of geological or geographic origin. Indeed, the satellite dark spot is what local Berbers call Waw al-Namus, 'Oasis of Mosquitoes'.

Geologically, it is a low volcanic caldera about 4km in diameter, with several small salty lakes inside, surrounded by a 5–10km wide dark black deposit of ash that stands out starkly against the yellow desert. The attached plume is simply dark sand and dust carried from this source downwind, deposited in a direction of prevailing winds. From this perspective, Waw al-Namus and its plume act as a long-term record of wind direction in this region, which can be seen from space.

However, Waw al-Namus and its plume are not the only wind records in the area. As you utilise higher resolution images — of the order of 1km or better, other, somewhat different long-term wind tracers are revealed at almost every place across the Sahara. These generally take the form of small-scale, long, parallel thin lines, as if being drawn across the desert by a giant pencil. One of the most spectacular places with long-term wind streamlines imprinted into the landscape can be found further southeast of Waw al-Namus, to the southeast of the Tibesti Mountains.

As you look at very detailed images like MSG HRV, or even better from the MODIS instrument, you can hardly fail to miss them. These features become even more interesting as you follow their large scale variability. Looking at the region between the Tibesti and Ennedi Mountains, northeast of the place where they are closest together (Borkou area), you can notice the obvious convergence of these streamlines as they enter the Borkou region from the northeast.

The low-level winds approaching the region from the northeast are squeezed together between the Tibesti (3,415m) and Ennedi (1,450m) Mountains, and have to pass through the bottleneck (Borkou area) between them. This speeds up the wind flow significantly, creating an effect resembling a jet-engine. As this local jet blows further south, exiting the Borkou bottleneck, by coincidence it passes just above the Bodélé depression — a dried salty lake, which serves as a very efficient source of small particles for local dust storms (see Dust storm on 5 January 2005).

How much these local Bodélé dust storms contribute to the large scale dust storms of the Sahara, and, therefore, how many of the Sahara's dust storms actually originate here, is a different story

Additional content

Met-8 RGB composite image (7 January 2005, 13:00 UTC)
Waw al-Namus on Google Earth (source: Google Earth)
Google Earth image with manually drawn wind vectors (source: M. Setvak)
Waw al-Namus from Space Shuttle (source: NASA)