Saharan Air Layer

The Saharan Air Layer (SAL) is an intensely dry, warm and sometimes dust-laden layer of the atmosphere which often overlies the cooler, more-humid surface air of the Atlantic Ocean. In the Sahara Desert region of North Africa, where it originates, it is the prevalent atmosphere, extending from the surface upwards several kilometers. As it drives, or is driven, out over the ocean, it is lifted above the denser marine air. This arrangement is an inversion where the temperature increases with height. 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 meters. These can be driven out to sea within the SAL as far west as North America.

In the case of Africa, winds blow twenty percent of dust from a Saharan storm out over the Atlantic Ocean, and twenty percent of that, or four percent of a single storm's dust, reaches all the way to the western Atlantic. The remainder settles out into the ocean or washes out of the air with rainfall. NASA scientists think that the July 2000 measurements made in Puerto Rico equaled about one-fifth of the total year's dust deposits. If these estimates hold true over the long term, then the entire state of Florida receives about three feet of dust every million years.

This phenomenon can happen at any time of year but is usually associated with the hot air found over the islands during the summer months, ranging in time scale from a few hours up to a week. "Calima" as it is called, is caused by a duststorm that is stirred up by high winds in the Sahara and is then driven over the Canary Islands by south easterly winds. The fine sand particles cause the air to become thick and visibility becomes rather like that experienced during a thick fog, depending on the severity. During the calima, every surface will be covered in fine reddish brown dust.

In July 2000 alone, nearly 8 million tons of dust from Africa's Sahara desert reached as far west as Puerto Rico. "If you figure that a pickup truck weighs 1 metric ton, that dust weighed as much as 8 million pickups," says NASA aerosol researcher Dr. Peter Colarco from the Goddard Space Flight Center.

These clouds of dust are visible in satellite photos.

Findings to date indicate that the iron-rich dust particles which often occur within the SAL reflect solar radiation, thus cooling the atmosphere. The particles also reduce the amount of sunlight reaching the ocean, thus reducing the amount of heating of the ocean. They also tend to increase condensation as they drift into the marine layer below, but not precipitation as the drops formed are too small to fall and tend not to readily coelesce. These tiny drops are subsequently more easily evaporated as they move into drier air laterally or dry air mixes down from the SAL aloft. Research on aerosols also shows that the presence of small particles in air tends to suppress winds. The SAL has also been observed to suppress the development of tropical cyclones, which may be related directly to these factors. The SAL is a subject of ongoing study and research.

References

* [http://www.aoml.noaa.gov/hrd/tcfaq/A17.html NOAA FAQ: Saharan Air Layer]
* [http://cimss.ssec.wisc.edu/tropic/real-time/wavetrak/sal.html Real Time SAL data]
* [http://www.logicalscience.com/skeptic_arguments/SAL.htm HA! Look at 2006! Where are the Hurricanes?]
* [http://www.stanford.edu/dept/news/pr/2007/pr-slowwind-012407.html Research: Aerosols Slow Wind]