MIAMI - The strength of a large air mass hanging off the west coast of Africa may mean that fewer tropical cyclones may develop in the eastern Atlantic.
The Saharan Air Layer is found in atmosphere above the desert latitudes (0 degrees to 30 degrees north) of the Northern Hemisphere, typically off of the western coast of Africa. This air is laden with particles like clay and dust that is prominently found in the Sahara desert. Since the winds blowing through this region, called the Trade Winds, lift the dust from the desert into the atmosphere, it acts as a buffer of dry air in this region. This dry air, called SAL (or Saharan Air Layer), will impede any tropical cyclone development because it hinders any vertical motion. The SAL layer can be as thick as 20,000 feet. This occurs because tropical cyclones need very warm and moist conditions to develop, along with strong convection and high amounts of vorticity, or "suction" similar to what is seen in the drains of our bathrooms. If the SAL is present and very intense, a tropical cyclone will not develop in this region. The dust will be carried westward from Africa to the eastern United States from the motion of the Trade Winds.
Currently, as shown by the METEOSAT-9 European meteorological monitoring satellite, the SAL is extremely strong over the Atlantic. It extends about approximately 3,000 miles from the western coast to almost Puerto Rico. The strongest part of the layer can be seen off of the western coast of Mauritania and Western Sahara in Africa. (Here's an image: http://tropic.ssec.wisc.edu/real-time/sal/splitE.jpg) The red in the image indicates the driest part of the atmosphere, meaning the least amount of vertical development is going to occur. The length of the layer also impacts the area in which tropical cyclone development may occur.
Why is this important?
Well, Florida is the hurricane capital of the United States. The stronger the SAL, the less likely we will see tropical cyclone development in the eastern Atlantic. Some of the strongest hurricanes in Florida's history originated as tropical waves off of the western coast of Africa. (Anybody remember Hurricane Andrew in 1992?)
While there are many other factors that attribute to the lack of development, such as surface high pressure and upper level wind shear, the SAL will further impede this development. (See another image: http://www.nhc.noaa.gov/gtwo/two_atl.gif)
In the current model run from the National Hurricane Center (NHC), it can be seen that the region is very quiet. Aside from the storms that are moving eastward in the Central Atlantic, there are no storms in the tropical Atlantic Ocean that threaten the United States, mostly because of the strong influence of the SAL.
However, there is a small region detected off of the eastern coast of Central America in the Gulf of Mexico that will be monitored for further development. Knowing about the presence of the SAL will help better forecast tropical cyclone development in the Atlantic Ocean. So, until next time, "Keep Calm and Watch the Tropics."
- Nature & Environment
- Natural Phenomena
- Saharan Air Layer