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Cooperative Institute for Meteorological Satellite Studies / University of Wisconsin-Madison
Tropical Cyclones
Saharan Air Layer (SAL)

Additional background information on the Saharan Air Layer (SAL) can be found at the NOAA/AOML/Hurricane Research Division's Saharan Air Layer Project and Tropical Cyclone FAQ page webpages.
 
The Saharan Air Layer's Dry Air: The Saharan Air Layer's dry air modifies very little as it traverses the North Atlantic. Figure 1 shows locations of several GPS dropwindsondes that were launched by NOAA's G-IV jet around the environment of Hurricane Danielle of 1998. Figure 2 shows composite vertical moisture profiles for SAL (dry) vs non-SAL (moist) dropwindsondes depicted in Figure 1 and clearly shows the extreme dryness of the SAL air to the north of Danielle.
 
The Saharan Air Layer's Low to Mid-Level Easterly Jet: The SAL's low to mid-level jet can dramatically increase the local vertical wind shear in the North Atlantic. This easterly jet typically moves at 20-45 kt and can generate vertical shear magnitudes as high as 40-60 kt. Figure 3 shows Hurricanes Isaac and Joyce of 2000 and the strong SAL induced vertical wind shear NE of Joyce.
 
Saharan Air Layer/Tropical Cyclone Interactions: Tropical cyclones (TCs) interact with the SAL in several ways. Some TCs are embedded in the SAL for their entire life cycle and often struggle to intensify beyond strong tropical storm strength (>=34 kt to <64 kt). Tropical Storm Chantal of 2001 is an example of this type of interaction (Java Loop 1). Other TCs like Joyce of 2000 can be overrun by the faster moving SAL and are quickly weakened (Java Loop 2). TCs that are embedded in the SAL early in their life cycle and later emerge from the SAL can often be seen undergoing a period of rapid intensification. Hurricane Erin of 2001 is an example of such an interaction (Java Loop 3).
 
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Reference:
Dunion, J.P., and C.S. Velden, 2004: The impact of the Saharan Air Layer on Atlantic tropical cyclone activity. Bull. Amer. Meteor. Soc., vol. 85, no. 3, 353-365.
 
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