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Saharan Air Layer - Product Description |
The Saharan Air Layer (SAL) suppresses Atlantic Tropical cyclones activity. The SAL forms over
African deserts during the late spring, summer, and early fall and has a typical vertical extent of 500 to 850
hPa in the North Atlantic. It is often associated with a mid-level easterly jet and usually contains substantial
mineral dust lifted from the dry desert surface. The low-level dust is a useful tracer for tracking the SAL's
strong low to mid-level wind surge and low humidity. These properties appear to inhibit the strengthening of weak
tropical waves or disturbances and to weaken pre-existing tropical cyclones. The SAL retains its Saharan
characteristics of warm, stable air near its base and dryness and dustiness throughout its depth as it is carried
as far west as the Caribbean Sea (~7000 km from the west African coast). It can cover an area of the Atlantic
slightly larger than the 48 contiguous United States.
The dry air contained within the SAL 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 vs non-SAL dropsondes targeted in Fig. 1 and clearly shows the extreme dryness of the SAL air to the north of Danielle. The SAL's low-level wind surge can dramatically increase the local vertical wind shear in the North Atlantic. The SAL surge typically moves at 20-35 kts 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. Tropical cyclones (TCs) interact with the SAL in several ways. Some TCs are embedded in the SAL for their entire life cycle and rarely 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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