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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 Tropical Cyclone FAQ page webpage.
Saharan Air Layer Outbreaks: SAL outbreaks occur from the late spring through early fall and cover extensive portions of the North Atlantic between the Sahara Desert, the West Indies and the United States (e.g., Florida and Texas). These outbreaks move off the coast of Africa every 3-5 days in a layer from ~550-850 hPa (~1,500-5500 m) and can cover an area the size of the continental U.S. (Movie Loop 1). SAL activity (described by the size and westward reach of the outbreaks) begins to rapidly increase in early June, peaks from mid-June through late July, and begins to subside in early August. Figure 1 shows the mean occurrences of moist tropical (MT), SAL, and midlatitude dry air intrusion (MLDAI) soundings in the Caribbean region from June to October (Dunion 2011). During the summer, the SAL affects ~40% of all Caribbean rawinsonde soundings and on average, over 20% of soundings during the peak months of the hurricane season (July-October, see Figure 1 legend).
The Saharan Air Layer's Warm Temperatures: The temperature at the base of the SAL (~800–900 hPa) is often 5°-10°C warmer than the typical moist tropical sounding (Dunion 2011). While the warmth of the air above the SAL’s base results from its origin over the Sahara, it is maintained by the absorption of solar radiation by the SAL's suspended mineral dust. Although the effect of the SAL's mineral dust on microphysical processes is not fully understood, the warming it induces produces a strong temperature inversion at its base that can stabilize the atmosphere and limit vertical motions through the SAL. The stability induced by this temperature inversion is partially captured by the stability index called convective inhibition (CIN), which describes how resistant the atmosphere is to vertical motions and convection. On average, CIN is ~40% higher (more stable) in the SAL compared to the typical moist tropical sounding (Dunion 2011). This enhanced stability acts to suppress the formation of clouds and thunderstorms in SAL outbreaks.
The Saharan Air Layer's Dry Air: The SAL’s dry air modifies very little as it travels across the North Atlantic and is typically 50% drier than the moist tropical atmosphere (Dunion 2011). The extremely dry air below the melting level in the SAL (~600-850 hPa) can act to suppress convection by enhancing evaporatively driven downdrafts that act to stabilize the environment. Satellite derived total precipitable water (TPW) is a useful tool for identifying dry air masses like the SAL. Dunion (2011) found that 45 mm is a reasonable TPW threshold value for discerning moist tropical air masses from environments with substantially dry air in the lower to middle levels (e.g., the SAL and midlatitude dry air intrusions).
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 20-50 kt easterly jet is typically located in the 600-800 hPa layer and can contribute vertical shear magnitudes as high as 25-75 kt. Figure 2 shows an example of a SAL outbreak covering much of the Caribbean on August 8, 2018 and includes a vertical wind shear analysis from UW-CIMSS. Shear values in the Caribbean ranged from 30-50 kt and were being enhanced by a 30-35 kt SAL easterly jet.
Saharan Air Layer/Tropical Cyclone Interactions: Tropical cyclones (TCs) and tropical disturbances can interact with the SAL in several ways. Some storms are embedded in the SAL for their entire life cycle and often struggle to intensify, while other can be overrun by the faster moving SAL. Some storms remain insulated from the inhibiting effects of the SAL and intensify despite the hostile environment surrounding them. Since SAL-TC interactions are complex and variable, specific interactions need to be assessed in detail on a case by case basis.
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Dunion, J.P., 2011: Re-Writing the Climatology of the Tropical North Atlantic and Caribbean Sea Atmosphere. J. Climate., 24, 893-908.
Dunion, J.P., and C.S. Velden, 2004: The impact of the Saharan Air Layer on Atlantic tropical cyclone activity. Bull. Amer. Meteor. Soc., 85 no. 3, 353-365.