The Saharan Air Layer and Its Impacts

A look at how dust can mitigate tropical activity in the Atlantic's MDR

The Saharan Air Layer (SAL, the dust that ejects from Africa’s West Coast) plays a significant role in suppressing tropical development in the Atlantic's Main Development Region (MDR), which spans the tropical Atlantic Ocean and Caribbean Sea (roughly 10°N to 20°N). The SAL is a mass of very dry, dusty air originating from the Sahara Desert in North Africa, typically transported westward across the Atlantic by trade winds during the hurricane season (June to November). Its characteristics create an environment hostile to tropical cyclone formation and intensification.

Let’s take a more detailed look at why the SAL hinders tropical development:

Dry Air Intrusion

• The SAL is characterized by extremely low relative humidity, often below 30-50%, compared to the moist air (70-90% relative humidity) needed for tropical cyclone development. Tropical systems thrive on high moisture content to fuel thunderstorm activity and maintain their convective structure.

• When the SAL interacts with a tropical wave or disturbance in the MDR, it introduces dry air into the system’s circulation. This dry air disrupts the moist convective processes by reducing the ability of air parcels to rise and condense, weakening or preventing the formation of the deep thunderstorms necessary for tropical cyclogenesis.

• For example, satellite imagery (e.g., water vapor channels) often shows SAL outbreaks as plumes of dark, dry air engulfing tropical waves, stifling their development.

Increased Atmospheric Stability

• The SAL is typically warmer than the surrounding tropical atmosphere due to solar heating of the desert surface before it’s transported westward. This warm air layer, often found between 850 hPa and 500 hPa (roughly 5,000 to 15,000 feet), creates a temperature inversion or stable layer.

• This stability inhibits the vertical motion of air, which is critical for the development of deep convection in tropical systems. Without robust upward motion, the latent heat release that drives tropical cyclone intensification is curtailed, effectively capping the growth of disturbances.

Wind Shear Enhancement

• The SAL is often associated with a strong mid-level jet, known as the African Easterly Jet (AEJ), which flows at altitudes around 10,000-15,000 feet. This jet can introduce moderate to strong vertical wind shear (changes in wind speed or direction with height) across the MDR.

• Tropical cyclones require low wind shear (typically less than 10-15 knots) to maintain their vertical structure and allow thunderstorms to organize around the center. The SAL’s mid-level jet can increase shear to 20-30 knots or more, tilting or shearing apart the developing circulation of a tropical wave, preventing it from organizing into a tropical depression or storm.

Dust-Induced Radiative Effects

• The SAL contains high concentrations of dust particles, which can influence the radiative balance of the atmosphere. Dust absorbs and scatters solar radiation, cooling the sea surface temperatures (SSTs) below the SAL by reducing the amount of sunlight reaching the ocean.

• Tropical cyclones require warm SSTs (typically above 26.5°C or 80°F) to sustain evaporation and convection. The cooling effect of SAL dust can lower SSTs slightly, reducing the energy available for tropical development.

• Additionally, dust particles can act as cloud condensation nuclei, altering cloud microphysics in ways that may suppress the formation of deep convective clouds critical for tropical cyclone development.

Interaction with Tropical Waves

• The MDR is a breeding ground for tropical waves (also called African Easterly Waves), which are the seedlings for many Atlantic hurricanes. These waves often emerge from the African continent and move westward, but when they encounter a strong SAL outbreak, their development is hindered.

• The SAL can envelop a tropical wave, starving it of moisture and introducing shear and stability. Meteorologists often observe this on satellite imagery, where SAL outbreaks appear as hazy, dusty regions that disrupt the wave’s cloud patterns and inhibit the organization of a closed low-pressure center.

Real-World Context and Observations

• Meteorologists track the SAL using satellite products, such as those from GOES-16 or Meteosat, which highlight dust and dry air through infrared and aerosol optical depth imagery. Tools like the University of Wisconsin’s CIMSS SAL product help forecasters identify SAL outbreaks and predict their impact on tropical development.

• For instance, during a typical hurricane season, SAL outbreaks are most prevalent in June and July, often delaying the onset of active tropical cyclone formation in the MDR until later in the season (August-September) when the SAL weakens or shifts northward.

• Historical data shows that strong SAL events can suppress entire weeks of tropical activity. For example, in July 2020, a robust SAL outbreak was noted for limiting tropical development despite otherwise favorable conditions like warm SSTs.

Broader Implications

• The SAL’s presence is a key factor in seasonal hurricane outlooks. When SAL activity is strong, as often seen during neutral or La Niña phases of ENSO, tropical cyclone activity in the MDR may be reduced. Conversely, during weaker SAL periods or when the SAL is confined to higher latitudes, tropical waves have a better chance to develop.

• Meteorologists also consider the SAL in short-term forecasts. For example, a tropical wave approaching the Lesser Antilles might be monitored for SAL interaction, with forecasters noting whether the wave can “outrun” the SAL or if it will be overtaken by dry air and shear.