Each year, Saharan dust is transported across the Atlantic from Africa to the Gulf Coast region of the US, typically during the summer months. Recently, a large concentration of dust has reached the southeastern US and the Caribbean Islands and is forecast to linger in the region for the near term. The dust plume is likely to minimize tropical storm activity, having a direct impact on hurricane season in the Atlantic basin for the time being. Additionally, an increase in dust will lead to reduced visibility and air quality, raising concerns for individuals with respiratory illness or who have developed coronavirus disease (COVID-19). Although this event is significant regarding the density and extent of the cloud, the Saharan dust plume is not a unique phenomenon and not a major cause for alarm.
Heightened Saharan Dust Plume Activity
Beginning the week of June 21, the large concentrations of dust reached the Caribbean islands,
bringing decreased visibility and poor air quality conditions. The US Virgin Islands and Puerto Rico reported approximately only 8 km (5 miles) of visibility at the airport June 22. Forecast models indicate that the dust will be transported further west, bringing hazy conditions and poor air quality across the eastern Yucatan Peninsula and Gulf Coast region through at least July 1. The plumes of dust will become less concentrated as they travel westward toward the US; however, some impacts are still possible.
Poor Air Quality Puts Individuals with Respiratory Disease at Risk
Higher concentrations of dust in the air is a cause for concern during the COVID-19 pandemic in which infected individuals’ respiratory systems are compromised. If conditions are poor, officials urge those with pre-existing respiratory issues, including COVID-19, to stay inside when possible. In addition to poor air quality, increased dust concentrations are associated with more vivid sunrises and sunsets due to more particles in the air to scatter the light particles, illuminating red and orange light.
Saharan Air Layer Affects Tropical Activity
The main impact of Saharan dust on large-scale weather patterns is how it hinders the formation of tropical storms in the Atlantic. Tropical disturbances form as a cluster of thunderstorms organizes in warm waters, typically of at least 26.5 degrees C (80 degrees F). Warmer waters correlate with a moister atmosphere, allowing for low-pressure systems to develop in the Atlantic basin. Warm waters provide a source of energy, and since warm air parcels are less dense than their surroundings, they rise and form a cloud base.
Typically, during the late spring or early summer, a large plume of Saharan dust, known as the Saharan Air Layer (SAL),
forms over the Saharan Desert of West Africa and gets carried westward in the Atlantic Ocean by easterly trade winds. If the Saharan dust is thick enough, it can carry to the Caribbean and southern and eastern regions of the US, as has been the case in June 2020.This large area of dry, dusty air infiltrates the middle and upper levels of the atmosphere at approximately 1,500-6,000 meters (5,000 to 20,000 feet) above the earth’s surface. This significantly inhibits development of tropical cyclones in the Atlantic basin for the following reasons:
- Dry air from the SAL creates what is known as a capping inversion in the upper levels of the atmosphere. This causes air to sink, rather than rise, and is unconducive for the development of thunderstorms, as it prevents clouds from forming.
- The SAL brings strong winds aloft, which leads to high amounts of speed and directional wind shear. High amounts of wind shear make it difficult for tropical systems to form since it interferes with the storm’s vertical structure.
It is estimated that 85 percent of all major hurricanes and 60 percent of all tropical systems form in a region known as the Main Development Region (MDR). This area is between 10 and 20 degrees N in latitude and stretches across the Atlantic from West Africa to the Caribbean. Forecast models indicate the Saharan dust plume will continue trekking westward across this region toward the southeastern US over the coming days after impacting the Caribbean. As a result, tropical activity will likely be limited in this region through at least early July, despite the active start to the season. However, 80 percent of all tropical activity historically occurs after August 15. WorldAware’s 2020 Atlantic Hurricane Forecast report is expecting an above-average season, suggesting that this is just a lull period for the time being.
Near-term Effects of Increased Saharan Dust in Gulf of Mexico and Atlantic Basin
While the June 2020 Saharan dust event is not a new development, the magnitude of this event is quite significant, as records indicate it is likely the most widespread event in the observed satellite. The likelihood of this event majorly impacting the US, Caribbean islands, and Mexico remains low; however, individuals who have developed COVID-19 or other respiratory illnesses should remain especially cautious. In addition, increased Saharan dust over the Atlantic does play a role in decreased tropical activity, which will remain calm through at least early July.
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