By Amanda Delaney
One of the main reasons there will a lull in the active Atlantic tropical cyclone season occurred earlier this year, as abundant dust blew east to west across the Atlantic Ocean from the Sahara Desert. This wasn’t factored into the Atlantic hurricane forecasts because it isn’t unusual to have dust plumes travel across the Atlantic while tropical cyclones attempt to develop over the Eastern Atlantic Ocean. In fact, the onset of tropical cyclone development and the dust plumes occur simultaneously each year.
Most tropical cyclone development that begins offshore Western Africa and tracks to the west occurs in a region known as the Intertropical Convergence Zone, or ITCZ. This feature, which can be found all year round to the north of the Equator, shifts farther north during the summer and closer to the Equator during winter. The ITCZ can be identified on satellite imagery as a band of showers and thunderstorms that move from east to west. During the late Spring and through early Autumn, the ITCZ shifts to the north over Central Africa, where clusters of showers and thunderstorms track west across the continent to the coast and offshore near the Cape Verde Islands, before continuing to the west toward the Caribbean Sea. These clusters of showers and thunderstorms come in waves, known as tropical waves, and will move offshore the African coast every 3-5 days and generally reach the Caribbean Sea approximately 7-10 days later.
Some of these tropical waves can move into a favorable environment that will allow the thunderstorms to strengthen and organize into a tropical cyclone under certain conditions: low or no stronger winds aloft, moist air, and warm sea surface temperatures (approximately 80°F or 26.7°C or more).
The ITCZ reaches its northern apex during late spring through early autumn. Tropical waves moving across Africa before reaching the coast generate more wind along their northern side. This happens to cross the Southern Sahara Desert, which allows dust to be picked up and blown toward and offshore the coast.
The dust is usually found approximately one mile above the sea surface but can extend as high as two to two and a half miles into the atmosphere. The larger dust plumes can spread as far west as the Caribbean Sea, Gulf of Mexico and Central America.
Although there is ongoing research about how the Sahara Dust interaction can either promote or hinder tropical cyclone development, moisture can cling to the dust in the air and promote more moisture higher in the atmosphere and therefore, tropical cyclone development.
However, if there is too much dust, then this will hinder tropical cyclone development. These large dust plumes (also known as the Sahara Air Layer or SAL) will move offshore the African coast and can suppress tropical cyclone development in several ways. The SAL is drier than the typical tropical atmosphere over the tropical waters by as much as 50 percent. This will generally hinder cloud formation.
The dust also absorbs sunlight, which will maintain warm air as the dust is crossing the Atlantic. The SAL’s arrival on land usually coincides with the warmest days in the summer. Stronger winds flowing from east to west form in the mid-levels of our atmosphere (between 6,000 and 15,000 feet) with the SAL. Not only does this transport the dust, but it also tilts any thunderstorms that may develop along the tropical waves, moving them away from the wave axis and any circulation that may attempt to develop along the tropical wave, thus disrupting the development of any tropical cyclones.
The SAL usually peaks during late June through mid-August before gradually waning through the autumn, when the ITCZ starts to shift back to the south. As a result, tropical waves will shift farther south, and associated higher winds will shift south of the Sahara Desert. Less dust is then picked up and won’t spread as far west across the Eastern Atlantic Ocean. As the dust diminishes, and as the sea surface temperatures have a chance to warm up over the Tropical Atlantic Ocean, tropical cyclone activity can increase over the Eastern and Central Atlantic Ocean, typically near the Cape Verde Islands around late July through August, usually peaking around mid-September when sea surface temperatures are highest over the Tropical Atlantic Ocean.
An exceptional case of the SAL occurred near the end of June 2020, when a large dust plume tracked to the west and covered much of the tropical Atlantic Ocean and Caribbean Sea, and extended as far west as eastern Texas and eastern Central America. This massive dust plume extended close to the surface, which reduced visibility and became a health hazard for people in these regions, especially over the Caribbean Sea. While this dust plume occurred, sea surface temperatures dropped by as much as 0.4 degrees due to the dust shielding sunlight. However, the warm air that came in with the dust plume allowed sea surface temperatures to rebound back to warmer than normal temperatures. This may have aided to a more active than normal tropical cyclone season for the rest of that year.
The weather features that allowed this dust plume to be so intense included stronger than normal tropical waves, more intense rainfall to the south of the Sahara Desert that increased winds near the desert, a weaker easterly mid-level jet offshore Western Africa that allowed the dust to accumulate offshore, then a strong high building over the North Atlantic that carried the dust farther west than normal.
This event allowed scientists to realize that Sahara dust contributes more to tropical cyclone development than initially thought. As a result, more research is being conducted to understand this relationship and how much IT could impact tropical cyclone development and rainfall amounts in these systems in the future.
Amanda Delaney is a senior meteorologist at Weather Routing Inc. (https://www.wriwx.com). WRI provides customized forecasts and online weather services to mariners worldwide.
