Not many people think about the possibility that things in western Africa can be transported in the atmosphere to the US, but that is exactly what is happening right now. Dust from the Sahara Desert is moving across the eastern part of the Atlantic Ocean right now and that air will likely move through the Caribbean then ultimately into the US. There is a lot of interest and questions about this event, so tonight I want to discuss what this means for Hurricane Season and our area.
What is a Saharan Air Layer?
A SAL or "Saharan Air Layer" is simply an air mass that contains a lot of dust and dry air from the Sahara Desert in western Africa. These SAL events occur almost every single year, so they are not unique to 2020. An SAL occurs when a storm system or group of t-storms kick up dust in the western part of Africa in the Sahara Desert. This can occur from something as simple as t-storms or, on a larger scale, from a low pressure system affecting the area. Notice the SAL expanding off the west coast of Africa right now. The color of the dust is VERY obvious from the GOES-16 satellite.
Once this dust gets suspended in the atmosphere, then it is carried eastward by the trade winds. If there is enough dust in a larger SAL, then that dust can travel all of the way across the Atlantic into the US. Latest model data shows this SAL exceptionally well traveling from Africa to the Caribbean and into the southern US. Notice, how thick the SAL is over the eastern Atlantic and how the layer makes it into the Gulf States, .but may dissipate before arrive in our area.
How Does This Affect Hurricane Season?
When an SAL occurs and it is as substantial as this one, it injects extremely dry air into the mid levels of the atmosphere. From the animation I showed above, you can image how this dry air will affect Hurricanes. The SAL normally shuts down hurricane season for a period while these are present. Think about what the SAL represents. Yes it has dust in it, but it represents very dry air that resided over the Sahara desert. We know hurricanes love moist air at the surface and mid levels, so the SAL is literally the opposite of what feeds hurricanes. Here is some good info about how SAL affects the tropics per NASA...
Life cycle of a hurricane
Scientists have long understood that convective waves of westward-traveling atmospheric disturbances from the north African coast can be the beginnings of tropical storms and hurricanes. Dunion said, “In the Atlantic, more than half of tropical storms and weak hurricanes, and 85 percent of major hurricanes—categories three, four, and five—come from Africa.” Scientists also know that a number of factors, including sea-surface temperatures, unstable atmosphere, and high water-vapor levels, can cause the waves to intensify and form storms.
Albers and Dunion are among more than one hundred researchers who participated in the NASA African Monsoon Multidisciplinary Analyses (NAMMA) campaign, a joint effort between NASA and the National Oceanic and Atmospheric Administration (NOAA), during the Atlantic hurricane season of 2006. Syed Ismail, a scientist at NASA Langley Research Center, said, “The objective of NAMMA was to see what role the Saharan dust aerosols play in the development of tropical disturbances, which could eventually become hurricanes in the Atlantic. The disturbances propagate from the coast of north Africa, and they get energized in the warm Atlantic climate. And then they sometimes develop into hurricanes.” The researchers suspected that Saharan dust storms sometimes prevent disturbance waves from intensifying into tropical storms and then hurricanes. That Saharan dust keenly interests Dunion, a research meteorologist from the NOAA Hurricane Research Division in Miami. He said, “The Saharan Air Layer is essentially a huge dust storm that can be the size of the continental United States. Every three to five days during the summertime, these storms roll off of the African coast.” As the dust storms move off northern Africa, convective waves develop farther to the south, pulling moisture up into the atmosphere.
This NASA satellite image shows a dust storm, hundreds of thousands of square miles in size, moving from the Saharan Air Layer over Africa into the eastern Atlantic Ocean. The image was captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, on the Terra and Aqua satellites, on February 26, 2000. (Courtesy NASA MODIS Rapid Response Team)
“We think a dust storm has three main components that can suppress a hurricane,” Dunion continued. “One, it’s got super-dry air. Hurricanes don’t like dry air in the middle parts of the atmosphere, and that’s exactly what the Saharan Air Layer has. A Saharan dust storm also has a very strong surge of air embedded within it, called the mid-level easterly jet, that can rip a storm apart that’s trying to develop. We call that vertical wind shear. And then the third piece is all this dust.”
Researchers think the dust itself suppresses cloud formation, playing a role in preventing tropical waves from becoming more intense. Ismail said, “We think that dust aerosols can affect tropical disturbances, sometimes even kill those disturbances. Dust inhibits convection, the process of moisture rising to the higher levels of the atmosphere, and then precipitating as rain. So these Saharan dust layers seem to have a blanketing influence on the development of convection.”
How Can Saharan Dust Affect Our Area?
This is legitimate and interesting question. Saharan Air Layers can and do make it into the US. When they arrive, we normally see those that suffer from allergies note an increase in problems, so if you have allergies then that would be of interest. SALs also have a notable affect on sunrises and sunsets. With the increased dust in the atmosphere, it affects sunsets like wildfires do by making the sky more colorful at sunrise and sunset. So the good would be more colorful sunsets, while the bad would be increased allergy issues for that that struggle with allergies.
