Saharan Dust
Atmosphere Daily Earth Science Environment Repost

Saharan dust heads towards the USA

It is the middle of June in 2020, and the dust is starting to fall. Not just any dust, but Saharan dust that journeyed up to 7,000 miles from the deserts of North Africa. High in the atmosphere, this dust blew across the Atlantic Ocean before landing on the USA Gulf Coast in Texas, Louisiana, Mississippi, and Florida. But this dust may go unnoticed except for colorful sunsets and the onset of asthma in those with respiratory problems.

This ‘rain of dust’ is not an anomaly because every year, unbeknownst to most people, somewhere between 400 million and 2.2 billion tons of tiny soil and mineral particles blow out of the North African deserts and head for the Caribbean, USA, and South America. The Sahara is the largest supplier of atmospheric dust on the planet. It accounts for half the world’s supply.

Winds sweep the dust particles from the Sahara’s surface between late spring and early fall, where they collect in a hot, dry air mass called the Saharan Air Layer.  This air layer extends from 5,000 to 20,000 feet above the desert surface, and strong winds blow the dusty air westward across the Atlantic Ocean. So, the summer months of June and July are usually the time of peak transport.

The picture below shows the density of atmospheric particles during a 2018 dust storm. The brown areas show a dust storm larger than the entire continental USA.

Source: NASA Earth Observatory

Dust tames the raging storms

The volumes of dust moving in the upper atmosphere are enough to affect regional weather. Scientists studying the Saharan atmospheric dust believe its presence helps suppress the formation of tropical storms and hurricanes. 

Three characteristics of the Saharan Air Layer inhibit hurricane formation. The first of these is dry air. The lack of moisture in the North African deserts makes the air extremely dry. But tropical storms thrive on moist air, not the dry air in the mid-atmosphere where the dust storms travel.

The second storm inhibitor is fast-moving air currents. These strong air surges, embedded in the middle of the dust storms, create wind shear that rips the storms apart when they try to form.

The last damping effect is the Saharan dust itself. The dust aerosols inhibit the convection process, where moisture rises high in the atmosphere to condense and form rain. Without moisture and convection, and then chopped off at their top by wind shear, tropical storms have a hard time getting a foothold.

What about the Amazon?

The USA Gulf Coast only gets a portion of the Saharan dust from North Africa. Caribbean islands collect the falling particles, and much of the Sahara’s former surface cover also ends up on the ocean floor. But, some of this sediment load makes its way to South America, settling over the Amazon Rain forest. Then the nutrient-rich dust from North Africa replenishes the rainforest soils that are continually depleted by heavy tropical rains.

The intensity of the summer Saharan dust storms inversely correlates with wet weather conditions in North Africa. Wetter weather in North Africa limits the size and intensity of dust storms. However, a hotter planet from global warming will tilt the trend towards larger and more frequent dust storms.

Much remains unknown about the complex connections between Saharan dust and the ecosystems it influences. However, research into how this dust affects primary ocean productivity is underway. Nutrients are a critical component of phytoplankton blooms at the base of the marine food chain. So airborne nutrients from the Sahara may drive increases in ocean bio-productivity for some regions.   

For now, though, if you are enjoying the Gulf Coast in June or July, try and catch a good sunset as the Saharan dust adds color to the evening sky.

(Read in Medium Magazine)


Sources:

Here Comes the Saharan Dust (Source: NASA Earth Observatory) – https://earthobservatory.nasa.gov/images/92358/here-comes-the-saharan-dust  Also:

Saharan dust versus Atlantic hurricanes (Source: NASA Earth Data) – https://earthdata.nasa.gov/learn/sensing-our-planet/saharan-dust-versus-atlantic-hurricanes  Also:

The pulsating nature of large-scale Saharan dust transport as a result of interplays between mid-latitude Rossby waves and the North African Dipole Intensity  (By E.Cuevas , A.J.Gómez-Peláez , S. Rodríguez , E.Terradellas , S.Basartc, R.D.García, O.E.García, S.Alonso-Péreze; Science Direct) – https://www.sciencedirect.com/science/article/pii/S1352231017305757  Also:

Understanding the Transport and Impact of African Dust on the Caribbean Basin (By Joseph M. Prospero and Olga L. Mayol-Bracero; American Meterological Society) – https://journals.ametsoc.org/bams/article/94/9/1329/90663/Understanding-the-Transport-and-Impact-of-African  Also:

Enormous Cloud of Dust From Africa’s Sahara Appears to Be Heading for U.S. (By ARISTOS GEORGIOU; Newsweek) – https://www.newsweek.com/enormous-cloud-dust-africa-sahara-texas-louisiana-florida-1511110  Also:

Feature Image: Dust Storm in Kajaki (Modified) – By Lance Cpl. William Kresse – https://www.dvidshub.net/image/958084, Public Domain, https://commons.wikimedia.org/w/index.php?curid=41097471  

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.