Saharan Dust

Several times a year, increased levels of Saharan dust move across our region, turning our typically blue skies into a dusty and hazy mess. The Sahara Desert emits more particles than any other desert in the world. More than half of the dust deposited in the oceans originates in North Africa. This Saharan dust affects the climate: among other things, it blocks or reflects sunlight, and it affects the formation of clouds and hurricanes. For those who have respiratory ailments, it becomes a time of sniffles and coughs as the dusty atmosphere affects your day-to-day life. Saharan dust isn’t all bad; however, as it provides nutrients for the world’s largest oxygen producers – phytoplankton in the Atlantic Oceans and the Amazon rainforest.

Dense Saharan Dust across the Northern Range during a Saharan Dust outbreak during early July 2018.
Dense Saharan Dust across the Northern Range during a Saharan Dust outbreak during early July 2018.

Saharan Dust Causes & Frequency

There are two main causes of Saharan dust transport, depending on the time of year. Harmattan Winds bring mild to moderate outbreaks across Trinidad and Tobago from November to the middle of March.

Thunderstorms across Central and Western Africa being mild to severe outbreaks across Trinidad and Tobago between April and November, with peak outbreaks occurring from June through August.

Harmattan Winds

The Harmattan is a season in the West African subcontinent that occurs between the end of November and the middle of March. During this season, a predominant northeasterly trade wind (dubbed the Harmattan Winds) blows from the Sahara Desert over Western Africa into the Gulf of Guinea.

The Harmattan Winds over Central & Western Africa (Source)
The Harmattan Winds over Central & Western Africa (Source)

During this period, a ridge of high pressure stays over the central Sahara Desert, and the Intertropical Convergence Zone (ITCZ) remains over the Gulf of Guinea. The Harmattan wind accelerates when it blows across the mountain massifs of Northwest Africa. If its speed is high enough and it blows over dust source regions, it lifts the dust and disperses it. Dust that makes it into the upper levels of the atmosphere can then get transported across the Atlantic Ocean and affect the Eastern Caribbean. These Saharan Dust outbreaks tend to be milder in the Eastern Caribbean than the dust outbreaks associated with West African thunderstorms driving dust into the upper atmosphere from April through November.

Thunderstorms Across Central Africa

Typically running from April through November, strong thunderstorms begin to develop across interior Central Africa and progress westward. These thunderstorms are due to the meeting of the dry northeasterly trades meeting the moist summer monsoon.

Diagram showing how thunderstorms push air into the atmosphere, beginning its journey across the Atlantic Ocean, adapted from Peter Knippertz (2014). Meteorological Aspects of Dust Storms
Diagram showing how thunderstorms push air into the atmosphere, beginning its journey across the Atlantic Ocean, adapted from Peter Knippertz (2014). Meteorological Aspects of Dust Storms

As strong winds move downward and outward from these thunderstorms, the wind kicks up dust as it moves across parts of the Saharan Desert and transports it into the upper atmosphere. This dust then moves across the Atlantic Ocean and affects the Caribbean, parts of Central American, and the Southern United States.

The emission and dispersion of dust are not limited to the two above causes, by can be affected by several meteorological phenomena: El Niño, the North Atlantic Oscillation, rainfall in the Sahel, the Sahara Heat Low, and the Intertropical Convergence Zone.

The Saharan Air Layer

Saharan Air Layer (SAL) is an extremely hot, dry and dust-laden layer of air originating over North Africa’s Sahara Desert, extending upwards from the surface for several kilometers.

The dust band is fed by strong, low to mid-level easterly winds that pull sand and dust particles into the atmosphere, generating the infamous annual dust haze.

Dusty, very dry, and warm, this layer of air is pushed westward by easterly winds or thunderstorms (see above). On reaching the West African coast or the Eastern Atlantic Ocean, rides over the cooler, more moist surface air of the Atlantic Ocean. This forms an atmospheric inversion layer or boundary: with warm, dry air aloft and cooler, moist air below.

When the SAL reaches the Atlantic Ocean, easterly trade winds carry the dust across at the lower and mid-levels, some of which, sometimes high concentrations, is deposited over Trinidad and Tobago and the Southern Caribbean.

Saharan Dust Monitoring & Forecasting

Saharan Dust is monitored and forecast through satellite imagery and numerical weather models. There are several dust models including NASA GEOS-5, University of Athens SKIRON Model and the EMCWF’s Copernicus Atmosphere Monitoring Service (CAMS) to name a few.

These models either model the aerosol optical thickness or the actual dust concentration within an atmospheric column or near the surface. The aerosol can be sea salt, dust, soot, and sulfates, but in this case, it looks at dust. Aerosols absorb or reflect energy (light), influencing temperatures in the atmosphere and on the ground. Satellites measure aerosols by how much light can pass through them. A thick layer of aerosols will block the ground from view, while a thin layer allows enough light through to see the ground. The measurement is called aerosol optical thickness. These models can also use aerosol optical depth, which is the degree to which aerosols prevent the transmission of light by absorption or scattering of light.

The lidar instrument aboard the CALIPSO satellite sends out pulses of light that bounce off particles in the atmosphere and back to the satellite. It distinguishes dust from other particles based on optical properties. Credit: NASA Goddard's Scientific Visualization Studio
The lidar instrument aboard the CALIPSO satellite sends out pulses of light that bounce off particles in the atmosphere and back to the satellite. It distinguishes dust from other particles based on optical properties. Credit: NASA Goddard’s Scientific Visualization Studio

False-colored visible satellite imagery is also a useful tool to visualize Saharan dust as it is transported across the Atlantic Ocean, as the dust particulates within the atmosphere look tan to brown.

haran dust across the Eastern Caribbean on July 9th 2017 as seen by the brown/tan colors from GOES-EAST Geocolor (CIRA) imagery across the islands. Credit: CIRA/RAMMB
Saharan dust across the Eastern Caribbean on July 9th, 2017 as seen by the brown/tan colors from GOES-EAST Geocolor (CIRA) imagery across the islands. Credit: CIRA/RAMMB

Lastly, the Cooperative Institute for Meteorological Satellite Studies’ (CIMSS) Saharan Air Layer Analysis is another product that can be used to determine and track Saharan Air across the Atlantic Basin. More information on this product can be found here.

Saharan Dust Effects

Health. High concentrations of dust with a diameter of fewer than 2.5 micrometers (PM2.5) have been correlated with increases in emergency room admissions for respiratory and cardiovascular disease in North America, Asia, and Europe. The primary health concern associated with Saharan dust is particulate matter, microscopic dust (PM2.5 & PM10) which can sidestep the lungs’ natural defenses. These tiny particles can contribute to cardiovascular problems as well as respiratory diseases such as asthma, especially in children and the elderly.

The dust particles also serve as a vehicle for the transport of known asthma triggers such as biological materials, including bacteria, viruses, fungal spores, and pollen. It has also been shown to transport various pollutants such as metals and pesticides. Dry skin and cracked lips are also typical symptoms of dense Saharan dust.

Visibility. Dust in our atmosphere ultimately causes hazy skies. Depending on the concentration, these hazy skies can significantly reduce visibility at airports, on the roadways, and on shipping lanes.

Weather. Saharan dust has several implications with regards to the weather.

  • Sunlight & Temperatures. The dust can reflect or block sunlight, causing lower than usual temperatures on the surface. However, this effect is only observed when concentrations are very unhealthy to hazardous.
  • Formation of Clouds & Tropical Systems. It can also affect the formation of clouds as the layer of air in which Saharan dust is transported across the Atlantic is quite dry, suppressing cloud development. This dry air lay (Saharan Air Layer) also suppresses and hinders the development of tropical cyclones. While this may sound like a benefit, tropical cyclones are necessary transfers of heat from oceans to the atmosphere. Without tropical cyclones, ocean temperatures will increase, bleaching corals and impacting predominant ocean currents which as a number of knock-on effects. Additionally, tropical cyclones can attribute a significant amount of rainfall for areas in which freshwater access is limited.
  • Locally Severe Showers & Storms. While having moist air at all levels of the atmosphere may seem beneficial for showers and thunderstorms, drier air at upper levels of the atmosphere can contribute to stronger thunderstorm development and wind gusts. As the Saharan Air Layer is a well-mixed layer of dry air between 5,000 ft and 15,000 ft, this provides conditions to be favorable for higher instability and convective wind gusts at the surface in strong thunderstorms. You can read more about this here.
  • Sunsets. With increased dust in the atmosphere, increased scattering of light occurs at sunset. This allows for more colorful sunsets.
  • Muddy Rainfall. While not as extreme as it sounds, vehicle owners who just washed their cars before rainfall during a Saharan Dust outbreak would notice this more than anyone. The dust mixes with the rainfall, and when it lands on the surface, it becomes a layer of dust and mud when the dust is particularly thick.
Muddy rainfall in Athens in 2010. Credit: Pantelis Saitas/EPA
Muddy rainfall in Athens in 2010. Credit: Pantelis Saitas/EPA

Geobiological.

  • The Amazon Rainforest. Approximately 400 to 700 million tons of dust are transported from the Sahara annually. This dust plays a crucial role in replenishing nutrients that are usually leached by rainfall in the Amazon Rainforest. Research has shown that this dust is rich in phosphorus, which is the main source of nutrients for the Amazon.
  • Plankton. Saharan dust is also a significant contributor of iron to marine algae. As a result of increased nutrients, the algae sink to the ocean floor or is eaten by planktonic organisms. Carbon dioxide can be sequestered into the seafloor sediments by planktonic organisms with sinking carbonate shells. The distribution of iron in the oceans is likely to be a way of trapping excess atmospheric carbon dioxide. On the one hand, global warming could increase winds and therefore dust production. However, more dust could enter the ocean, potentially providing a natural mitigator to those effects. (Source) Phytoplankton in the Caribbean and along the southeastern U.S. coast feeds on this iron as well. It allows these organisms to thrive and produce a significant amount of oxygen. (Source)
  • Bacteria Blooms. Notably 12 different species of pathogenic bacteria from the genus Vibrio — use these nutrient infusions to create blooms of their own. Marine Vibrio species also play a role in many diseases of ocean organisms. There are more Vibrio in the water when it’s warmer. Filter-feeding shellfish accumulate Vibrio parahaemolyticus and Vibrio vulnificus in their meat. This causes the majority of seafood-related sickness and death in the U.S. Other Vibrio species are known to be associated with diseased coral that is already facing so many other environmental stressors. Vibrio disease and mortality have even placed an economic strain on the fishing and shrimp farming industries.

Safety & Preparedness

Saharan Dust Precautions
Saharan Dust Precautions
EPA Air Quality Index (AQI) Risk Levels
EPA Air Quality Index (AQI) Risk Levels

The EPA’s AQI risk levels allow the level of health concern to be clearly communicated to the public.

Most outbreaks in recent years across Trinidad and Tobago are typically moderate to unhealthy for sensitive groups. Only a few times in 2018, the air quality index neared or fell in the unhealthy category.

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