Saharan Dust Continues To Affect T&T, Windward Islands As La Soufrière Erupts

As forecast, a surge of Saharan Dust arrived across the Windward Islands late Saturday. Saharan Dust is forecast to remain across the region through the next 10 days bringing additional hazy skies and reduced air quality, exacerbating the impacts of the La Soufrière eruption.

On Sunday afternoon the Environmental Management Agency (EMA) air quality monitoring stations are recording AQI values at levels that are moderate across Trinidad and Tobago. These measurements are based on PM2.5 (particulates the size of 2.5 micrometers and smaller, usually associated with increases in Saharan Dust, vehicle exhaust, and smoke) and PM10 particulates. Across the Lesser Antilles, mainly islands south of the French Antilles are experiencing degraded air quality.

Visibility at Piarco International Airport has been reduced to 7 kilometers while across Tobago, visibility is at 10 kilometers.

The Saharan Dust Forecast

Over the next seven to ten days, the air quality will fluctuate between levels that are moderate to unhealthy for sensitive groups.

A high-pressure system dominating, across the Atlantic, is producing strong easterly trade winds. These winds have become a conveyor belt for a significant and prolonged surge of Saharan Dust through mid-April.

00Z Sunday 11th April 2021, NASA GEOS-5 Dust Extinction Model Monitoring Tropical Atlantic Dust Aerosol Optical Depth showing Saharan Dust.
00Z Sunday 11th April 2021, NASA GEOS-5 Dust Extinction Model Monitoring Tropical Atlantic Dust Aerosol Optical Depth showing Saharan Dust.

What does this mean for you?

Members of sensitive groups may experience health effects. The general public is not likely to be affected. There is an increasing likelihood of respiratory symptoms in sensitive individuals, aggravation of heart or lung disease, and premature mortality in persons with cardiopulmonary disease & the elderly.

We’re in a period where a ridge of high pressure stays over the central Sahara Desert. The Intertropical Convergence Zone (ITCZ) also 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. The plumes of dust eventually affect the Eastern Caribbean.

Saharan Dust outbreaks from December through early April tend to be milder in the Eastern Caribbean compared to late April through November. During the latter period, those dust outbreaks are associated with West African thunderstorms driving dust into the upper atmosphere.

For mariners, an “Ashfall Advisory” is in effect for the marine waters surrounding St. Vincent and extending as far east as east of Barbados from the National Hurricane Center Tropical Analysis and Forecast Branch. Mariners are encouraged to contact the National Hurricane Center at 305-229-4424 if they encounter volcanic ash or floating volcanic debris.

Can La Soufrière’s ashfall impact T&T?

For ash to travel southward, winds will have to originate from the north. This is a highly unusual circumstance only seen when powerful low-pressure systems such as tropical cyclones move across the region, influencing wind speeds and directions across the Windward Islands.

Diagram explaining why Barbados is getting ash, even though St. Vincent is to our west!

Posted by Barbados Meteorological Services on Saturday, April 10, 2021

Through the next 14 days, models indicate mostly easterly winds below the 500 millibar level and mostly westerly to southwesterly winds above the 500 millibar level. Hence the islands most at risk for ashfall remain, St. Vincent, St. Lucia, Barbados, and Martinique. If winds at the mid to upper levels of the atmosphere shift toward the south-southwest, ashfall will be possible across Grenada’s dependencies including Carriacou and Petit Martinique, as well as several small, largely uninhabited islands. This occurred overnight from Saturday 10th April 2021 into Sunday 11th April 2021.

Similarly, as ash drifts further and further southeast over time, it is possible eastern areas of Tobago experience some ashfall by next week (April 12th-19th). For Trinidad, it still remains highly unlikely ash may travel as far south as the island. If an ash cloud does drift this far south, it will be highly dispersed in the upper atmosphere, remaining mainly an aviation hazard with little to no impacts on the ground.

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