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Between June 14 and 28 of 2020, a dust plume of record-breaking proportions
crossed the Atlantic from the Sahara Desert.
It degraded air quality in large areas of the Caribbean and southeastern US.
Now, the Caribbean isn’t a stranger to Saharan dust because it regularly
crosses the Atlantic from Africa, between late spring and early fall,
especially during June and July.
And some ecosystems even rely on this dust to thrive.
But this wasn’t an ordinary dust plume,
it was around the size of the continental United States!
Its strength and size earned the storm the nickname “Godzilla.”
Since then, researchers from the University of Kansas have been
looking into what caused such a colossal storm.
By looking at satellite data, researchers were able to
watch as the dust storm built up.
And they found that at the beginning of the storm,
more dust entered the atmosphere than normal.
See, at the start of the storm, the wind speeds in northern Africa were among
the highest they had been in 42 years, picking up a huge amount of dust.
But dust isn’t weightless.
Without wind to keep all that dust airborne, it would eventually fall back down.
Which can be a bit of a hiccup if you want to move dust across the Atlantic.
And that’s where things get interesting.
Because that dust stumbled into a “perfect storm” of three different air patterns
that were strong at the same time.
Let’s look at each pattern, in turn, starting with the African easterly jet, or AEJ.
This wind pattern is a bit different from the
local high wind speeds that normally pick up dust.
The AEJ comes from the east, over Africa, and moves west into the Atlantic.
It’s basically always around and exists because of the temperature gradient
between the Sahara Desert and the water in the Gulf of Guinea.
Since the desert is so much hotter than the ocean, hot air rises over the desert,
and cooler air rushes in to take its place.
The hot air moves westward, carrying dust particles with it.
Its wind speeds peak between May and June because those are the months
where the temperature differences are the greatest.
And it’s the AEJ that’s usually responsible for bringing
African dust across the Atlantic every year.
Now at the time of the Godzilla dust storm, the AEJ was much stronger than usual
because of a high-pressure system that had moved into northwest Africa.
Air naturally moves from areas of high pressure to areas of low pressure.
So, having a high-pressure system right at the coast compared to the
low air pressure at the Sahara made it easier to move the dust around.
And this added to the effect of the temperature gradient.
This helped the AEJ catch a large amount of dust that had been brought
into the atmosphere and transport it over longer distances.
Now next up is the North Atlantic Subtropical High,
or NASH, which is found in the Atlantic Ocean.
NASH is a semi-permanent center of high atmospheric pressure.
It’s one of several semi-permanent high and low-pressure areas worldwide.
They form because of temperature differences, ocean currents,
and the tilt and rotation of the Earth.
Basically, because this is a part of the world where the water is
especially cold and the land is warm, air tends to sink, leading to high pressure.
Air sinking down from the upper atmosphere generally carries less water,
so an area of high pressure normally means clear skies and good weather.
So NASH is responsible for the dryness of the Sahara and the Mediterranean
region and in the summer, it brings warm, subtropical air to the southeastern US.
Normally, NASH and AEJ work together to bring dust across the Atlantic,
but in an ordinary year, most of that dust falls into the ocean,
bringing a manageable amount of dust across.
But in the summer of 2020, NASH was anomalously strong.
Now this isn’t that odd.
Weather can be unpredictable, and any number of things
can cause these patterns to fluctuate.
But because it was so strong, it caught this massive amount of dust from the AEJ
and transported it onward, without dropping it into the ocean.
It passed that dust on to the Caribbean low-level jet, or the CLLJ.
The CLLJ is an off-shoot of NASH, which is located between
the islands in the Caribbean and the northern part of South America.
It’s strongest in February and July and weakest in May and October.
At the time of the dust storm, the CLLJ wasn’t just closing in on its strongest point,
it was also feeding off of the really strong NASH, so it was enhanced.
In fact, the CLLJ that week was the second strongest one ever recorded for
that part of June in 42 years, probably because NASH
was also really strong that week.
This intense CLLJ was able to pick up the dust from NASH and drive it home,
dropping the dust all over the Caribbean and the US.
So to recap, all three patterns were much stronger
than normal around the same time, creating the perfect conditions for them
to pass a huge amount of dust between them.
But scientists are concerned this might happen again.
Because while this event was rare, we could be seeing
more storms like this in the future.
See, these weather patterns are influenced by temperature gradients,
and land warms faster than water,
so researchers worry that rising temperatures could make them stronger.
And studies suggest that if there’s ever a large amount of dust,
conditions will be right to transport that dust across the ocean more often.
So it’s still possible that we could see more dust storms in the future
due to climate change, but scientists are working towards being able to
predict them more accurately...So that we can better prepare for the next Godzilla.
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