Why massive Saharan dust plumes are blowing into the US

was originally published by Wiredand is reproduced here as part
of the Climate Desk

The pandemic is still raging, the
Arctic is burning up
, and microplastics are polluting
every corner of the Earth
, but do try to take a deep breath.
Actually, belay that, especially if you live in the southern United
States. A plume of dust thousands of miles long has blown from the
Sahara across the Atlantic,
suffocating Puerto Rico in a haze
before continuing across the
Gulf of Mexico. Yesterday, it arrived in Texas and Louisiana.

It’s normal for Saharan dust to blow into the Americas — in
fact, the phosphorus it carries
is a reliable fertilizer of the Amazon rainforest
. The dust
makes the journey year after year, starting around mid-June and
tapering off around mid-August. The good news is, the dust plumes
can deflate newly forming hurricanes they might encounter on the
way over. But the bad news is that dust is a respiratory irritant,
and we could use fewer of those during the COVID-19 pandemic. Also,
the current plume is particularly dense, and it’s not alone: The
African desert is now releasing another that’s working its way
across the Atlantic and will arrive in a few days. Still more could
be on the way as the summer goes on.

A satellite captures a dust plume leaving Africa on June 19.

En route to the continental United States, the plume struck
Puerto Rico on Saturday, cutting visibility down to 3 miles. It’s
the worst Saharan dust event the island has seen in 15, maybe 20
years, says Olga Mayol-Bracero, an atmospheric chemist at the
University of Puerto Rico. Her air-analyzing instruments were
working in real time, detecting the component elements of the
desert dust. “We were quite surprised, seeing such high values
for all these different parameters — we had never seen that,â€
Mayol-Bracero says. “So it was quite shocking.â€

How does Saharan dust make it all the way across an ocean?
It’s a lesson in atmospheric science.

Because it’s a desert, the Sahara is loaded with particulate
matter, from coarse sand down to the tiniest of dirt specks, none
of which is very well anchored to the ground. By contrast, the lush
rainforests to the south of the Sahara have trees that both block
the wind and hold on to the soil with their roots, keeping all the
muck from taking to the air. The conflict between these two
atmospheric regions is what births the plumes that blow clear
across the Atlantic.

The dust plume arrived in the Caribbean a few days after it left

The Sahara is notoriously dry and hot. But down south, around
the Gulf of Guinea, it’s much cooler and wetter, on account of
its proximity to the equator. “The setup between those two —
the hot to the north and the cool, moist to the south — sets up a
wind circulation that can become very strong, and it can actually
scour the surface of the desert,†says Steven Miller, deputy
director of the Cooperative Institute for Research in the
Atmosphere at Colorado State University, which is monitoring the
plumes. (You can watch the dust’s progress from a satellite

with this neat tool
. Look for the gray clouds on the map.)

At the same time, a mile above the desert a 2-mile-thick mass of
hot, dry air called the Saharan Air
, or SAL, has formed. This happens reliably every summer,
blowing east toward the Americas. The process creates “pulsesâ€
of warm, dry, dusty air traveling along the SAL that cycle every
three to five days, says Miller. So if you take a look at the GIF
below, you can see the first plume that’s reached the southern
US, and the new plume currently kicking off from the Sahara. Each
plume takes about three days to cross the ocean.

Here you can see one plume moving through the Caribbean, while
another leaves Africa. VIDEO: CSU/CIRA/NOAA

Looking at these images, you might notice that the plumes are
traveling suspiciously
like hurricanes do across the Atlantic
— and, indeed, this is
where things get extra interesting. The SAL is about 50 percent
drier than the surrounding air, and 5 to 10 degrees Celsius hotter,
and it’s unloading plume after plume. “When that kicks into
high gear, and you’ve got these pulses after pulses of really
strong Saharan air, that’s what kind of inhibits the tropical
storm formation, which forms in these easterly winds as well,â€
says Miller. In other words, these dust plumes actually counteract
the generation of hurricanes.

That’s also because of the contrast between wetter air and
drier air. Tropical storms derive their energy from wet air.
“When you get dry air mixing in, it can weaken the storm, and it
creates these downdrafts and inhibits the convection that starts to
get organized to create hurricanes,†Miller says.

Think of this convection like boiling a pot of water. At the
bottom of the pot, the water gets much hotter than the water at the
surface, which is in contact with the air. This contrast creates
convection — boil some rice and you’ll notice that the grains
cycle between the top and the bottom of the pot. “But if you have
the opposite situation set up, where you have the warm water above
cool water, then it’s what we call a stable situation —
there’s no mixing that happens,†says Miller. Warm air, after
all, wants to rise, and cold air wants to sink. “When you have
the Saharan Air Layer moving across, it’s kind of like that.
You’ve got this warmer air moving across the Atlantic Ocean,
which is a cooler ocean surface. You have this cool air underneath
warm air, and then the atmosphere in that case is very

It doesn’t help matters for any budding hurricanes that the
dust in the SAL is absorbing heat from the sun as it travels across
the Atlantic, creating still more atmospheric stability. Even worse
for hurricanes, they need a calm environment in order to start
spinning, but the SAL is barrelling in with 50-mile-per-hour winds.
“It tilts and it bends the tropical cyclone vortex as you go up
in height, and it decouples and disrupts the storm’s internal
‘heat engine,’ as we call it,†says Miller. “What the storm
wants is just a nice vertically aligned vortex so it can transfer
heat and moisture from the surface upward and out.â€

Forecast models can predict where the dust might land in the
Americas, just like scientists would do
with an approaching hurricane
. Miller reckons that the plume
currently working through the southern United States could
eventually make it to him in Colorado, albeit in a diminished form.
That’s because of gravity: As the plume makes its way across the
Atlantic, the larger particles fall out first, leaving the smaller
particles to make landfall.

Air sampling stations throughout the US gather this particulate
material for scientists to study. “What we typically see is that
the concentrations are highest in the southeast, more toward
Florida,†says Jenny Hand, senior research scientist at the
Cooperative Institute for Research in the Atmosphere. “And as it
moves farther north, the concentrations will go down, just as it
sort of settles out, diffuses, and gets moved around. But we do see
those impacts up into the Ohio River Valley pretty regularly in our

So what does that mean for respiratory health, especially with
COVID-19 being a respiratory disease? “Yeah, it’s not good,â€
says Hand. “Especially now.â€

When you inhale dust, it travels deep into your lungs,
triggering an inflammatory immune response. If your lungs are
healthy, maybe this will manifest as a mild cough. “But for
others who have chronic inflammatory lung conditions, such as
asthma or emphysema, this extra burden of inflammation can tip them
over into severe breathing trouble,†says W. Graham Carlos of the
Indiana University School of Medicine and Eskenazi Health. “We
know, for example, that in many parts of the world that are
afflicted with sand and dust storm events, such as the Middle East,
we see more asthma and asthma attacks.†He advises that people
with respiratory conditions stay indoors until the plume passes. If
you have to go outside, he says, wear an N95 mask: “That type of
mask filters those fine particles, fine enough to travel in the air
across the Atlantic Ocean.â€

Carlos adds that researchers can’t yet say whether inhaling
the Saharan dust might predispose people to contracting COVID-19 or
make the illness worse. “I would caution, though, that COVID is
also an inflammatory condition in the lungs, and that’s in fact
why people are needing ventilators and hospitals are surging,†he
says. “So this could add insult to injury. In other words, you
might have a low-grade inflammatory condition from the dust plume,
and then if you were to get COVID on top of that, it may be

As the weather cools in Africa starting in mid-August, that
temperature differential between the desert and the forests to the
south will weaken, zapping the SAL conveyor belt. The dust clouds
will stop rolling across the Atlantic. Then we can all go back to
just worrying about COVID-19 and microplastics and a melting

This story was originally published by Grist with the headline
Why massive Saharan dust plumes are blowing into the US
on Jun
27, 2020.

Source: FS – All – Science – News
Why massive Saharan dust plumes are blowing into the