When the tropics are quiet, there’s an easy culprit meteorologists – including yours truly – love to point their fingers to: Saharan dust.
It’s an easy story to tell.
Big outbreaks of sand from the deserts of the Sahara are lofted to heights of 10,000 or 15,000 feet and travel westward in giant plumes – at times as large as the lower 48 states – over 4,000 miles from Africa through the Caribbean and Florida, occasionally reaching as far west as Texas, turning skies milky orange along the way.
It makes sense that these nation-sized outbreaks of desert sand would act as kryptonite for fledgling hurricanes that feed on plentiful moisture in the air – not exactly something we find in arid desserts.
Storms don’t form in the desert, so neither should hurricanes. It’s an effective marketing campaign. When all else fails, blame it on the dust.
But the story of dust and hurricanes is as hazy as the sand-sprinkled horizon.
The Saharan Air Layer or SAL – the scientific term specific for the warm, dry, dust-laden layer of the atmosphere extending 5,000 feet to 15,000 feet up – is ever-present during the hurricane season.
The dusty SAL usually ramps up during the spring and peaks over the tropical Atlantic in late June or early July. The great plumes that roll off Africa every three to five days – like a colossal ocean wave crashing ashore – ride on the backs of the same tropical disturbances that often grow into our strongest hurricanes.
Naturally, when our would-be tropical systems get mixed up in these great dust outbreaks, it stands to reason their development odds, like the surrounding humidity, drop.
Ongoing research, however, suggests it’s not that simple.
Scientists have known about this dusty airmass since at least the late 1960s, when field work near the easternmost Caribbean islands collected the first comprehensive data on airborne dust, largely through a network of early weather balloons.
In years since, we’ve added extensive data collected by instrument packages dropped from research aircraft like the hurricane hunters. More recently – since about the early 2000s – the advent of high-resolution satellites has painted an even clearer picture of the sands blowing in the winds over the Atlantic.
There’s no disputing dust can be detrimental to hurricanes.
Dr. Jason Dunion, a hurricane scientist based out of the government’s hurricane research lab on Virginia Key, has conducted extensive research on the Saharan Air Layer. In his seminal 2004 paper on Saharan dust and hurricanes, Dunion and his colleagues found once it infiltrates a hurricane’s core, dry Saharan air can increase sinking motions, extinguishing thunderstorms and throwing a hurricane off balance.
Additionally, a jet of strong winds on the southern edge of these dust plumes increases hostile wind shear nearby that can inhibit storm formation. By the mid-2000s, the adverse role of Saharan dust on hurricanes seemed increasingly apparent.
But at the dawn of a new decade, research emerged that began to call into question the true influence of dust on Atlantic hurricane activity.
Higher fidelity satellites, new to the scene at the turn of the 21st century, by 2010 allowed scientists to better sort through the dust-up. In earlier studies, when less refined satellite observations were available, dry air often masqueraded as dust, when often the dry air originated from other sources not related to Saharan dust.
Additionally, hostile wind shear near the edge of the dust plumes was found to be generally at a distance from the main hurricane formation zone, suggesting little influence on storm organization.
Fluctuations in intensity were largely due to other unrelated factors.
According to the provocative new research, the enhanced low-level winds on the leading edge of the dust outbreaks – believed earlier to deter storm development – may actually increase background spin to the south and help promote storm formation.
Other papers supported these claims, including a study published in late 2020 which uncovered – after analyzing over 50 tropical cyclones over 14 years – as many strengthening storms in the presence of the Saharan Air Layer as weakening storms.
In other words, just because the dust is high nearby doesn’t mean a storm won’t form or strengthen. Guilt by association in the case of dust is a dubious claim.
So what’s the verdict? Is dust to blame when the tropics go quiet? Not necessarily, and we probably give dust more credit (or blame) than it deserves.
Most tropical cyclones at some point encounter the Saharan Air Layer. Their seedlings, after all, are what loft and transport the dust from northern Africa into the Atlantic in the first place.
A dusty Atlantic is an easy explanation when storms don’t materialize, but it isn’t always accurate. A dustier Atlantic may signal more robust disturbances moving from Africa that can increase tropical development.
Before Dorian formed at the end of August in 2019, the tropical Atlantic was off to one of its least active starts in 20 years despite record low dust cover in July and August.
On the other hand, by the end of August 2005, the Atlantic already recorded one Category 4 and two Category 5 hurricanes, even though dust cover was running above average.
It’s not enough to say the Atlantic is dusty and therefore it’s quiet.
Other factors like rising or sinking air, upper-level winds, and the configuration of warm and cool Atlantic waters are more important to regulating overall tropical activity.
On a storm-by-storm basis, though, we know dust, when ingested into the core of a hurricane, can make its engine sputter. This is especially true when strong upper-level winds close to a hurricane’s core help to open the door for the Saharan Air Layer’s entrance.
We needn’t dismiss the dust, which can have indirect impacts on hurricanes as well, including the cooling of ocean waters during the summer. But before we blame it all on the dust, let’s first try to dig a little deeper.