In May, the National Oceanic and Atmospheric Administration (NOAA) forecast an above-normal Atlantic hurricane season, with up to 16 named storms and 10 full-blown hurricanes, six of them Category 3 or greater.
But with the August-through-October peak nearly over and only nine named storms to date – five of which reached hurricane status – this year's season, which ends Nov. 30, looks pretty low-key.
Many scientists credit the unexpected arrival of an El Niño, a periodic warming of the eastern Pacific that suppresses hurricane formation. But it may have been something else, too: desert dust.
A new study in Geophysical Research Letters suggests that large amounts of hot, dry, dust-laden air coming off West Africa may have squelched Atlantic hurricanes.
Nascent hurricanes need warm water – at least 80 degrees F. – and relatively calm atmospheric conditions to form. The warm, moist air rises, billowing into clouds and eventually forming storms. In summer, these conditions prevail over the ocean off West Africa, where 85 percent of all major hurricanes originate.
Some years, however, powerful winds blow off West Africa, carrying Saharan dust as far west as Central America and the Amazon basin. Scientists have long suspected that these massive dust storms, often covering an area the size of the lower 48 states, can stymie hurricane formation in the Atlantic.
"It's almost like moving the Sahara desert over the ocean," says Amato Evan, lead author of the study and an assistant researcher at the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin in Madison. This stops hurricane formation in several ways.
Saharan dust "absorbs solar energy just like a rock under a tree during the summertime," says Jason Dunion, field program director at the Hurricane Research Division of NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami and a coauthor of the study. The heated dust further warms the air, which prevents the moist ocean air below from rising, squashing the storm.
Airborne particles can serve as nuclei for water droplets to form, creating precipitation and clouds. But the sheer number of granules in these storms inhibits this process. No single particle can accumulate enough water to form a droplet.
Most important, the dryness of the Saharan air – up to 50 percent drier than ocean air – creates a strong wind shear. "When you have air that's that dry, it's almost like getting out of a shower on a cold morning," says Dunion. "The moisture gets ripped right off you." The powerful wind effectively lops the top off "seedling" storms, stopping them before they get started.
But lots of dust does not always mean fewer hurricanes.
In 2005, the year of Katrina and Rita and the most active hurricane season on record, more Saharan dust arrived in the Caribbean than at any time during the previous 30 years, says Joseph Prospero, director of the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami.
"It goes completely contrary to our argument," Mr. Prospero says, adding that many other factors, like the location of high-pressure systems over the Atlantic and El Niño in the Pacific, affect hurricane formation.
Prospero, who has measured Saharan dust in the Caribbean since the 1960s, saw it increase four-fold in the 1970s. Saharan dust varies directly with rainfall in the Sahel, a belt of land just below the Sahara Desert running the width of Africa, he says. Not coincidentally, the Sahel has suffered drought conditions since the 1970s.
The drought "is largely attributable to rising world temperatures and, specifically, increased temperatures in the Arabian Sea and Atlantic Ocean," he says. If carbon-dioxide emissions from burning fossil fuels are responsible for rising global temperatures, "then this increased dust that we're seeing is anthropogenic." In other words, humans may be to blame.
But human activity of another form may also have an impact, says Natalie Mahowald, a scientist who studies mineral aerosols at the National Center for Atmospheric Research in Boulder, Colo. Not only does dust coming off West Africa vary with rainfall in the Sahel, she points out, but with farming and pasture use as well.
"The biggest source of dust may be natural, but that doesn't mean all the dust is natural," she says.
As for the future, Ms. Mahowald foresees less dust, not more. Many climate models – hers included – predict higher global temperatures will lead to a wetter Sahara. Add to that the fertilization effects of higher levels of CO2, which may allow vegetation to colonize environments that were too hostile before – like the Sahara – and we might be in for "a less dusty future," she says.
The world's largest desert, the Sahara, currently dumps up to 200 million tons of dust into the North Atlantic annually. Scientists blame this dust, which may land as far west as Central America, for everything from melting glaciers in the Alps to the decline of Caribbean corals.
Because it is fine and reddish in color, residents of the southeastern United States, especially those in south Florida, often confuse Saharan dust with haze and smog. On bad days, authorities in Puerto Rico issue air-quality alerts because of the dust.
But the dust's most significant role may be as fertilizer.
Blowing off arid land, the dust is rich in iron and phosphorus – minerals often leached from soil in wetter climes. Scientists have blamed the fertilizing effects of this dust for toxic algal blooms in the Gulf of Mexico, even as they credit the dust for enriching soil in Bermuda.
But the greatest single beneficiary may be the Amazon rain forest, which scientists estimate receives one-third to one-half – up to 13 million tons – of all the Saharan dust that floats into the Western Hemisphere.
In fact, some scientists think that the Amazon rain forest may grow and shrink in direct proportion to the expansion and contraction of the Sahara Desert.
On different occasions during the Pleistocene era, which ended 10,000 years ago, green savanna dominated the Sahara and grassland – not rain forest – covered parts of the Amazon basin. Separated by an ocean but connected by dust, the two ecosystems may move as one.