A burst of African dust in June, pouring over tropical regions of the Atlantic, appears to be a major reason this hurricane season has been remarkably subdued so far.
What also might be a plus for Florida, the most storm-battered state in the nation: the dust might flare up on a cyclical basis. If so, potentially, future seasons might be tempered as well, said Jason Dunion, a research meteorologist for the University of Miami.
"There definitely seems to be a link, when you have more dust, you have fewer tropical cyclones," he said. "We might see more consistent and larger dust outbreaks."
So far, only two hurricanes have emerged this year, Humberto and Ingrid, and both were relatively weak Category 1 systems. Normally, five hurricanes, including two major ones, have developed by the first week in October.
Scientists say vast amounts of dry air have worked to keep hurricanes from forming, as tropical systems require warm, moist conditions to intensify. As a result, August and September, when the most powerful storms tend to emerge near Africa, were oddly quiet this year.
Experts still aren't sure what created all the dry air, but Saharan dust outbreaks are emerging as a likely factor. Early in the season, an enormous amount of dust floated over the Atlantic region were storms normally develop, making the tropical atmosphere the driest it's been in three decades.
"That's remarkable, because since the 1980s there have been several remarkably dry years," said Dunion, who also works for the National Oceanic and Atmospheric Administration.
Saharan dust outbreaks occur when clouds of particles are lifted from the parched desert region of north Africa and carried west by the winds. The dust clouds most commonly arise in July and August and can spread across the entire Southeast as far as Texas.
It's possible that strong high pressure over Africa generated the winds that produced the abnormal amount of dust. It's also possible that a natural cycle might be at play, as the same thing happened in the early 1980s, drying out the Atlantic for several years in that decade, Dunion said.
"There may be some kind of background cycle, and we just don't understand it," he said. "It's possible that we could return to a regime like the one in the '80s."
Another atmospheric feature that helped keep things calm this year was the Bermuda High, an area of high-pressure that normally sits over the western Atlantic during the hurricane season; it steered the dust over the tropics, Dunion said.
Now that it's October, when storms commonly form in the Caribbean, the Gulf of Mexico and the western Atlantic, it's unlikely African dust will weaken any storms that appear during the rest of the season, Dunion said.
Phil Klotzbach, the Colorado State University climatologist who, along with William Gray, develops seasonal predictions, also thinks African dust is in part responsible for making this year a breeze, so far.
"Unfortunately, there's no nice database we can refer to say, exactly, how much dust was in the air this summer and fall," he said. "But, based on what I've seen, I would say that the Saharan dust outbreaks this year were stronger and penetrated farther south and west than normal."
So far this year, there have been 11 Saharan dust outbreaks strong enough float across the Atlantic and reach South Florida and the Caribbean, said Joseph Prospero, a professor of marine and atmospheric chemistry at the University of Miami's Rosenstiel School of Marine and Atmospheric Science. He noted that Africa produces three-quarters of the dust particles that circulate around the globe.
"It is a major phenomenon," Prospero said. "It is a huge amount of material that is being lifted as dust from Africa."
Meanwhile, the dust caused some elevated air quality readings in South Florida over the summer, but nothing dangerous, local officials said.
Monica Pognon, an air quality supervisor under the Broward County Planning and Environmental Regulation Division, said several bouts of rain apparently kept the air clean.
"We had some readings that were higher than normal," she said. "But we didn't have to put out any health advisories."
"We didn't see anything exceptional this summer, compared to previous years," added Tim O'Connor, spokesman for the Palm Beach County Department of Health.