TAMPA — Wind shears, water vapor and temperature, El Nino and La Nina all play a role in the formation of hurricanes in the Atlantic Ocean.
This year, the Sahara Desert appears to have played a part as well.
The vast desert that stretches across North Africa spat out a massive plume of dust in a June wind storm that blew particles over the eastern Atlantic Ocean, blanketing the cauldron where tropical storms and hurricanes bubble up.
Some scientists and meteorologists credit the dust particles and the drying effect they create with impeding the formation of tropical cyclones and creating one of the calmest Atlantic hurricane seasons in decades. The season is mostly over, but only two hurricanes have formed, both of them minor.
Exactly why forecasters were so dramatically wrong – most of the major hurricane prognosticators predicted an above-average season – is the subject of much speculation in the hurricane-predicting community.
Part of the mystery is that sand from the Sahara blowing over the Atlantic is far from unusual. The pattern traditionally grows stronger in the early summer and into July, said Jason Dunion, a hurricane research analyst who works for the National Oceanic and Atmospheric Administration’s Atlantic Oceanographic and Meteorological Laboratory in Miami.
But this year’s June plume was exceptional, he said, and that translated into unusually dry air in the part of the Atlantic that is the main breeding ground for hurricanes. Such levels of dryness have not been since the mid-1980s, he said.
Whether that inhibits the formation of hurricanes, he said, well, “That’s the million-dollar question.”
Could it be that the amount of dust in the air keeps hurricanes from forming?
Perhaps, Dunion said, but the theory needs more study. The one scientific certainty, he said, was the amount of Saharan dust in the air over the region where storms are born during the peak of the season.
“One of the essential ingredients to get hurricanes going is moisture in the air, particularly in the middle areas of the atmosphere.”
Moisture, combined with other factors, he said, “gets hurricanes bubbling and brewing. Drying that air out too much stifles development.”
Another scientific certainty: The number of hurricanes in the Atlantic so far this year is way, way below normal.
Hurricane Humberto formed early in September in the Atlantic and petered out in open water. Hurricane Ingrid formed in the Caribbean and made landfall along the northeastern coast of Mexico a month ago. The season, which ends on Nov. 30, is on the downswing, having passed its peak in September. November averages only about one named storm every two years, forecasters say.
Besides the two hurricanes this year, there were 10 named tropical storms. Meteorologists say the number of tropical storms is about average, but where they formed is unusual.
Five of the 10 storms formed in the Caribbean. Typically, they form there early in the season, then shift to the eastern Atlantic during the peak of the season.
But this year, there’s been a dearth of hurricanes swirling along Hurricane Alley, that stretch of open water between West Africa and the Caribbean.
Forecasters in May predicted a 70 percent chance of an above normal season and just a 5 percent chance of a below-normal season, said Dennis Feltgen, spokesman for the National Hurricane Center in Miami. As many as 19 named storms were predicted, with up to nine becoming hurricanes. Three of those would become major – Category 3 or higher – hurricanes, they said.
So far this year: zero major hurricanes.
If the season ends without a major hurricane, it will mark the first time that has happened since 1994.
There are several theories cited for the lack of hurricanes, Feltgen said; the Saharan dust factor is just one.
There is also the Bermuda High, an area of high pressure along the eastern seaboard that has steered dry air into the tropics and over the hurricane birthing grounds. That, coupled with the dry air associated with the African dust, likely contributed to the lack of storms, he said.
Feltgen was reluctant to credit the Sahara as the only reason hurricanes have failed to form this year.
“There was some Saharan dust,” he said, “but it doesn’t cover 100 percent of the tropical season. If a lot of dust happens to be in an area where hurricanes are forming, that could be one of several factors.”
Someday, he said, Saharan dust may be factored in when meteorologists make hurricane season predictions.
Scientists are just beginning to study the relationship between Saharan dust over the Atlantic and the formation of tropical cyclones, said Joseph Prospero, a professor of marine and atmospheric chemistry at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.
“That is a very active topic of discussion,” he said. “The outbreaks of African dust have a clearly detectable impact. Increased dust in the air means reduced sunlight on the ocean surface, lower temperature and reduced water vapor emission, all of which play a role in tropical cyclones.”
He said the presence of a hot, dry layer of dusty air at an elevated altitude diminishes atmospheric conditions that lead to tropical cyclone formation.
But, he noted, “There are many other factors linked to these dust events that could suppress tropical cyclone development. In short, this is a very active area of research today.”