An infusion of very dry air over the Atlantic Ocean has kept the 2013 hurricane season from being the stormy summer that forecasters expected it to be in June.
"A lot of people are scratching their heads right now," said Keith Blackwell, professor of meteorology at the University of South Alabama in Mobile. "Everybody was wrong in their long-range predictions."
A high-pressure system known as the Azores-Bermuda High that often parks over the Atlantic Ocean during the summer is the reason so much dry air has moved over the Atlantic, scientists say. The Azores-Bermuda High often has a significant influence on the direction tropical storms take as they move northward.
But this year, that high-pressure system pumped dry air from the Sahara over the Atlantic, and that air quashed tropical storm formation, said meteorologist Jeff Masters, director of the private weather forecasting website Weather Underground.
In addition, an extreme drought in Brazil is creating more dry air over the Atlantic, and that also helped suppress tropical storm formation, Masters said.
Hurricanes and tropical storms draw their energy from warm, moist air and seawater that has been heated to at least 80 degrees Fahrenheit (27 degrees Celsius). Atmospheric winds known as wind shear inhibit the formation of tropical storms, so storms tend to flourish when those winds are minimal.
Warm waters and no wind shear are factors that usually indicate an active summer hurricane season.
The factors that usually are conducive to hurricane formation were in place in June when seasonal forecasts were issued by Colorado State University and the National Oceanic and Atmospheric Administration, said Michael Laca, producer of the website TropMet.
Both CSU and NOAA predicted that as many as 20 named tropical storms would form from June 1 to November 30. The forecasts also predicted that as many as 11 of those storms could become hurricanes with winds of at least 74 miles per hour (119 kilometers per hour). As many as six of those hurricanes were forecast to intensify into major hurricanes with winds exceeding 110 miles per hour (177 kilometers per hour).
But meteorologists also correctly predicted that a weather phenomenon known as El Niño would not develop, Laca said. An El Niño (see video) occurs when waters off the Pacific coast of South America are unusually warm.
Among other things, an El Niño can cause wind shear over the Atlantic Ocean that disrupts hurricane formation.
Although there are still more than two months remaining before the hurricane season ends November 30, the period when storms are most likely to form has passed. As of today, only nine named tropical storms have formed, and only one of those storms became a hurricane. That hurricane did not intensify into a major storm.
And while Mexico was hit by two hurricanes – one on its Atlantic coast, the other on its Pacific coast – neither of those storms intensified into major hurricanes. The flooding from the storms was nonetheless devastating, with 110 people dead by latest count. (See "Pictures: Mexico Hurricanes Pack a Rare Double Whammy.")
South Alabama's Blackwell also noted that only about half the usual number of storms have formed in the Pacific Ocean, and storm formation in the Indian Ocean is down by about 25 percent.
Hard to Predict
While many factors that influence tropical storm formation were apparent at the beginning of the summer, the presence of so much dry air is not something that can be factored into a long-range seasonal forecast.
In seasonal forecasting, "you can see large-scale signals which can lead you in the right direction, but seeing something like dry air months out is pretty much impossible," said Greg Nordstrom, a meteorology instructor at Mississippi State University in Starkville.
Blackwell thinks other, less evident factors may have also contributed to the unexpectedly quiet hurricane season. And it may be awhile before meteorologists understand what all of those factors were.
"This will be the subject of a lot of research in the coming months," Blackwell said. "Whatever happened this year, it may be completely outside the indicators that normally allow some kind of multi-month prediction."
But while this year's peak period of hurricane formation has passed, it is possible for extremely powerful hurricanes to form in October. Hurricane Wilma, the most powerful hurricane on record for the Atlantic Basin, formed in October 2005. And Hurricane Sandy, which devastated much of the U.S. Atlantic coast in 2012, formed at the end of October.
"I think we'll get one, maybe two more [tropical storms], most likely in the third week of October," said Weather Underground's Masters. "I think we'll probably get something during the last half of October."