for National Geographic News
As the 2009 Atlantic hurricane season comes to an end November 30, it will be remembered as one of the quietest in almost two decades, meteorologists say.
That's because persistent, upper-level winds linked to El Niño—unusually warm waters that sometimes form off the northwestern coast of South America—hampered tropical storm formation. Just 9 storms took shape, instead of an average of 15.
During El Niño, the winds—known as the jet stream—shift southward and disrupt hurricane formation and development in the Atlantic Basin, which includes the Caribbean Sea and the Gulf of Mexico.
Jeff Masters, meteorological director for the Web site Weather Underground, said cooler water temperatures in parts of the Atlantic also may have inhibited storm formation in June and July.
Hurricanes draw their energy from ocean waters that have been heated to at least 80 degrees Fahrenheit (26.6 degrees Celsius).
A total of nine tropical storms with winds of at least 35 miles (56.3 kilometers) an hour formed this season. Three storms became hurricanes with winds of at least 74 miles (119.1 kilometers) an hour, and two developed into major hurricanes with winds exceeding 110 miles (177 kilometers) an hour.
Those totals were well below yearly averages since 1995, when meteorologists think an ongoing period of increased hurricane activity began in the Atlantic.
Hurricane seasons since 1995 have averaged 15 named storms, 8 hurricanes, and 4 major hurricanes.
There were still a few oddities, including Hurricane Ida, the first storm since 1985 to make landfall on the U.S. Gulf Coast in November.
Ida began as a tropical depression in the southwestern Caribbean on November 4. It struck Nicaragua, then became a Category 2 hurricane with winds exceeding 100 miles (161 kilometers) an hour as it moved from the Caribbean into the Gulf of Mexico.
(Watch a video of hurricane destruction.)
"It was remarkable that Ida was able to form and move through the region that had been extremely hostile to hurricane formation," said Keith Blackwell, a meteorologist at the University of South Alabama's Coastal Weather Research Center in Mobile.
Blackwell said Ida had benefited from an "oasis of favorable conditions."
Ida weakened as it churned through the Gulf, and made it landfall as a tropical storm near Bon Secour, Alabama, on November 9.
While the El Nio kept a lid on Atlantic hurricanes, it was a major factor in a very active season in the Pacific.
Blackwell said the eastern and central Pacific, which had 20 named storms, was "really, really energized this year."
One of those storms was Hurricane Rick, a Category 5 storm that became the second-most-powerful hurricane on record for the Pacific. Weather Underground's Masters said the El Niño created warmer waters in the Pacific, and this helped fuel the active season. But Masters and other meteorologists don't think El Niño will continue into the 2010 season. "I think we'll be back to the typical type of season we've been seeing since 1995," he said.
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