Hurricane season puzzles experts

From a Reuters' story by Michael Christie posted on the Planet Ark Web site:

MIAMI - Judge the 2007 Atlantic hurricane season by the 13 storms so far, and it looks like a relatively busy year. But look at the number of days a hurricane has swirled in the Atlantic, or use other measures of a storm season's ferocity, and 2007 has been surprisingly benign.

Hurricane experts had predicted the season to be above-average because of warm Atlantic sea surface temperatures, the continuance of a decades-long natural period of increased storm activity, and the development of La Nina weather conditions in the Pacific.

Many tropical waves, often a precursor of a tropical storm, developed in the Atlantic over the busiest weeks of the season between September and early October, and eight named tropical storms formed in September -- matching a record for the month.

But apart from maximum-strength Hurricane Felix, which slammed into Central America on Sept. 4, most were exceedingly brief or weak, meaning September only registered 3.5 days with a hurricane.

One noted hurricane forecasting team at Colorado State University had predicted 20 hurricane days that month.

This year's storms caused relatively little damage and casualties especially compared to the havoc inflicted in 2005, when Hurricane Katrina flooded New Orleans, Wilma pummeled the Mexican resort of Cancun and Florida, and Rita hit the Texas-Louisiana border area.

The main reason for the low number of hurricane days this year has been high vertical wind shear -- the difference in windspeeds at different altitudes -- which tears storms apart while they try to form, hurricane experts said.

Scientists are puzzled. A periodic cooling in sea temperatures in the eastern equatorial Pacific, known as La Nina, is supposed to reduce shear over the Atlantic.

"It's like everything else with hurricanes; every now and then the scientists just have to scratch their heads," said Jeff Masters, co-founder of the Weather Underground Web site.