Mixed Atlantic hurricane season puzzles experts

MIAMI (Reuters) - 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 September 4, most were exceedingly brief or weak, meaning September only registered 3.5 days with a hurricane.

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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.

END OF SEASON SURPRISE?

The U.S. National Oceanic and Atmospheric Administration said on Tuesday the La Nina phenomenon had definitely kicked in and would be weak to moderate this winter.

That could make the end of the six-month hurricane season, which began on June 1, a little busier than one might otherwise expect because a normal increase in late-season wind shear might be suppressed by La Nina, experts said.

Gerry Bell of NOAA's Climate Prediction Center, which issues the U.S. government's hurricane season forecasts and had called for between 13 and 16 named storms this year, said there was no anomaly in the total number of storms.

"We've had 13 named storms, so that's certainly above normal," Bell said. "Where we've been a bit low is on the hurricanes."

There have officially only been four hurricanes this season but many experts expect Tropical Storm Karen to be upgraded to a hurricane in a post-season analysis, pushing the number to five. The long-term average is for 10 to 11 named storms and six hurricanes per season.

Bell said that in addition to the shear, this year had seen a lot of northwesterly flow in the upper atmosphere that had brought dry air over the tropical Atlantic where many storms form. Storms don't like dry air as they need moisture to grow.

"But the season isn't over," Bell cautioned.

But James Elsner, a Florida State University professor of geography who analyzes hurricane forecasting, said: "We are getting to the end. If something doesn't happen in the next two weeks it's basically over."

"I think what we have to question is why there was so much enthusiasm for this season (in terms of the activity predicted)," he said.