Saturday, November 28, 2009

Climate change may weaken El Niño's hurricane buffering effect

Curtis Morgan in the Miami Herald: Despite underwater mortgages, failed Ponzi schemes and jailed politicians, the beleaguered citizenry of South Florida does have a few things to be thankful for this holiday season. … Hurricane season is about to end, meekly.

….Unfortunately, experts see this year's quieter-than-normal hurricane season, which officially ends Monday, as a fortunate break owed largely to the weather pattern known as El Niño. “Unless we have a special event like El Niño, we still are in a period where we are going to have active seasons,'' said James Franklin, chief of hurricane specialists at the National Hurricane Center in Miami.”

If that's not enough to dull the relief of dodging another tropical bullet, a new study co-authored by a University of Miami scientist suggests climate change may sap the strength when El Niños form in the future -- at least some of the time.

An El Niño occurs periodically when ocean temperatures rise in the eastern Pacific Ocean. That creates an atmospheric ripple effect reaching around the globe in the form of strong upper level wind shear that can weaken and shred tropical cyclones as they move across the Atlantic Ocean. Shear took its toll on virtually every storm this year, from Ana to Ida.

But Ben Kirtman, a professor of meteorology and physical oceanography at UM's Rosenstiel School of Marine and Atmospheric Studies, said climate data indicates that El Niño may be forming more frequently in the central Pacific, a 4,000-mile move to the west that could rob its ``protective shield'' of power. “When it is shifted further to the east, it has a nice effect on the shear that tends to suppress our hurricanes,'' Kirtman said. ``When El Niño happens to shift further to the west, we might expect less of an influence.''….

…But at least El Niño maintained its clout this year. The average six-month hurricane season in the Atlantic basin produces 10 named storms and six hurricanes, two of them major. This year's total: nine named storms and only three hurricanes, with Bill and Fred both reaching major Category Three-plus strength of 111-mph winds or higher….

The tracks of pre-1900 hurricanes that cross Florida

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