WeatherWatch | Global warming: This is a switch

Coming soon to a coastal property near you: rising seas . . . 150-m.p.h. winds . . . disappearing beaches!

For residents of the Atlantic and Gulf Coasts, the recent global-warming scenarios have been more terrorizing than horror-movie promos.

Put simply, a warmer world means warmer oceans and thus more fuel for hurricanes.

But now comes a plot twist.

A warming planet also could have a dampening effect on hurricanes, National Oceanic and Atmospheric Administration researchers reported last week.

"These two things are acting in the opposite sense," said coauthor Brian Soden, who works out of the University of Miami's Rosenstiel School of Marine and Atmospheric Science in Virginia Key, Fla.

His words will hardly be the last on a topic of monumental interest to coastal residents - indeed, to all taxpayers, who are still footing the bill for the catastrophic 2005 hurricane season.

Soden and lead author Gabriel Vecchi, a research oceanographer at NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, believe it is still unclear whether one warming effect would overpower or neutralize the other.

"Things are more complex," said Frank D. Marks Jr., director of NOAA's Hurricane Research Division, who holds that hurricane intensity is tied to more than just slight increases in ocean temperatures. "There are a lot of issues."

The study, published in Geophysical Research Letters, is believed to be the first to argue that global warming might result in weaker storms.

How might that happen?

Bear this in mind: To grow into a monster, the tepid air in a tropical storm has to ascend into the cooler high atmosphere. Blocking the rise dampens the storm.

General ocean warming would increase temperatures in the upper atmosphere. Warmer air rises over cooler, and a warm layer aloft would block the ascent.

In addition are the complex effects of a phenomenon known as "vertical wind shear."

In this scenario, warmer ocean surfaces would lead to large-scale changes in tropical precipitation patterns. Stronger west-to-east winds in the upper atmosphere over the eastern Pacific and subtropical Atlantic would blow across the tops of nascent storms before they could mature.

Vertical wind shear has been a significant factor in quiet hurricane seasons that coincided with El Niño, an abnormal warming of surface waters in the tropical Pacific. The warming causes shifts in storminess that displace drying, upper-level winds. Those winds invade the Atlantic Basin and stop tropical storms from blossoming. "The mechanism that leads to an increase in shear is similar to what happens during an El Niño event," said Soden.

El Niño is episodic, occurring every three to seven years, and is confined to an area of the central Pacific. It has a dampening effect only in the Atlantic Basin.

A more permanent warming of both the Atlantic and the Pacific would affect shear in ways that aren't entirely clear, but Vecchi said a variety of computer models suggested that significant impacts on future tropical-storm seasons were almost certain.

"We were surprised how much of a common feature it was across models," he said.

In interviews last week, he and Soden said they were well aware of the inherent paradox of attributing both the weakening and strengthening of hurricanes to global warming.

"I don't think this piece will quiet the discussion," said Vecchi. "It will have the opposite effect."

Animations of warming's impact on storms are at http://go.philly.com/earthEndText

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