Hurricanes to Be Sapped, Not Strengthened, by Warming?

Sara Goudarzi
for National Geographic News

April 18, 2007

Climate change could lead to stronger wind shear, a weather pattern that weakens hurricanes, a new study says. Some previous, widely publicized studies have linked global warming to stronger hurricanes.

(See "Warming Oceans Are Fueling Stronger Hurricanes, Study Finds" [March 16, 2006] and "Global Warming Link to Hurricane Intensity Questioned" [July 28, 2006].)

Previous simulations had found that as global atmospheric temperatures rise, sea surface temperatures rise as well.

And because warm ocean waters fuel hurricanes, that temperature rise was predicted to increase hurricanes (interactive: how hurricanes form).

The models, however, also project that the difference in wind speed and direction—an effect known as wind shear—will also increase due to rising temperatures.

An increase in wind shear could counteract the effect of rising sea surface temperatures and actually inhibit hurricane formation.

A Hundred Years of Hurricanes

In the tropical Atlantic, near-surface winds generally blow from east to west while winds in the upper atmosphere blow from west to east.

If the difference in the wind speed and direction between the two layers ramps up, the resulting mechanism could put the brakes on storms by tearing them apart.

"The models project that that difference should get larger in a warming world," said Gabriel Vecchi, an oceanographer at the U.S. Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey.

Vecchi is the lead author of a paper detailing the new hurricane model published today in the journal Geophysical Research Letters.

"We're not arguing at all that hurricanes are going to disappear under global warming," Vecchi said.

The increase in wind shear is not an isolated phenomenon, and so must be put into the context of a warming world where there is an increase in hurricane activity as well.

"We just don't know how the two of them will combine."

Vecchi and Brian J. Soden of the University of Miami used 18 climate models to assess factors linked to hurricane formation from year 2000 to 2100.

The different simulations mimic how certain systems would interact with each other under various climate scenarios.

Specifically, the researchers looked at how the wind shear over the Atlantic ties to the Pacific Walker circulation, a pattern of airflow above the equatorial Pacific Ocean that affects climate around the globe.

The results showed that, in response to rising temperatures, there would be a slowing of the Pacific Walker circulation, which would increase wind shear.

Averaging Results

Kerry A. Emanuel is a professor of atmospheric science at the Massachusetts Institute of Technology.

He said, "I think it's a very interesting study, and the technique they're using is pretty good."

"The only reservation I have about it is, to do this, [the researchers] took the output from 18 climate models and they kind of averaged these results," said Emanuel, who was not involved in the study.

They're "averaging a lot of things together to get a consensus, which is almost like treating these models democratically. Not all models are created equal. Some of these models are pretty awful."

Emanuel has no doubts, though, that an increase in wind shear could have profound effects on hurricanes.

In a computer simulation he ran last year, Emanuel uniformly increased the wind shear by 10 percent, finding that it caused hurricane power to decrease by almost 12 percent.

"[This] is consistent with what [Vecchi and Soden] showed," Emanuel explained. "The problem is that the atmosphere that you get from averaging these results may not be the atmosphere that any one model could ever produce."

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