Altered Atlantic winds may hamper hurricanes

Increased vertical wind shear–a tearing action which can pull a storm apart and is caused by differences in wind speed or direction with altitude–historically has been associated with reduced hurricane activity and intensity. While other studies have linked global warming to greater hurricane intensity, this study is the first to identify changes in wind shear that could counteract that effect, the scientists say.

“Wind shear is one of the dominant controls to hurricane activity, and the models project substantial increases in the Atlantic,” says oceanographer Gabriel A. Vecchi of the National Oceanic and Atmospheric Administration (NOAA) in Princeton, New Jersey.

Using 18 different models, he and Brian J. Soden of the University of Miami, Florida, assessed changes in environmental factors linked to hurricane formation and intensity. In particular, they investigated potential variation in vertical wind shear over the tropical Atlantic and its ties to the Pacific Walker circulation. That vast loop of winds influences climate across much of the globe and varies in concert with El Nino and La Nina oscillations. In the new work, the models mostly predict a slowing of the Pacific Walker circulation, leading to greater wind shear throughout much of the tropical Atlantic.

“The impact on hurricane activity of the projected shear change could be as large — and in the opposite sense — as that of the warming oceans,” Vecchi says. In other regions, such as the western tropical Pacific, the study finds that global warming renders the environment more favorable for hurricanes.

Vecchi and Soden report their findings today in Geophysical Research Letters, a journal of the American Geophysical Union.

The simulations incorporated a mid-range emissions scenario from the Intergovernmental Panel on Climate Change Fourth Assessment — the latest of those assessments. According to the emissions scenario, the concentration of atmospheric carbon dioxide stabilizes at 720 parts per million by the year 2100. The wind-shear study examines two 20-year periods during this century: 2001-2020 and 2081-2100.

Vecchi notes that projections of increased wind shear found in the study are confined to the tropical Atlantic and East Pacific.

Moreover, factors besides global warming also contribute to change in Atlantic wind shear. The new simulations, Vecchi adds, provide “one piece of the puzzle” of how increased wind shear may affect hurricane activity.

For animations and still images depicting wind shear's impact on a hurricane, please visit the NOAA web site at: http://gfdl.noaa.gov/~gav/ipcc_viz.html