While some research has linked global warming with increased intensity of hurricanes, a recent study has found that warming might also contribute to vertical wind shear, which has long been viewed as an inhibitor of hurricane development.

"Global climate model simulations for the 21st century indicate a robust increase in vertical wind shear in the tropical Atlantic and East Pacific oceans, which could inhibit the development or intensification of hurricanes in these regions," said the National Oceanic and Atmospheric Administration (NOAA). "Historically, increased vertical wind shear has been associated with reduced hurricane activity and intensity."

The findings are reported in a study by scientists at NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, NJ, and the Rosenstiel School of Marine and Atmospheric Science at the University of Miami. They said this is the first study to indicate that changes to vertical wind shear in future climate projections would likely diminish the frequency and intensity of hurricanes.

"Wind shear is one of the dominant controls of hurricane activity, and the models project substantial increases in the Atlantic," said Gabriel Vecchi, the paper's lead author and a NOAA research oceanographer. "Based on historical relationships, the impact of the projected shear change could be comparable in magnitude as that of the warming oceans -- with the opposite effect."

Examining possible impacts of anthropogenic greenhouse warming on hurricane activity, the researchers used climate modeling to assess large-scale environmental factors tied to hurricane formation and intensity. They focused on projected changes in vertical wind shear over the tropical Atlantic and how those changes tie to the Pacific Walker circulation. The Walker circulation is a vast loop of winds that influences climate across much of the globe and varies during El Nino and La Nina oscillations.

Earlier this month, the Colorado State University hurricane forecast team suggested that dissipating El Nino conditions late this past winter suggest that vertical wind shear will be less prominent this hurricane season and less of an inhibitor of hurricanes (see Daily GPI, April 4).

By reviewing results from 18 different models, the authors identified a robust increase in wind shear over the tropical Atlantic and East Pacific attributable to global warming. Multiple models, in close agreement, project long-term increases in vertical wind shear, which are closely linked to a weakened Walker circulation over this century. The research suggests that the increase in vertical wind shear could inhibit both hurricane development and intensification.

"The models send a fairly clear message that we can't dismiss vertical wind shear as we look at the long-term effect of global warming on hurricanes," Vecchi said. He added that the model projections of an increased wind shear are confined to the tropical Atlantic and East Pacific, and that factors other than global warming contribute to change in Atlantic wind shear. "This doesn't settle the issue," he said. "It's one piece of the puzzle that will contribute to what is an incredibly active field of research."

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