Wind shear may cancel climate's effect on hurricanes

18:15 18 April 2007
NewScientist.com news service
Phil McKenna
Hurricane activity in the Atlantic may not increase as a result of global warming, according to a new study focusing on changes in tropical wind patterns.
The findings appear to contradict a number of recent studies linking warming waters in the region to an increase in hurricane intensity and frequency. The new study suggests that increases in vertical wind sheer – differences in wind direction and speed between the upper and lower levels of the atmosphere – caused by climate change could counter-balance the affects of warming waters.
“Based on this study, there is no evidence for a strong increase in hurricane activity in the Atlantic over the next century due to global warming," says Brian Soden at the University of Miami in Florida, US, who carried out the research with colleagues.
Hurricane forecasters have long known that increases in vertical wind shear make it harder for tropical storms to form and to increase in intensity. After examining 18 different climate models, Soden's team predict a “robust increase” in vertical sheer over the Atlantic as result of climate change.
Walker circulation
Upper-atmosphere westerly winds will likely intensify “shearing” against easterly winds in the lower atmosphere, Soden says. Changes in the Walker circulation – an enormous wind pattern over the Pacific, thought to have wide-reaching affects on global weather patterns – will cause this increase in shearing, the researchers say.
“We are seeing a slowing and an eastern shift in the Walker circulation which we are projecting will cause a long term increase in shear over the Atlantic,” says Gabriel Vecchi of the National Oceanic and Atmospheric Administration in Princeton, New Jersey, who took part in the study.
Vecchi says similar short-term shifts in the Walker circulation during El Niño years have caused increased shear over the tropical Atlantic, though exactly how one affects the other remains unclear. Soden notes that shear will not increase in all areas but says its affect on the Atlantic could be significant.
Conflicting models
“Exactly how much increased shear will counter the affects of warming water is hard to tell,” Soden admits, “but the magnitude of changes we are seeing [in shear] are comparable to those of warming oceans.”
Hurricane expert Kerry Emanuel, at MIT in the US, disagrees: “Sensitivity to shear may be overestimated,” he says of the study. Emanuel published a modelling study in October 2006 (Journal of Climate study, DOI: 10.1175/JCLI3908.1) suggesting a 10% increase in water temperature would increase hurricane intensity by 65%, whereas a 10% increase in shear would decreases hurricane intensity by only 12%.
Soden says the new findings do not challenge ideas on climate change. “It simply adds another factor that has to be considered when examining global warming’s affects on hurricanes.”
Journal reference: Geophysical Research Letters (DOI: 10.1029/2006GL028905)