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Adverse wind conditions may reduce the frequency of tropical cyclones due to global warming — but those forming will be stronger, researchers say.

The findings are contrary to the widely held theory that global warming will increase the frequency of tropical cyclones, known as hurricanes in the Atlantic Ocean.

A team led by David Nolan from the University of Miami in the United States developed new computer-based models to predict the impact of warmer ocean temperatures and wind conditions on tropical cyclones.        

The models showed that, as expected, the frequency of tropical storms increased with warmer ocean temperatures. But when 'wind shear' — the change in wind direction and speed with height — was studied they found that, as water temperature increased, the shear helped suppress formation of hurricanes to a greater extent.

"We expected to find that when oceans warm, hurricanes would form more quickly, even in the presence of adverse wind conditions we call wind shear," says Nolan. "Instead we found the opposite: adverse wind shear did more damage to developing storms when sea surface temperature was increasing."

Ocean temperatures and wind shear are considered the two most important factors for predicting hurricanes, both in regular weather prediction studies and climate change studies.

Nolan says the new models are complementary to existing ones, which are able to simulate global weather but do not simulate hurricanes accurately. In contrast, the new models are able to simulate hurricanes in small localities.

The study was published in Geophysical Research Letters (July 2008). The researchers hope their model will lead to a better understanding of the relationship between climate and global hurricane activity.

In a separate study, reported in the August issue of the same journal, Brazilian and Indian scientists noted a decrease in easterly wind shear in the tropics, favouring the formation of strong cyclones.

The study notes that, for the first time in recorded history, a category five tropical hurricane formed in June 2007 together with two more severe tropical storms over the northern Indian Ocean.

Tropical cyclones in the Bay of Bengal region usually occur in the non-monsoon months. But the researchers suggest that if the weakening trend continues, there could be more severe cyclones, even during the summer monsoon months of June to September.


Geophysical Research Letters, doi 10.1029/2008GL034147 (2008)

Geophysical Research Letters, doi 10.1029/2008GL034729 (2008)