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UW researcher contributes to study linking warmer seas to stronger hurricanes

The Capital Times  —  9/04/2008 5:13 pm

A University of Wisconsin-Madison researcher contributed to a new study that bolsters the theory that global warming might be contributing to stronger hurricanes in the Atlantic Ocean over the last 30 years.

The study, published Thursday in the journal Nature, was led by Florida State University geography researcher James Elsner, with UW-Madison research scientist James Kossin and FSU researcher Thomas Jagger contributing.

The researchers used global satellite data to find that the strongest tropical cyclones -- which include hurricanes, typhoons and tropical storms -- are getting stronger, and that ocean temperatures play a role in the trend.

"As seas warm, the ocean has more energy that can be converted to tropical cyclone wind," Elsner said in a written statement.

The study does not prove the "heat-engine" theory of cyclone intensity, but it does show that the data are quite consistent with it, Elsner said.

Kerry Emanuel of the Massachusetts Institute of Technology first suggested the possible connection between global warming and increases in tropical cyclone intensity in 2005. He linked the increased intensity of storms to the heating of the oceans, which has been attributed to global warming. Emanuel's theory is that the intake of warm air near the ocean surface and the exhaust of colder air above the cyclone is what drives a hurricane.

Critics argued that the data were not reliable enough to make assertions about the relationship between climate change and hurricanes. Moreover, scientists did not see an upward trend in total tropical cyclones annually. They occur about 90 times per year worldwide.

Elsner's team addressed both issues by using satellite-derived tropical cyclone wind speeds as opposed to the observational record and by focusing on the highest wind speeds of the strongest tropical cyclones each year.

They looked at the storms that were closest to their maximum possible intensity. Under the heat-engine theory, every storm loses some energy through inefficiency, and that loss limits the storm's potential. The MPI represents the storm's maximum potential under ideal environmental conditions, for instance, without wind shear that might prevent a hurricane from strengthening.

"We speculated that you might not see a trend in the intensity of typical hurricanes due to environmental factors, but if the heat-engine theory is correct, you should see a trend in the intensity of hurricanes at or near their MPI," Elsner said.

The researchers found that the strongest tropical cyclones are getting stronger, particularly over the North Atlantic and Indian oceans. Wind speeds for the strongest tropical storms increased from an average of 140 mph in 1981 to 156 mph in 2006, while the ocean temperature, averaged globally over the all regions where tropical cyclones form, increased from 28.2 degrees Celsius to 28.5 degrees Celsius during this period.

"By creating a better, more consistent historical data set, we've been able to weed out quality issues that introduce a lot of uncertainty," Kossin said. "Then, by looking only at the strongest tropical cyclones, where the relationship between storms and climate is most pronounced, we are able to observe the increasing trends in storm intensity that both the theory and models say should be there."

The research was supported financially by the National Science Foundation and the Risk Prediction Initiative of the Bermuda Institute for Ocean Studies.

The Capital Times  —  9/04/2008 5:13 pm