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. Other factors being equal, the warmer the ocean, the warmer the intake of air. This heat-engine theory of how hurricanes increase their intensity is well accepted, but there are many environmental factors, such as wind shear, that might prevent a hurricane from strengthening, Elsner said.

To address that problem, Elsner’s team looked at a subset of hurricanes that are closest to their maximum possible intensity (MPI).

Under the heat-engine theory, every storm will lose some energy through inefficiency, and that loss will limit the storm’s potential.

The MPI represents the storm’s maximum potential under ideal environmental conditions.

“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.

“On average, the strongest storms are closest to their MPI.”

The researchers created a data set from satellite observations of hurricane intensity of all tropical cyclones around the globe and looked at the maximum wind speeds for each one during a 25-year period.

Tropical cyclones, which include hurricanes, typhoons and tropical storms, occur on average about 90 times per year worldwide.

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.

... contd.