COLLEGE STATION -- Texas A&M University atmospheric scientist Robert Korty is using computer models to study how hurricanes behave in climates ranging from the last ice age to the predicted warmer world of the future. With hurricane season set to start June 1, he may soon have more examples to study.
The first obstacle to understanding any relationships between climate and hurricanes, Korty says, is that there are only 30 years worth of good data. People have been dealing with hurricanes for ages, but before satellites, a storm in a remote part of the ocean was recorded only if a ship passed by. Scientists can make more observations now, and while not as good as direct measurements, satellite data can tell a lot about hurricane intensity, he observes.
“Satellite images can tell us temperature and cloud heights, which correlate with storm strength,” Korty says.
Increasing ocean temperatures are expected to increase hurricane intensity, but it’s hard to say just how much stronger. To better understand this relationship, Korty simulates storms in both warm and cool climates. He feeds temperature data from climate research models into higher-resolution, regional models similar to those used in hurricane forecasting. This allows Korty to turn the temperature down to that of the last ice age, or up to temperatures predicted 100 years from now. Early indications from these simulations showed Korty that hurricanes are expected to be stronger in a warmer climate. But surprisingly, he observes, storms may have been more frequent in a cold climate despite being more commonly associated with warm water. The Texas A&M researcher says atmospheric circulation has more to do with setting areas of storm formation than ocean temperatures, and this structure is present in both hot and cold climates.
“The large-scale structure of the atmosphere is set by the speed the Earth spins,” Korty notes. Humidity, he says, is the key difference between these two climate scenarios.
The air needs a certain level of moisture for hurricanes to form, and air five kilometers above the ground in the tropics near the equator is relatively dry. The saturation point is higher in a hot climate and lower in a cold one, so while warmer air holds more moisture, gathering clusters of thunderstorms have to add more water to the atmosphere when it’s hot.
“It takes longer for a hurricane to form, which makes it more susceptible to things like wind shear,” Korty says, adding that this means hurricanes form more easily in cold climates, but they usually end up weaker.
Destructive hurricanes like Katrina and Ike and the prospect of a warming climate are driving interest in how climate affects hurricanes. With a short history of observations and lots of seasonal variability, predicting how storms will behave 100 years from now is difficult. But by simulating storms in a variety of climates, Korty is working to make that task easier.
“We want to find out how nature sets the environment for tropical storms and see if there are flaws in our understanding,” Korty says.