ODU's Tuleya is Author of Study Predicting More Intense Hurricanes by Century's End

A team of scientists including Robert Tuleya of Old Dominion University's Center for Coastal Physical Oceanography (CCPO) turned up cautionary results from a study that paired an ensemble of climate-change projection models with tried and true models that the United States government uses in hurricane forecasting.

The scientists' paper about Atlantic hurricanes, which appeared last month in Science magazine, warns that the number of intense, category 4 and 5 hurricanes could double by the end of the 21st century. Furthermore, the strong hurricane activity may target highly populated areas in the eastern United States.

Morris Bender, a scientist with the National Oceanic and Atmospheric Administration (NOAA), is the lead author of the article. He and other authors, Thomas Knutson, Joseph Sirutis, Gabriel Vecchi, Stephen Garner and Isaac Held, are with NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, N.J.

Tuleya joined ODU after he retired in 2002 from GFDL, where he served as head of the Hurricane Dynamics Group. He and Bender worked together in the 1990s to develop the GFDL hurricane model.

At ODU as an adjunct professor and self-supporting research professional, Tuleya has continued to work on contract with NOAA to improve the GFDL hurricane model and to help develop the next generation Hurricane Weather and Forecasting (HWRF) model, which is now used as guidance by the National Hurricane Center. Altogether, he has devoted nearly 40 years of his career to devising computer models that predict tropical storm formation, intensity and movement of present day storms and investigating the impact of climate change on tropical storms in the future.

"This paper reports results of the most objective and thorough investigation of its kind to date," Tuleya said in an interview.

The title of the article in Science, which is one of the world's most prestigious scientific journals, is "Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes." Results reported are in line with other studies suggesting that the overall frequency of Atlantic tropical storms and hurricanes could decrease as the climate warms.

For example, a recent GFDL study using a regional atmospheric model called ZETAC predicted a reduction by 18 to 27 percent in the total number of Atlantic tropical storms and hurricanes by the end of the century if the climate warms according to the projections of the Intergovernmental Panel on Climate Change.

But the authors contend that many models, including the regional ZETAC, are incapable of simulating high-intensity hurricanes. Their new modeling and simulation strategy - employing the so called "operational" GFDL hurricane model that that has been used in one form or another for the past 15 years to track and predict the intensity of tropical storms and hurricanes - simulated a more realistic distribution of intense hurricane winds than the ZETAC regional model in a control period of 1980-2006.

"We explored the influence of future global warming on Atlantic hurricanes with a downscaling strategy by using an operational hurricane-prediction model," the researchers write in the article. "The model projects nearly a doubling of the frequency of category 4 and 5 storms by the end of the 21st century…when the downscaling is based on the ensemble mean of 18 global climate-change projections."

Several of the climate-change projection models in the ensemble of 18 were run individually, and the authors note that one predicted that climate warming over the century will bring a significant drop in the number of tropical storms as well as all categories of hurricanes. That model includes a relatively large projected increase in wind shear in the south Atlantic and Caribbean Sea. Recent studies have shown that global warming increases wind shear, which can inhibit the development or intensification of tropical storms and hurricanes.

The authors also say that their results indicate that the influence of global warming caused by humans may not have yet had an impact on hurricane intensity. They don't expect that impact to push up intensities until the latter half of the century.

"A big question has involved the observed increasing frequency or intensity over the last 20 years," Tuleya said. "Some are attributing this to anthropogenic (human-caused) global warming, but it appears that a good bit of this signal may be natural variability."

The paths of these intense hurricanes that are predicted to come more frequently in the latter part of the century tend to be across the western Atlantic north of latitude 20 degrees north. This would put much of the eastern United States in jeopardy.

One analysis cited by the researchers found that category 4 and 5 hurricanes accounted for only 6 percent of the hurricanes with United States landfalls from 1900 to 2005, but caused nearly half of the country's hurricane damage during that period. In the latest study, the ensemble model as well as two of the models that were run individually predicted greater hurricane damage potential in the United States by the century's end even though the overall number of storms is projected to decline.

The authors point out that their findings are dependent on the global climate models used to provide the environmental conditions for our downscaling experiments. "Future studies should reassess our findings using both updated climate model projections and improved hurricane simulation models," they write.