The findings can be found in Geophysical Research Letters online and in print in the September 3, 2009 issue.

“As the size of landfalling hurricanes from the Gulf of Mexico increases, we’re seeing more tornadoes than we did in the past that can occur up to two days and several hundred miles inland from the landfall location,” said James Belanger, doctoral student in the School of Earth and Atmospheric Sciences at Georgia Tech and lead author of the paper.

Currently, it’s well known that when hurricanes hit land, there’s a risk that tornadoes may form in the area. Until now, no one has quantified that risk because observations of tornadoes were too sporadic prior to the installation of the NEXRAD Doppler Radar Network in 1995. Belanger along with co-authors Judith Curry, professor and chair of the School of Earth and Atmospheric Sciences at Tech and research scientist Carlos Hoyos, decided to see if they could create a model using the more reliable tornado record that’s existed since 1995.

The model that they developed for hurricane-induced tornadoes uses four factors that serve as good predictors of tornado activity: size, intensity, track direction and whether there’s a strong gradient of moisture at midlevels in the storm’s environment.

“The size of a tropical cyclone basically sets the domain over which tornadoes can form. So a larger storm that has more exposure over land has a higher propensity for producing tornadoes than a smaller one, on average,” said Belanger.

The team looked at 127 tropical cyclones from 1948 up to the 2008 hurricane season and went further back to 1920 modifying their model to account for the type of data collected at that time. They found that since 1995 there has been a 35 percent percent increase in the size of tropical cyclones from the Gulf compared to the previous active period of storms from 1948-1964, which has lead to a doubling in the number of tornadoes produced per storm. The number of hurricane-induced tornadoes during the 2004 and 2005 hurricane seasons is unprecedented in the historical record since 1920, according to the model.

“The beauty of the model is that not only can we use it to reconstruct the observational record, but we can also use it as a forecasting tool,” said Belanger.

To test how well it predicted the number of tornadoes associated with a given hurricane, they input the intensity of the storm at landfall, it’s size, track and moisture at mid-levels, and were able to generate a forecast of how many tornadoes formed from the hurricane. They found that for Hurricane Ike in 2008, their model predicted exactly the number of tornadoes that occurred, 33. For Hurricane Katrina in 2005, the model predicted 56 tornadoes, and 58 were observed.

The team’s next steps are to take a look to see how hurricane size, not just intensity (as indicated by the Safir-Simpson scale), affects the damage experienced by residents.

“Storm surge, rain and flooding are all connected to the size of the storm,” said Curry. “Yet, size is an underappreciated factor associated with damage from hurricanes. So its important to develop a better understanding of what controls hurricane size and how size influences hurricane damage. The great damage in Galveston from Hurricane Ike in 2008 was inconsistent with Category 2 wind speeds at landfall, but it was the large size that caused the big storm surge that did most of the damage.”

Ocean temperatures averaged 62.5 degrees Fahrenheit in the June-August period, 1 degree higher than normal.

Last month also saw the warmest August sea-surface temperatures  on record at an average of 62.4 degrees, also 1 degree higher than usual, NOAA’s National Climatic Data Center said.

The combined land and water temperature worldwide for June-August was 61.2 degrees, third warmest in 129 years of recordkeeping. For August it was 58.2 degrees, fourth warmest.

“Large portions of the world’s land mass observed warmer-than-average temperatures in August,” NOAA said in a statement. “The warmest departures occurred across Australia, Europe, parts of the Middle East, northwestern Africa, and southern South America. Both Australia and New Zealand had their warmest August since their records began.”

Meteorologists said there’s a combination of forces at work: A natural El Nino system just getting started on top of worsening man-made carbon emissions tied to global warming, and a dash of random weather variations.

The resulting ocean heat is already harming threatened coral reefs. It could also hasten the melting of Arctic sea ice and help hurricanes strengthen.

The Atlantic Ocean’s hurricane season peaks from mid-August to late October and averages five to six hurricanes per year.

Hurricanes begin as tropical disturbances in warm ocean waters with surface temperatures of at least 80 degrees Fahrenheit (26.5 degrees Celsius). These low pressure systems are fed by energy from the warm seas. If a storm achieves wind speeds of 38 miles (61 kilometers) an hour, it becomes known as a tropical depression. A tropical depression becomes a tropical storm, and is given a name, when its sustained wind speeds top 39 miles (63 kilometers) an hour. When a storm’s sustained wind speeds reach 74 miles (119 kilometers) an hour it becomes a hurricane and earns a category rating of 1 to 5 on the Saffir-Simpson scale.

Hurricanes are enormous heat engines that generate energy on a staggering scale. They draw heat from warm, moist ocean air and release it through condensation of water vapor in thunderstorms.

Hurricanes spin around a low-pressure center known as the “eye.” Sinking air makes this 20- to 30-mile-wide (32- to 48-kilometer-wide) area notoriously calm. But the eye is surrounded by a circular “eye wall” that hosts the storm’s strongest winds and rain.

These storms bring destruction ashore in many different ways. When a hurricane makes landfall it often produces a devastating storm surge that can reach 20 feet (6 meters) high and extend nearly 100 miles (161 kilometers). Ninety percent of all hurricane deaths result from storm surges.

A hurricane’s high winds are also destructive and may spawn tornadoes. Torrential rains cause further damage by spawning floods and landslides, which may occur many miles inland.

The best defense against a hurricane is an accurate forecast that gives people time to get out of its way. The National Hurricane Center issues hurricane watches for storms that may endanger communities, and hurricane warnings for storms that will make landfall within 24 hours.

Some studies reveal that tropical storms around the world are intensifying, with computer models suggesting a shift toward extreme intensity. A big question on many minds is, “Does the warming of the earth have a direct effect on the strength of hurricanes?” Opinions are varied.

Scientists caution that one must consider questions of climate change over decades, even centuries. A particularly rough hurricane season or two cannot be blamed on global warming.

Preliminary evidence suggests that, once hurricanes form, they will be stronger if the oceans are warmer. However, much uncertainty exists about whether hurricanes and other storms will become more frequent.

According to the Geophysical Fluid Dynamics Laboratory, which assesses natural climate variability, “The strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the earth’s climate is warmed by increasing levels of greenhouse gases in the atmosphere. Although we cannot say at present whether more or fewer hurricanes will occur in the future with global warming, the hurricanes that do occur near the end of the 21st century are expected to be stronger and have significantly more intense rainfall than under present day climate conditions.” This is based upon an anticipated increase of energy from higher sea surface temperatures.

A study published in the Journal of Climate indicates that an 80-year buildup of atmospheric CO2 at 1% per year (compounded) leads to roughly a one-half category increase in potential hurricane intensity on the Saffir-Simpson scale, and an 18% increase in precipitation near the hurricane’s core. Of course, if emissions of CO2, along with other greenhouse gases, were to be higher, then hurricanes could potentially become even stronger.

However, many other scientists are quick to point out that since the 1940s, there has been an overall decrease in hurricane activity. According to the United Nations Environment Program of the World Meteorological Organization, “Reliable data…since the 1940s indicate that the peak strength of the strongest hurricanes has not changed, and the mean maximum intensity of all hurricanes has decreased” CNSNews.

In September 2004, in response to some labeling the busy hurricane season a byproduct of global warming, a group of climatologists, scientists, professors and other experts in climate change stated, “Computer simulations suggest that in a warmer world most of the warming would occur in the Polar Regions. Atmospheric circulation, which crucially affects storms, is driven primarily by the temperature difference, or gradient, between the tropics and the poles.

“Warmer polar regions would reduce this gradient and thus lessen the overall intensity or frequency or both of storms-not just tropical storms but mid-latitude winter storms as well (such as blizzards and northeasters).

“Again, longer periods of history bear this out. In the past, warmer periods have seen a decline in the number and severity of storms. This is well-documented in scientific journals for data extending back centuries or even millennia. If the surface temperature of the planet rises further in the future, it is likely that these declines will continue” (ibid.).

Some researchers believe other factors-including La Nina and other big weather systems-will overpower any effect global warming might have on hurricanes.