Will Our Luck Run Out? Major Hurricane Gap in U.S.

By Andrew Freedman

When the 2012 North Atlantic hurricane season starts on June 1, the U.S. will have gone a record 2,412 days since the last major hurricane — with sustained winds of 111 mph or greater —  struck these shores. The last storm to strike with such fury was Hurricane Wilma, which made landfall in Southwest Florida in October 2005.

Will our luck run out this year? If it does, it’s not likely to be pretty. After all, a lot has changed since 2005. For one thing, there are now more people, with more property, living and working in vulnerable coastal areas than there were seven years ago. The population of Florida alone grew by 17 percent between 2000 and 2010, with about a million people added since 2005, according to U.S. Census data. And the U.S. economy is also a lot shakier, and less capable of absorbing the shock from a devastating natural disaster.

Chart created by U. Colorado researcher Roger Pielke Jr., showing the record gap between landfall dates of major hurricanes in the U.S. Click on image for larger version.

But why has the country been so lucky, and what does this major hurricane drought suggest about the upcoming hurricane season, which is off to an unusually active start?

According to hurricane researchers, the spell of relative calm between major hurricanes is mainly due to the random variability that is inherent in the weather and climate. Historical records of landfalling storms in the U.S. show no clear, statistically significant trend over time, despite changes to the environment in which hurricanes are forming, such as global warming.

Once they form over the warm waters of the North Atlantic Basin, hurricanes are steered by both internal dynamics and the weather systems that surround them. Large-scale climate features, like the so-called “Bermuda High” that sprawls out across the Central North Atlantic during the summer, also help determine whether a hurricane will churn westward toward the U.S. coast, or curve harmlessly out to sea.

The clockwise flow of air around that High helps steer Atlantic hurricanes west towards the Caribbean, and then bends them northwest, and eventually northward. Where that turn takes place is crucial for determining a storm’s eventual path. A storm that curves very late is more likely to make landfall in the U.S. than one that turns earlier and farther offshore.

Hurricane tracks during the 2010 season. Colors indicate intensity, with orange colors corresponding to major hurricane intensity. Credit: Wikimedia Commons.

The 2010 hurricane season is a great example of how large-scale climate factors can work in the country’s favor. Typically, about 25 percent of the total number of hurricanes that form in the North Atlantic Basin make landfall in the U.S., but in 2010, the five major hurricanes that formed took one look at the U.S. mainland and sped off in another direction. So did the other seven weaker hurricanes that season.

A study published last year in the journal Geophysical Research Letters shed some light on why the U.S. went unscathed in 2010. According to a research team led by Chunzai Wang of NOAA, the area of warm water that typically supports hurricane development was unusually broad that year, about twice its typical size. This helped spark tropical storms and hurricanes farther east than they typically form, making it harder to reach the country without being deflected by a weather system first.

The broad expanse of warm water that year also nudged the Bermuda High out of its normal position in the Central Atlantic, like a soccer goalie drawn away from the net, and this helped change the steering currents in the upper atmosphere. As a result, there was a predominantly offshore flow of air along the East Coast in 2010, which acted as a roadblock to any hurricane that dare approach the area.

Of course, the U.S. hasn’t avoided every tropical storm or hurricane since 2005, just the most powerful ones. It doesn’t take a major hurricane to cause a lot of death and destruction. Hurricane Ike, a strong Category 2 storm, laid waste to the Texas coastline near Galveston in 2008, for example.

And last year, Hurricane Irene caused at least 45 deaths and more than $7.8 billion in damage. And it was only a tropical storm when it passed over Manhattan and headed up the Connecticut River Valley. But what Irene lacked in wind speed, she made up for in rainfall rates, causing deadly flooding in the Mid-Atlantic and Northeast.

Irene proved that there “really is no such thing as a weak hurricane,” said outgoing National Hurricane Center director Bill Read.

Nearby countries have been hit hard in recent years as well. “While the U.S. has been spared, the same cannot be said for Belize, Mexico, Nicaragua, Cuba, etc.,” said Brian McNoldy, a senior research associate at the University of Miami's Rosenstiel School of Marine and Atmospheric Science.

As the Climate Warms, Where Will the Hurricanes Go?

The question of how climate change may alter hurricane tracks looms large over any discussion of the seven-year major hurricane drought. Studies have shown that a combination of natural climate variability and global warming have warmed sea surface temperatures in the North Atlantic, which should affect hurricane activity, and there is evidence that climate change is altering the jet stream in the Northern Hemisphere, which may affect storm tracks.

Forecasters have more skill now at predicting the short-term track of hurricanes, as seen with Hurricane Irene last year.

While the scientific community has come to a consensus that with global warming, hurricanes are likely to become somewhat stronger, wetter and less frequent, there is very little evidence that storm tracks have already shifted, or even whether they are going to change in appreciable ways.

Tom Knutson, who works at NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J., said that there is a large amount of variability in the historical record of landfalling storms, and researchers have more confidence making global projections than they do about what will happen in the Atlantic specifically.

Kerry Emanuel, a meteorology professor at MIT, said the seven-year gap between major hurricanes in the U.S. is most likely just due to chance. “Seven years is simply far too short to see global warming signals in U.S. landfalling hurricane statistics of any kind,” he said via email.

Emanuel said simulations using sophisticated computer models show that as the climate warms there may be a northeastward shift in the locations where hurricanes tend to form, but he cautioned that there is a great deal of uncertainty in such findings. “This is consistent with a generally greater frequency of landfalls in the Mid-Atlantic and Northeast, and a diminution in the western Gulf [of Mexico]. But there is a great deal of variation from one model to the next . . . ,” he said.

According to NOAA’s 2012 Atlantic hurricane outlook, released May 24, the upcoming hurricane season is likely to feature “near normal” levels of storm activity, with a 70 percent chance of having nine to 15 named storms, of which one to three of them will become major hurricanes. It's not yet clear how the major steering currents will set up this year, so where most of the storms will go is anybody's guess.