WASHINGTON, D.C.--When the weather turns hot and sticky, hurricanes force thousands of Americans to batten down their hatches. Some storms ravage states along the Gulf of Mexico, while others menace the Eastern Seaboard. Now, researchers have proposed that a climate pattern over the Atlantic Ocean steers most hurricanes toward one region or the other, but not both, for decades at a time.
Climatologists already knew that El Niños in the Pacific seem to suppress major Atlantic hurricanes, while the cooler La Niña seasons spur them on. That may explain the number of storms, but it doesn't predict where they'll strike. So climatologist James Elsner of Florida State University in Tallahassee and his colleagues looked for ties between hurricane tracks and the North Atlantic Oscillation (NAO), a seesaw shift in atmospheric pressures over the ocean (Science, 12 February 1999, p. 948). When the NAO is intense, a fair-weather ridge of high pressure dominates the north-central Atlantic. During a weak NAO, this "Bermuda High" sags southwest toward Florida.
The team compared records of this atmospheric pattern with records of hurricane landfall during the last 150 years and found a suggestive statistical link, Elsner reported on 19 February at the annual meeting of the American Association for the Advancement of Science, which publishes ScienceNOW. In years of strong NAOs, most major hurricanes curved along the southern margin of the high-pressure ridge and then turned north to plow into the East Coast. But when the Bermuda High drifted farther south, it deflected big storms through the Caribbean and into the Gulf of Mexico. Elsner will also describe his results in an upcoming issue of the Journal of Climate.
Although the NAO pattern waxes and wanes, Elsner notes, it often lingers for decades at a time. This may explain why the biggest storms tend to strike either the Gulf Coast or the Atlantic Coast in batches that persist for 20 to 30 years, he says.
"The evidence seems strong" for a link between the NAO and preferred hurricane tracks, says climatologist Kerry Emanuel of the Massachusetts Institute of Technology in Cambridge. However, he notes, "until more rigorous statistical tests are done, I think the jury is still out." If his team can prove its case, Elsner says, forecasters could use Atlantic climate to gauge which stretches of coastline may bear the brunt of storms during a given season.