Image: Frances and Ivan
An infrared satellite image from the National Oceanic and Atmospheric Administration shows Hurricane Frances at upper left over Florida on Sunday, with Hurricane Ivan at lower right, about 800 miles east-southeast of Barbados.
By Alan Boyle Science editor
updated 9/10/2004 11:30:45 AM ET 2004-09-10T15:30:45

First, Florida was hit by Hurricane Charley. Then came Hurricane Frances. Now there's Hurricane Ivan. What's behind this year's upsurge in violent storms?

Some studies, based on computer modeling, indicate that hurricanes could become more intense if sea surface temperatures rise in accordance with long-term climate change — but scientists say it's too soon to blame global warming this time around.

Rather, they say, the weather pendulum is swinging against Florida after decades when the state experienced fewer hurricanes than normal.

“The people of Florida have been very unlucky this year, but from a 38-year perspective, they've been lucky,” said Colorado State University climate researcher William Gray, who has issued annual hurricane forecasts for the past 21 years.

This year's string of storms is in line with Gray's prediction for an unusually active hurricane season, but even he admits that the past month was more active than he expected, with eight named tropical storms (Alex through Hermine) in August.

"We predicted above average for August, but nothing like this," he told

The culprit, Gray and other weather forecasters say, is a pattern of atmospheric and ocean flow known as the Atlantic thermohaline circulation — the same pattern that provided the plot twist in the global-warming disaster movie "The Day After Tomorrow." Since 1995, a shift in that pattern has resulted in the warmer Atlantic seas that fuel stronger hurricanes.

"These are natural things," Gray stressed. "Greenland ice-core data show that North Atlantic temperatures swing back and forth on these 30- to 40-year time scales. The people who say humans are causing this hurricane activity — typically they're the ones who don't know anything about tropical cyclones."

Gray counted up how often major hurricanes — that is, a Category 3 storm or above — made landfall in Florida. "In the 40 years from 1926 to 1965 there were 14 major landfalling storms that hit," he said. "In the 38 years since then, there was only one: Andrew."

Most researchers agree with Gray that hurricane activity rises and falls over the course of decades. Thus, the hurricane pattern could merely be going back to the active phase experienced during the mid-20th century.

Yet another factor added to Florida's bad luck this year: Over the past decade or so, a low-pressure trough in the Atlantic has helped curve hurricanes to the north and away from the Florida coast.

"This year, the trough is not there," Gray said. "We tend to have more of a ridge, and there's more westward steering."

The latest forecast holds some small consolation: The El Niño weather pattern in the Pacific has been linked to a less active hurricane season in the Atlantic, and a weak El Niño may be in the works. Under those conditions, "October tends to be below average" in hurricane activity, Gray said.

But Gray said the longer term could well promise more of what Florida has been experiencing this year, because it takes decades for the weather pendulum to swing back.

"If the future is like the past, it should go for 30 to 40 years," he said. "We've been in it for 10 years now, so who knows? Maybe another 20 or 25 years."

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