There's nothing like a monster bigger than Florida to throw a scare into people, and Hurricane Floyd's 140-mph winds threw one from Miami to Montauk last week. Officials from governors on down were suddenly staunch believers in the precautionary principle, declaring states of emergency, closing schools, deploying more than 8,000 National Guardsmen and ordering the largest evacuation (more than 3 million people) in the country's history--before a single leaf trembled. Lines of cars hundreds of miles long snaked inland from coastal cities like Savannah, Ga., and Myrtle Beach, S.C., Barrier islands emptied, and enough plywood was nailed over windows to erect a small town.
When Floyd finally materialized into something more than a mesmerizing swirl on The Weather Channel, however, it turned out to be extremely wet but not that wild. New Jersey and North Carolina suffered the worst flooding, with stranded motorists rescued by scuba divers and people plucked from rooftops and trees by helicopter. But even though Floyd fell short of the hype, it was far more than a fizzle. It was, scientists say, a dry run.
Although Floyd will not be remembered as the storm of the century (it caused more than 30 deaths, most in traffic accidents, and billions of dollars in damage), the tests of emergency preparedness that it triggered will come in handy. That's because the United States has entered into a period of Atlantic hurricane activity more intense than any since the 1960s, says meteorologist William Gray of Colorado State University. The years from 1900 to 1925 were meteorologically quiescent, the mid-1940s to mid-1960s were tempestuous and the 30 years since were quiet again. The results of the most recent lull in the storms were all too predictable: developers who thought the laws of nature had been repealed built houses on beaches and barrier islands; people who had never experienced a Category 4 or 5 hurricane, the strongest kind, bought them. The effect: Texas to Maine, 47 percent of the population lives in coastal counties. Combine that with the arrival of a hyperhurricane period, says Gray, and "it is inevitable that we are going to see, in the years ahead, hurricane damage like we've never seen before."
A hurricane is a heated spiral fed by warm air flowing in at the earth's surface. The storm that would grow into Floyd took shape off the west coast of Africa in early September as a tropical depression, or an area of low pressure surrounded by slowly swirling clouds. Warm sea-surface temperatures whipped it into a tropical storm by Sept. 8, and a full-fledged hurricane two days later. It grew into a powerful Category 4 because it picked up additional heat from the sea surface and did not encounter upper-level westerly winds that clip the top off storms. Floyd got an extra boost of energy near the Dominican Republic, where it raced over an 88-degree patch of water.
Then it demonstrated the downside of being second: as Floyd approached North America it ran into a patch of cold water left behind by Hurricane Dennis, whose gusts had churned up the ocean depths. This frigid water weakened Floyd as it traveled north. "Off the Bahamas, Floyd was a low Category 5 storm, but by the time it came ashore it was barely a 100-mph hurricane," says meteorologist Robert Gall of the National Center for Atmospheric Research. "No one really knew it was going to do that." And once Floyd made landfall it ran into a cold, dry air mass coming in from the west, which slowed its winds and wrung out so much of its water so fast that parts of New Jersey got 15 inches of rain.
The next hurricane might do even worse. Gray dates the beginning of a new active period back to 1995. He says it all begins with a "conveyor belt" of water in the North Atlantic. Surface water flows north, cooling as it goes. Cooler water is also denser, so by the time it reaches Iceland it sinks. The greater the amount of cold water deep down, the less there is available to circulate on the surface toward Europe and Africa. The absence of that pool of cool does two things: it keeps the Atlantic slightly warmer than it would otherwise be, providing warmth to fuel storms, and it weakens the trade winds that blow from Africa to the Caribbean. The trades cause the wind shear that can cut off a hurricane at its knees. Less shear and warmer water add up to more hurricanes. This hurricane-friendly regime brought us 12 storms in 1969, for instance, including Camille (256 deaths and $12.2 billion in damage). It typically lasts decades.
Now that scientists have assembled a short list of ingredients for a hurricane, the search is on for what provides those ingredients. A hurricane needs, first, a warm sea surface of 73 degrees Fahrenheit or more, with the layer of warm water at least 180 feet deep, says climate researcher Tom Murphree of the Naval Postgraduate School in Monterey, Calif. Hurricanes also need cooperative winds. In particular, upper-troposphere winds must be in sync with low-altitude trade winds blowing west from Africa. That is, they must blow in the same direction at about the same speed. Otherwise, says Murphree, you get wind shear, which knocks apart the spiral storm, much as sticking your hand in the swirl of water above a bathtub drain destroys it. That's why El Nino years are death to hurricanes. El Nino kicks up westerlies at 40,000 feet, blowing in the opposite direction of the lower-level trades. This produces more shear and fewer Atlantic hurricanes. La Nina years, the climatological opposite of El Nino, bring just the opposite. The 40,000-foot winds blow in the same direction as the trades, so there is no shear. Hurricanes hold together. After a spell dominated by El Nino, the planet is now in the grip of a hurricane-making La Nina, says Murphree, and we may be returning to conditions last seen in the late 1960s and 1970s.
The biggest unknown is how global climate change, principally the warming that Earth is now undergoing, will affect the frequency and severity of hurricanes. Ten years ago the answer seemed straightforward: a warmer planet, with more heat in the seas and atmosphere, would surely spawn more storms because storms feed on heat. Now things look more complicated. As the world warms, predicts the climate model developed by NASA's Goddard Institute for Space Sciences, minor storms off West Africa spin faster. A faster spin means that a greater number of storms could bulk up into hurricanes, says Columbia University's Leonard Druyan. Although about 60 cyclonic systems develop off west Africa every year, only a few of them form hurricanes. "But our research," says Druyan, "points to an increase in the frequency of tropical storms with global warming."
That is hotly debated because the ocean and atmosphere are buffeted by so many countervailing forces. For instance, global warming seems to heat the upper atmosphere more than the lower. That leaves less of a temperature difference between the upper and lower atmosphere. That temperature gradient fuels storms, so a smaller gradient might mean fewer storms. But a warmer world is also a world with more evaporation. Evaporation releases heat back into lower levels of the atmosphere, restoring the temperature gradient. On balance, a warmer Earth might indeed be a stormier Earth.
If there are more severe storms in the near future, predicting their intensity and path becomes even more crucial. The National Oceanic and Atmospheric Administration is now test-driving a computer model for predicting the strength of storms. It simulates how the hurricane interacts with the ocean below. "The hurricane extracts energy from the ocean, and the sea is cooled by that interaction," robbing the storm of a source of energy, says NOAA's David Evans. Taking that into account should correct a problem that plagues existing weather models: they tend to overestimate how long a storm will stay strong, as they did with Floyd.
Predicting hurricane paths is still not an exact science. In 1965 the average error between predicted path and actual path was 155 miles 24 hours ahead; now it is about 100 miles, says Gall. That improvement reflects, among other things, the use of dropsondes released from Gulfstream IV jets. Dropsondes are sensors that collect data on wind and air conditions around a storm, much like the little spheres tossed into the tornado in the movie "Twister," and thus reveal the strength and direction of the forces pushing it. Gall is optimistic that tracking errors will fall to 60 miles within 10 years.
The forecasters would do well to refine their predictions. And governments and disaster agencies would do well to practice their evacuation drills and other preparedness plans. The current storm surge, Gray says, seems to have begun in 1995. If it follows the pattern of the recent past, it could last 30 more years.