s people within the reach of El Niño know, the exchange of energy between the ocean and atmosphere can have effects that ripple across oceans and continents over periods of months and years. But sometimes the interplay of sea and sky gives birth to more violent, short-lived phenomena that play out over minutes to hours.
Warm water, combined with just the right conditions aloft in the atmosphere, can give rise to a spinning column of air -- a vortex. Tornadoes bloom on the plains of North America in the infamous "Tornado Alley" stretching from the Midwest to Texas. But the seas have their own tornado alleys.
Tornadoes born on the water are called waterspouts. Like land-based tornadoes, they feed on warm, moist, unstable air. Where winds shear past each other, a vortex may develop. A waterspout can form when air rising from the warm ocean surface gets tangled up with the vortex, stretching it thousands of feet to the base of a cloud. This causes the air to rotate faster and faster, like a spinning figure skater as she pulls her arms in toward her body.
Typically, waterspouts live from 2 to 20 minutes and can produce brief bursts of wind up to hurricane force. Though their winds are not usually as powerful and deadly as those of their land-lubber cousins, waterspouts can still destroy: In 1980, a waterspout over San Antonio Bay in Texas chewed up a shrimp boat and then flipped it over and sank it.
Atlantic hurricanes, the ones that plague the Caribbean and Eastern United States every year from June to November, are born as clusters of thunderstorms in the tropical seas west of Africa. Blown by the trade winds, these hurricane "seedlings" scud across the ocean, drawing energy from the warm water. Cyclones can form when storms reach waters where the temperature of the sea surface rises to 80 degrees Fahrenheit.
As the temperature in the center of the storm rises, more water evaporates and the atmospheric pressure at the storm's center drops. The higher pressure air at the edges of the storm rushes in toward the center, like water spiraling down a bathtub drain. In the Northern Hemisphere, forces generated by Earth's rotation deflect the air into a counterclockwise direction. When the winds reach 38 mph, the disturbance is called a tropical storm. At 74 mph and greater, it is a full-fledged hurricane.
In an average year, about ten tropical storms will form in the Atlantic, but only six of those will graduate to becoming a hurricane. What happens aloft is just as important to the formation of cyclones as the warm water that they feed on. In El Niño years, for instance, unusually strong winds at high altitude literally decapitate the tops of developing hurricanes, which can tower 8 to 10 miles into the sky.
Each year, a team of researchers at Colorado State University led by meteorologist William Gray offer a prediction for the coming Atlantic hurricane season. They use a statistical technique based on the number of storms seen in the past given certain conditions. Gray's "hurricane predictors" include the presence of El Niño. From 1991 to 1994, repeated El Niños fostered wind conditions that helped to suppress hurricane formation. Other predictors include the amount of warm-water fuel available in the Caribbean and the climate of West Africa, where hurricane seedlings first sprout.
The forecast for 1999 calls for a relatively active Atlantic hurricane season, with 14 named storms, 9 hurricanes, and 4 intense hurricanes -- approximately as intense as the 1998 season. Gray has been relatively successful over the years. In 1998, he predicted 14 named storms and 9 hurricanes; the season saw 14 storms and 10 hurricanes.
-- By Daniel Pendick