Huge hurricanes on the horizon?

Science World, Oct 6, 1997

Ilana Apelker was fast asleep in a "shelter" in her mom's bedroom closet when Hurricane Andrew struck on August 24, 1992. "I woke up to the sound of a train whistle blowing through my house," recalls the 17-year-old from Miami. Fierce winds blew through cracks in the windows and drenching rain seeped beneath the doors. "A big palm tree cracked and landed on the roof," Ilana says. "Our carport canopy blew away."

Though Andrew hit five years ago, Florida is still recovering. The hurricane claimed more than 50 lives. Damages totaled $26.5 billion, making it the costliest storm in U.S. history. And more trouble may loom on the horizon. William Gray, a noted meteorologist at Colorado State University, predicts that this hurricane season, which runs from June through November, will be unusually turbulent.

Gray's forecast: 11 tropical storms, which are spiraling masses of thunder-clouds, hundreds of miles wide, that form over tropical oceans. Gray predicts that seven of these tropical storms will grow into hurricanes, with winds surpassing 118 kph (7a. mph). Three of these hurricanes will be as intense as Andrew, with winds reaching 176 kph (110 mph) or greater.

FRIGHTFUL FORECAST

If Gray's predictions come true, 1997 will can the worst three-year run of hurricanes ever recorded. Already, 20 hurricanes have stirred up the Atlantic Ocean in the past two years, well above the normal average of six hurricanes a year. (Hurricanes, also called cyclones, can form over the Pacific Ocean, too, but they rarely hit the West Coast.)

Gray doesn't peer into a crystal ball to make his predictions. He relies on a formula that considers several stormy "ingredients" that are brewing across the globe. Eerily, Gray's forecasts have been on target 10 of the past 12 years. In 1992, for instance, he predicted only one intense hurricane. That hurricane was Andrew.

This year, Gray warns, many of the conditions needed to start hurricanes swirling are present. Warmer sea temperatures are among the most important of these conditions.

Warm water is the "lifeblood" of hurricanes, according to John Knaff, a meteorologist on Gray's team. In recent years, the surface temperature of tropical Atlantic waters has been higher than normal, rising above 26.5 [degrees] C (80 [degrees] F). Warm water evaporates and rises, lowering the air pressure. Cooler air rushes in to replace the rising warm air, creating winds.

Meanwhile, as the moist air rises, it condenses to form thunderclouds and rain. This process releases heat into the atmosphere and pumps energy into "seedling" storms, or small storms that form off the coast of western Africa. Eventually, the winds and clouds spinning around the low-pressure center form the eye of the storm, the calm center in the middle.

During dry years in western Africa, fewer tropical storms develop into hurricanes over the Atlantic. When rainfall is heavier in western Africa, stronger and better organized seedling storms form, and they pick up moisture as they travel west across the Atlantic (see map below). This year has been a wet one.

EL NINO'S BACK!

Certain climate conditions can discourage the formation of hurricanes, however. El Nino, a massive warm ocean current in the eastern Pacific, can stop hurricanes from developing far away in the Atlantic.

El Nino occurs every two to seven years when trade winds weaken. (Trade winds are low-altitude winds that ships follow as they cross the oceans.) When trade winds slow down, warm water near Australia drifts eastward to South America, disrupting weather patterns worldwide. In June, the National Oceanic and Atmospheric Administration announced that a strong El Nino was developing.

But El Nino has a good side as well as a bad side. The warm water current heats the air over the Pacific, creating strong, high-altitude winds that blow toward the Atlantic. These winds shear off the tops of hurricanes, stopping them in their tracks.

Will El Nino prevent hurricanes and steer Gray's forecast off course? Gray says no. "The other factors we look at in the Atlantic are so positive for hurricane development that I do not think El Nino will play a dominant role in reducing storm activity this year," he says.

NEW CYCLONE CYCLE!

Even more thunderous, Gray believes this season could be the beginning of a 10- to 20-year cycle of increased hurricane activity. He theorizes that a global ocean current, known as the "Great Ocean Conveyor Belt," is picking up speed, as it does every two to three decades. The stronger current warms the North Atlantic, particularly tropical waters. These changes subsequently alter the atmospheric conditions around Africa where seedling storms form. That makes it easier for them to turn into hurricanes.

But some scientists are hesitant to confirm long-term predictions about hurricanes. "I don't think we have a clue," says Ants Leetmaa, an oceanographer with the National Weather Service. "If you look at hurricane activity over the past 100 years, we've seen variations from decade to decade. I don't think we have good hypotheses for what causes these trends."