African rains foretell stronger hurricanes.

African Rains Foretell Stronger Hurricanes

When Hurricane Hugo slammed into South Carolina last year, it jarred a fading memory among Eastern Seaboard residents, who had passed through almost two decades without feeling the fury of a devastating storm. The next decade will bring no such reprieve, predicts one noted meteorologist.

Looking back over historical weather records, William M. Gray of Colorado State University in Fort Collins found that strong hurricanes tend to lash the eastern United States and Caribbean when summer rainfall increases in the western portion of the Sahel, a semiarid region south of the Sahara desert. Because of recent changes in Sahel weather, Gray suggests the eastern states and the Caribbean will likely face growing numbers of intense hurricanes in the next decade or two.

"The relationship is very strong between intense storms and West African rainfall," says Gray, who describes his work in the Sept. 14 SCIENCE.

For example, 13 strong hurricanes (category 3 or higher) struck the eastern United States or the Florida peninsula between 1947 and 1969, when the Sahel was wetter than normal, Gray notes. In contrast, only one category 3 hurricane (Gloria) hit the region during the Sahel's debilitating drought from 1970 to 1987.

According to weather records dating back a century, the Sahel climate has swung between wet and dry periods, each lasting a decade or two. The region faced drought from 1900 through 1914, grew wet from 1915 through 1935, and then dried up again from 1936 through 1946, says Gray.

Rainfall in the Sahel increased during 1988 and 1989, possibly signaling the end of the most recent drought. If so, warns Gray, the number of intense hurricanes hitting the Eastern Seaboard and Caribbean will probably rise dramatically. The intense hurricanes of 1988 and 1989 -- Hugo, Gilbert and Joan -- may represent the beginning of the shift, he says.

conditions in West Africa do not appear closely linked to the number of strong hurricanes in the Gulf of Mexico, or to the number of less intense East Coast hurricanes, Gray's analyses indicate. This suggests that a wetter Sahel may not spell an increase in the absolute number of storms striking the United States or Caribbean, he says.

Previous studies have not turned up a relationship between Sahel rainfall and Eastern Seaboard hurricanes, Gray observes, although he and other scientists say the new evidence of a connection does not surprise them. Moist summertime conditions in the Sahel tend to reduce the strength of key winds over West Africa, and Gray suggests this might allow better development of low-pressure systems there. As these pressure waves travel through the atmosphere toward the United States, they can develop into tropical depressions and then into hurricanes.

The proposed relationship between African rainfall and hurricanes could provide important clues for meteorologists seeking to understand how hurricanes develop, says Jerry Jarrell, deputy director of the National Hurricane Center in Coral Gables, Fla. "We know almost nothing about what causes storms to form and what determines whether they'll be strong or large," he says.

Whether or not the relationship holds in the future, Gray's prediction of strong storms in coming years dovetails with the National Hurricane Center's efforts to keep East Coast residents prepared for the return of hurricanes that rival those of the '50s and '60s.

"Within everybody's memory, they haven't felt these big ones and they haven't seen the destruction," says Jarrell. He notes that some political forces are now trying to weaken southern Florida's building codes, which were strengthened in the wake of several severe storms during the 1940s and 1950s.