If a new analysis of the link between weather patterns in West Africa and the tropical Atlantic is correct, communities along the U.S. East Coast that experienced relatively few dangerous hurricanes in the 1970s and 1980s may face more frequent killer storms in the next two decades or so.
The analysis by William M. Gray of Colorado State University, an expert on tropical cyclones, found that when rain in the western Sahel region of Africa is plentiful, more strong hurricanes develop in the Atlantic and strike the United States.
The years from 1947 through 1969 were just such a time. In that period, the study found, 13 hurricanes with winds of more than 110 miles an hour slammed into the East Coast and Florida.
But when there is drought in the Western Sahel, fewer strong Atlantic hurricanes develop. According to the study, that was the case in the Sahel from 1970 through 1987, when only one storm with peak winds of more than 110 miles an hour struck the East Coast and Florida.
The study suggests that this relatively calm period is ending and that a more violent period is about to begin or may already have begun. And the potential for damage is greater now, Mr. Gray said.
"There are more people and more development along the Atlantic Coast," he said. "There is more property to damage."
Officials at the National Hurricane Center of the National Weather Service in Coral Gables, Fla., say they take the Gray study seriously and welcome it because it may help to combat complacency among coastal dwellers who have gotten used to relatively benign hurricane seasons. Some of them "are beginning to think something's different, and maybe we shouldn't be as concerned," said Jerry Jarrell, deputy director of the center. The Gray study, he said, is a signal that it is time to "get serious again."
The connection between West African rainfall and the frequency and strength of hurricanes goes beyond statistics and is rooted in physical relationships, according to Mr. Gray's report in the Sept. 14 issue of the journal Science.
While Mr. Gray and his colleagues are still trying to understand the physical mechanisms involved, they believe that the shifts in the rainfall patterns are related to long-term, periodic changes in the circulation of ocean currents that are part of broader movements in the complex, ever-shifting chains of atmospheric linkage in the global climate system.
In "wet" years, they believe, changes in sea-surface temperatures favor the development of a stronger summer monsoon over West Africa.
This, in turn, creates a more favorable environment for the development of the low-pressure disturbances, or "waves," that originate over West Africa and that can evolve into hurricanes as they move out into the Atlantic.
At the same time, Mr. Gray believes, prevailing winds blowing over West Africa from the east become weaker. This allows the waves to move more slowly to the west, and they become bigger and better organized in pre-hurricane cloud patterns.
In dry Sahel years, according to the Gray analysis, the waves tend to be smaller and less well organized, to move faster and to dissipate more quickly as they move out over the ocean toward America.
In wet years, according to this analysis, the waves also encounter an environment more favorable for further strengthening as they move out over the Atlantic.
This environment includes relatively low air pressures at the surface, warmer water and wind patterns favorable for hurricane development.
Forecasters at the Climate Analysis Center at Camp Springs, Md., say the findings could ultimately help improve long-term predictions of overall hurricane activity.
Mr. Gray analyzed long-term rainfall data for the last 12 decades. The data include figures from 38 observation stations in the Western Sahel, a region of sub-Saharan Africa characterized generally by marginally adequate precipitation.
In examining trends for the summer monsoon season, when most rain falls, he found that the region alternated between wet and dry spells that were 10 to 30 years long.
He found further that far fewer intense hurricanes developed in the Atlantic Ocean in dry Sahel periods than in wet ones. Intense hurricanes were defined as those rated in categories 3 through 5 of the Saffir-Simpson hurricane intensity scale.
Category 3 hurricanes have peak winds of 111 to 130 miles an hour; category 4, of 131 to 155 miles an hour; and category 5, of more than 155 miles an hour.
The vast majority of damage caused by hurricanes results from relatively infrequent but very powerful storms in these top categories, the study report said.
In the 18-year Sahel dry period from 1970 through 1987, only one storm in categories 3 to 5 made landfall in the Eastern United States and Florida: Hurricane Gloria, a category 3 storm, in 1985.