The more it rains in West Africa, the more often intense hurricanes strike the United States, according to a new theory published today that links weather patterns across the Atlantic.
Moreover, after nearly two decades of drought in West Africa, it appears the rains are returning, increasing the likelihood that more destructive hurricanes will strike the United States and the Carribean in coming years.
According to studies done by William Gray of Colorado State University and published in today's issue of the journal Science, the frequency of killer hurricanes was high during the period from 1947 until 1969, when plentiful rains fell on the Sahel region of West Africa, which lies on the southern and western fringe of the Sahara Desert. The same area suffered a severe drought from 1970 until 1987 and the number of intense hurricanes dropped significantly.
Gray asserts that the connection between African rain and Western Atlantic hurricanes is strongest when one looks at the most destructive storms -- those in which winds exceeded 110 mph. Hurricane Hugo, which last year killed 68 people in the U.S. and Carribean and caused an estimated $7 billion in damage, was one of these.
Moreover, Gray has also found a link between African rain and the number of destructive storms that make landfall in the United States. When it's wet in West Africa, Gray says the potential damage from severe storms in the United States is two to four times higher than during dry periods. Gray adds that the association is even more striking for hurricanes that strike Florida.
If Gray is correct in his analysis, the United States and the Carribean may be in for intense hurricane activity in the decade ahead. The Sahel is now in a wet period and the number of hurricanes this year has been higher than average.
During this century, the alternating wet and dry periods in West Africa appear to last 10 to 20 years. Since 1947, there have been 23 wet years followed, beginning in 1970, by 18 dry years. The drought appears to have ended in 1987, and hurricane activity increased in 1988 and 1989.
Gray postulates that increased rain over the Sahel region releases heat into the atmosphere, thus driving more intense wind and weather patterns that give rise to more hurricanes. It is well known that heat is released when water vapor condenses and forms clouds.
Hurricane forecasters say they are intensly interested in Gray's work, but some remain skeptical about a direct relationship between African rain and hurricane predictibility.
"He is a master at bringing out relationships, but he needs to conduct strict statistical tests to make sure his associations hold," said Miles Lawrence, meterologist at the National Oceanic and Atmospheric Administration (NOAA) National Hurricane Center in Miami.
Other researchers who have examined the same data agreed there appears to be a connection between rain in the Sahel and hurricanes. But the scientists believe the connection holds for all types of hurricanes: big ones and small ones, regardless of whether they make landfall in the United States or disappear harmlessly over the Atlantic Ocean.
"If you look at all hurricanes in the Atlantic basin, it does appear that hurricanes are related to Sahel rainfall. But if you look at storms that hit the United States, it's less clear. Even less so for Florida," said Stanley Rosenthal, director of the hurricane research division at NOAA Atlantic Oceanographic and Meterological Laboratory in Miami.
Rosenthal said that many climatic events have been associated with hurricane formation over the years, including the warming of the Pacific Ocean off the coast of South America, known as El Ninåo.