Boston, Mass. -- The two jet streams that flow over West Africa during the summer months may contribute to the extended drought that has plagued this already semi-arid area, according to Penn State researchers.
"We are trying to explain the West African drought, in part, by looking at the correlations between precipitation and the jet streams," says Colleen Mikovitz, graduate student in meteorology in the College of Earth and Mineral Sciences.
The area affected by drought since the 1970s is the Sahel, a savannah area lying just south of the Sahara Desert. Countries in the Sahel include Ghana, Niger, Senegal, Sierra Leone, Liberia and Burkina Faso, where the rainy season usually runs between May and September.
"While the drought that began 30 years ago continues, we are looking at data from 1979 through 1994," Mikovitz told attendees today (June 3) at the spring meeting of the American Geophysical Union meeting in Boston, Mass. "It appears that there is a correlation between the strength of the jet streams in this area and precipitation."
Two jet streams flow over the Sahel, the African Easterly Jet, which originates over the continent and the Tropical Easterly Jet, which originates over the Indian Ocean and is part of the circulation pattern of the Indian Monsoon. The Tropical Jet is higher and flows at about 40,000 feet above the Earth's surface and the African Jet is at about 10,000 feet.
Mikovitz and Dr. Gregory S. Jenkins, assistant professor of meteorology, looked at the average precipitation per year and the long-term averages and compared them with the patterns of the jets. They found that the Tropical Easterly jet was mostly weaker during the dry periods.
"The pattern with the African Easterly jet was not as clear, but tends to be slightly stronger especially during very dry years," says Mikovitz. "There seems to be some connection between the jets and precipitation."
The Penn State researchers also looked at the radiation emitted from clouds to see if there was a difference seen in radiation during dry years.
"If less radiation is emitted, it may indicate that there was more convective rainfall, bigger storms or more clouds," says Mikovitz.
The researchers noted that the month of May was not a good predictor of whether or not it would be a drought year.
"Some years have a real wet May, but the whole year is dry," says Mikovitz.
The 1950s and 1960s were very wet for the Sahel, followed by a period of some normal years. Researchers do not know what happened in 1979 to switch the area into drought. The researchers plan to look at the effects of El Nino, the North American Oscillation and other phenomenon to see how they affect the jets and the drought in the Sahel. They would also like to look at the easterly atmospheric waves that bring most of the rainfall and rain squalls and are also the start of Atlantic hurricanes.
"We would like to be able to predict droughts," says Mikovitz. "To tell residents when it will be an especially dry year so that perhaps they can change their farming methods and be better prepared."
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