Abstract
This paper investigates the African monsoon circulations which determine main rainfall areas in the 1979 northern summer, and focuses on the different causes of rain between East and West Africa, by using the FGGE level II-c precipitation and snow data set and the FGGE level III-b data set.
When the rainbelt reaches its most northerly position over tropical Africa, the main rainfall areas exist within the surface southwesterlies. In the zone of 7°-17°N, the non-rainfall area appears between Lake Chad and Mt. Marra even in the wet air mass (i.e., equatorial westerlies). The main rainbelt is divided by this non-rainfall area into the rainfall areas in West Africa and west of the Ethio- pian Plateau.
The main rainbelt in West Africa roughly corresponds to the mid-tropospheric easterly jet. The two maximum axes of the 850mb υ power spectra in the 3.1-4.4 day period are found along approx. 8°N and 20°N. West of 5°E, the main rainbelt exists between them, while east of it, the southern axis is obscure and the main rainbelt is concentrated along approx. 13°N, about 5 degrees south of the northern axis. This axis disappears between Lake Chad and Mt. Marra where the non-rainfall area appears.
The main rainfall area west of the Ethiopian Plateau corresponds to the thickest zone of equatorial westerlies. The 850mb υ power spectra in the 3.1-4.4 day period is unclear in East Africa. Orographic disturbances in the equatorial westerlies are therefore responsible for the rain. In the zone of 2°-7°N, the main rainfall area south of the mountains in Cameroun corresponds to the southern one of the two thicker equatorial westerlies in their latitudinal profile.
