The intensity, location and structure of the tropical rainbelt over west Africa as factors in interannual variability

This study examines the nature of the tropical rainbelt over west Africa in 4 years corresponding to the four most common patterns of rainfall anomalies in the region. These include 2 years that were anomalously wet in the Sahel, 1955 and 1950, and 2 years, 1983 and 1984, that were anomalously dry. The year 1955 is ubiquitously wet, while a rainfall dipole prevails in 1950, with abnormally high rainfall in the Sahelian latitudes but drought conditions in the Guinea Coast latitudes to the south. The year 1983 is ubiquitously dry and 1984 is a year with a rainfall dipole but opposite ill phase (i.e. Sahel dry, Guinea Coast wet). The main results of the study are that the contrast between wet and dry years is associated with a weakening and contraction of the tropical rainbelt. In addition, a north/south displacement of the rainbelt is associated with the dipole patterns, with a north ward/southward shift bringing good rains to the Sahel/Guinea Coast. These shifts are associated with north/south shifts in the latitude of the African Easterly Jet (AEJ) over west Africa. The latitudinal position and extent of the rainbelt correspond to the latitudinal position and extent of an immense 'tongue' of strong ascending motion. The core of the ascending motion is bound by the axes of the AEJ on the north and the Tropical Easterly Jet (TEJ) in the south. This importance of these two jets in determining the character of the summer rainy season implies that the Sahelian region provides the link between the Sahel and global scale phenomena. Copyright (c) 2008 Royal Meteorological Society