Wednesday, 12 January 2005: 1:45 PM
A new perspective on drought in West Africa
The droughts that affect West Africa are extremely large in spatial extent and tend to fall into one of two types of spatial configuration. In one case, most of West Africa, from the Saharan fringe to the Guinea Coast, is affected. In the second, there is a dipole pattern with rainfall anomalies of opposite sign prevailing on either side of roughly 10 degrees north latitude. An examination of seasonal trends and circulation features show that the particular spatial pattern that results is determined two factors: changes in the latitudinal location of the tropical "rainbelt" over West Africa (frequently termed the West African monsoon, or the ITCZ rainfall) and the overall intensity of this rainbelt. The pattern of drought throughout appears to be linked to both constricted latitudinal migration and reduced intensity. The dipole is associated with constricted latitudinal migration but normal or above average intensity. An unusual but interesting pattern is one of above rainfall throughout the rainfall. It appears to be linked to increased intensity but a stalling of the seasonal migration in August only. In such cases, high rainfall occurs primarily in the months of July and September, with August being anomalously dry. Seasonal migration of the rainbelt is closely linked to the latitudinal location of the African Easterly Jet, which in turn is closely linked to SSTs in the Gulf of Guinea. Changes in intensity of the rainbelt are more difficult to explain, but appear to be linked to intensity of the Tropical Easterly Jet and its location with respect to the African Easterly Jet. The dynamics of the two jets are critical in determining rainfall intensity over West Africa.