Guinean and sahelian rainfall anomaly indices at annual and monthly scales (1933-1990)
Résumé
The annual and monthly rainfall variability in north tropical Africa are analysed by principal component analysis in order to detect the main coherent modes. The Sahel constitutes the first mode, although it is divided close to 1O"W into an Atlantic section and a Continental section. The Guinean zone is less coherent, and is divided into distinct modes. The subequatorial area never formed a distinct mode. Rainfall variability for the coherent areas is constructed from rainfall anomaly indices (RAI), based on the coherent areas, and a rainfall typology taking into account the spatial pattern of the rainfall anomaly fields and the intensity of the anomaly. Their evolution (mainly since 1950) is synchronous on the Sahelian and the Guinean zones with the opposition between the 1950s and 1960s (wetter than normal) and the 1970s and the 1980s (drier than normal). In particular, the area of drought, which has plagued the Sahel since 1968, extends to Guinean Africa, mainly after the mid-70s. The typology allows us to investigate the relationships at both the annual and the monthly scale.