Recherches sur la pluviométrie de la corne orientale de l'Afrique

Résumé 0

The Eastern Horn of Africa (Somalia, Ethiopia, Djibouti and Kenya) exhibits strong rainfall anomalies, considering the latitudinal position of the region. The mean annual rainfall map shows a vigorous meridian contrast between the rainy Western Highlands of Kenya and Ethiopia, and the deficiency of the lowlands of Somalia, the Eastern parts of Kenya and Ethiopia, and the Red Sea coast. This opposition is mainly related to the northern summer season, during which the different caracteristics of the wet "west-african" monsoon and the dry divergent "indian" monsoon are clearly emphasized. They induce single-maximum summer rainfall regimes, and double-peak spring and autumn regimes, respectively. The extension of the summer rainfall maximum as far south as Western Kenya constitutes another anomaly. A contrast also exists between the (unexpected) winter rainfall regimes of the Red Sea Coast, and the summer rainfall regimes of the Ethiopian Plateau. The local and regional influences of relief and sea proximity (the latter generally up to 30 or 50 kms from the coast), and their associated wind circulations, play an important part in the rainfall patterns. That is the case (at least partially) for the coastal winter rains of the Red Sea and the Gulf of Aden, the summer rains along the coasts of Kenya and Southern Somalia, or shelter effects in the Rift Valley. The inter-annual variability is generally high, particularly in the eastern half of the region, where the variations are especially related to the autumn rains. In many of the multiple-peak rainfall areas, there is also a large inter-annual variability in the seasonal repartition of rains, compared to mean regimes. However, year to year persistence of rainfall anomalies is shown to be higher in some of the western continental regions, in particular the Ethiopian Highlands, although there are seldom clear trends. Periodic droughts have affected the region, the worst hit area being Ethiopia, where severe famines have been reported in 1888-92, 1972-74 or 1982-85, among others. According to the few long-term records available, rainfall variations differ from place to place, but one can discern general patterns, such as low rainfall in the 40's, very high rainfall in the 60's, and two dry periods in the early 70's and 80's, separated by above-normal rainfall years. Although comprehensive study of spatial variations is still to be carried out, it seems that Ethiopian rainfall patterns (especially in Eritrea) are often quite different from those of the other regions. This is also shown in the analysis of rainfall periodicities, which indicate 4 peak frequencies: - 2 to 2.5 years, mainly in the Indian Ocean coastal areas; - 3 to 4 years, weak and rare; - 5 to 6 years, common throughout the Horn of Africa (except in Eritrea); - 10 to 12 years, essentially significant in Addis-Ababa.