African Water Development Report 2006 - United Nations Economic ...

ican countries cannot afford either to incur orto prevent. Since the late 1960s, droughts havecaused much suffering in Africa. Millions of Africanshave died of starvation and related causes.The hardest-hit areas include Ethiopia and theSahel region on the southern edge of the Sahara.(World Book, Inc., 2003).Anomalies and Trends in Rainfall PatternsThe variability of the climatic flux over the continenthas been based on the continental air(El Sayem ,1986; Zahran, 1986).Surveys on African rainfall carried out by Klaus(1977) and Nicholson (1985, 1986) have indicateda marked coherence of rainfall variations overlarge portions of the continent, a characteristicfound to be especially strong in the semi-arid regionssouth of the Sahara. With this approach,Nicholson (1986) identified various patterns ofrainfall anomaly fields that illustrate a strongtendency for synchronous fluctuations along thetropical and temperate margins of the Sahara.From the foregoing analysis, it can be inferredMANAGING RISKSFigure 10.3: Mean Position of the Africa Intertropical Convergence Zone ITCZWest RegionEast RegionSource: NOAA, 2005mass circulation controlled by the Inter TropicalConvergence Zone. As a result, areas below Lat10°N receive rainfall around March - April whilethose up to Lat 20°N receive rain in June throughSeptember. Nicholson (1986), however, pointsout that variations in Sahel rainfall are generallyrelated to changes in the intensity of the rainyseason rather than to its onset or length as theITCZ hypothesis would claim. This means that,in this case, extreme rainfall events cannot bemeaningfully predicted through the anomalousexcursions of the ITCZ as proposed by Kraus(1977), which has been used quite extensively foranalysis of rainfall variations in tropical Africathat the rainfall pattern over Africa exhibits veryhigh variabilities over time and are quite oftenaccentuated with positive and negative anomalies.Positive anomalies occur when there aresignificant departures of rainfall above the longtermmean and negative when below. This variabilityassumes higher dimensions, especially inareas lying in the transition from tropical climateto the arid conditions of the North. It becomesapparent that investigations into the behaviouralpatterns of rainfall must be directed toward theclimatic and atmospheric mechanisms at smallsynoptic scales and mesoscales that could trigger231