One and a half centuries of rainfall variability in West Africa and its links to large-scale factors

The Sahel region of West Africa is long-known for the extreme droughts that have plagued the region. The most severe conditions of the 20th century occurred through the three decades from 1968 to 1997. There has been considerable debate about whether the rainfall conditions in the region have returned to the wetter state that prevailed prior to 1968. This paper examines Sahel rainfall over a period of 160 years, using an extensive gauge data set and compares it with conditions to the south along the Guinea Coast, where roughly 140 years of gauge data are available. Preliminary analysis of the data set shows several characteristics of the interannual variability and of recent conditions. 1) There has been no sustained period of above average rainfall, such as occurred prior to 1968. 2) Interannual variability has been large. 3) Peak rainfall has in some cases shifted from August to July. 4) The relationship between the Sahel and Guinea Coast shifts within the season, such that the regions tend to be in-phase in the early and late season but out-of-phase duration the peak months of the season. 5) A major shift in the relationship between these two regions occurred in the 1960s. 6) There have been decadal scale changes in the months that contribute most to interannual variability. 7) These indicate that during some intervals interannual variability is related most strongly to the start and end of the season, while during most of the 20th century it was related mainly to conditions during the months of August and September. A comparison of the regional time series with large-scale circulation parameters is also presented and the rainfall characteristics demonstrated are interpreted in the context of changes in the large-scale factors controlling interannual variability in the Sahel.