Natural climatic variability and Sahelian rainfall trends

Abstract

Output from a 500-year-long simulation of a global climatic model coupled to a mixed-layer oceanic model has been used to investigate rainfall variability over the Sahel. As part of the natural climatic variability of this run, the model was able to reproduce a period of above average rainfall followed by an extensive drought, which has been a feature of the Sahel over the last few decades. These rainfall changes are shown to be related to characteristic Pacific Ocean sea surface temperature (SST) anomalies. The basic features of the simulated SST, rainfall and wind changes in the Sahelian region are in agreement with observations. Large-scale teleconnection patterns were identified via 1-point correlation maps, which demonstrated the dominance of the Pacific region. Concentrating on the specifics associated with the extended period of anomalous Sahelian rainfall, a contemporaneous period of exceptional SST anomalies was identified in the Pacific Ocean. A marked anti-correlation was identified between the long-term trends in these SST anomalies and Sahelian rainfall. The mechanism connecting these Pacific SST anomalies to the Sahelian region was identified to be the Pacific North America (PNA) oscillation. Quite different PNA patterns occurred for the extended periods of above and below average Sahelian rainfall. The PNA oscillation was found to influence the Sahelian rainfall via its impact on the North Atlantic Oscillation. The particularly extended period of Sahelian rainfall anomalies was additionally influenced by a synchronised temporal variation of the PNA oscillation and the North Atlantic Oscillation at this time. Thus, this particular simulation suggests that the recent anomalous Sahelian rainfall can be attributed to natural climatic variability originating with SST anomalies in the Pacific Ocean.