Rainfall observations in the Sahel region of West Africa show significant variability at the time scale of decades. Here we explore the mechanisms of this low-frequency variability using a coupled biosphere-atmosphere model which includes explicit representation of vegetation dynamics. By forcing the model with the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1898-1997, numerical experiments on the climate variability of West Africa have been carried out. The results of these experiments suggest that vegetation dynamics is a significant process in shaping the natural variability of the Sahel rainfall. The response of the regional climate system to large-scale forcings is significantly regulated by vegetation dynamics. The relatively slow response of vegetation to changes in the atmosphere acts to enhance the low-frequency rainfall variability. The regional climate system over West Africa has several climate regimes coexisting under the current precessional forcing. Climate transitions between different regimes act as another mechanism contributing to the low-frequency rainfall variability. Climate persistence at one regime and climate transition towards another collectively compose a distinct type of multi-decadal variability.
