The Role of Vegetation-Climate Interaction and Interannual Variability in Shaping the African Savanna

Using a coupled atmosphere-land-vegetation model of intermediate complexity, we explore how vegetation-climate interaction and internal climate variability might influence the vegetation distribution in Africa. When the model is forced by observed climatological sea surface temperature (SST), multiple equilibrium states are found in the climatically sensitive zones, namely, the Sahel and southern Africa. Depending on the initial condition, these regions can settle on either a forest-like or a desert-like vegetation cover. When forced by interannually varying SST that corresponds to observed wet and dry years in the Sahel, these stable equilibria are driven toward an intermediate grass-like vegetation due to the nonlinearity in the coupled system. These results point to a three-way interaction involving feedbacks by vegetation, the mean climate, and interannual or longer time scale variability arising from other slow components of the climate system, such as the ocean. Both vegetation and interannual variability thus play an active role in shaping the subtropical Savanna.