The role of vegetation-climate interaction and interannual variability in shaping the African savanna

Using a coupled atmosphere-land-vegetation model of intermediate complexity , the authors explore how vegetation-climate interaction and internal clima te variability might influence the vegetation distribution in Africa. When the model is forced by observed climatological sea surface temperature (SST ), positive feedbacks from vegetation changes tend to increase the spatial gradient between desert regions and forest regions at the expense of savann a regions. When interannual variation of SST is included. the climate varia bility tends to reduce rainfall and vegetation in the wetter regions and to increase them in the drier regions along this gradient, resulting in a smo other desert-forest transition. This effect is most dramatically demonstrat ed in a model parameter regime for which multiple equilibria (either a dese rtlike or a forestlike Sahel) can exist when strong vegetation -climate fee dbacks are allowed. However, the presence of a variable SST drives the dese rtlike state and the forestlike state toward an intermediate grasslike stat e, because of nonlinearities in the coupled system. Both vegetation and int erannual variability thus play active roles in shaping the subtropical sava nna ecosystem.