On the stability of the atmosphere-vegetation system in the Sahara/Sahel region

A conceptual model has been developed for the analysis of atmosphere-vegeta tion interaction in subtropical deserts. The model can exhibit multiple sta ble states-in the system: a "desert" equilibrium with low precipitation and absent vegetation and a "green" equilibrium with moderate precipitation an d permanent vegetation cover. The conceptual model is applied to interpret the results of two climate-vegetation models: a comprehensive coupled atmos phere-biome model and a simple hox model. In both applications, two stable states exist for the western Sahara/Sahel region for the present-day climat e, and the only green equilibrium is found for the mid-Holocene climate. Th e latter agrees well with paleoreconstructions of Sahara/Sahel climate and vegetation. It is shown that for present-day climate the green equilibrium is less probable than the desert equilibrium, and this explains the existen ce of the Sahara desert as it is today. The difference in albedo between th e desert and vegetation cover appears to be the main parameter that control s an existence of multiple stable states. The Charney's mechanism of self-s tabilization of subtropical deserts is generalized by accounting for atmosp heric hydrology, the heat and moisture exchange at the side boundaries, and taking into account the dynamic properties of the surface. The generalized mechanism explains the self-stabilization of both desert and vegetation in the western Sahara/Sahel region, The role of surface roughness in climate- vegetation interaction is shown to be of secondary importance in comparison with albedo. Furthermore, for the high albedo, precipitation increases wit h increasing roughness while, for the low albedo, the opposite is found.