Positive and negative feedbacks in the vegetation impact on the Sahel Drought

Abstract

In this contribution, the vegetation feedback mechanisms on the Sahel drought are revisited and further analysed using the intermediate complexity Atmospheric General Circulation model SPEEDY coupled to the VEGAS dynamic vegetation model. It is found that the albedo feedback provides a strongly positive feedback to the atmosphere-vegetation system. The albedo increase leads to a reduction in net surface radiation that is mainly compensated by reduced evaporation (latent heat loss), providing one major source of the total rainfall response. Another positive feedback is provided by the pressure response to the reduced atmospheric heating in the Sahel region that leads to a high pressure in the Saharan region, thus weakening the Saharan heat low and being favourable for diverging north-easterly wind anomalies in the Sahel region. The major negative feedback is provided by the vegetation impact on evaporation, which limits the response in soil wetness and is thus stabilizing the coupled atmosphere-vegetation system.