Impact of greenhouse warming on the West African summer monsoon

The Meteo-France climate model has been used to run a time-dependent climate change experiment in order to study the impact of increasing amounts of greenhouse gases and aerosols on the West African monsoon system. A transient climate simulation of 150 years has been performed with a coupled ocean/sea-ice/ atmosphere model including stratospheric ozone with the greenhouse gas concentrations changing annually according to one of the new scenarios of the Intergovernmental Panel on Climate Change. The simulated climate change has been analyzed as the difference between two 30-year tiine-slices (1980-2010 and 2070-2100 respectively) with particular attention paid to the hydrological cycle. The climate of the recent period. validated in detail by comparison to reanalyses, shows that the model is able to reproduce a realistic seasonal cycle of the African monsoon, The precipitation changes simulated at the end of the twentyfirst century show an enhanced monsoon precipitation over West Africa, The analysis of the atmospheric branch of the water cycle highlights some of the parameters that control these precipitation anomalies: changes in precipitable water, water vapour recycling, moisture convergence and precipitation efficiency. These changes of the hydrological processes also cause corresponding changes in the thermal state of the atmosphere such as a smaller warming of the surface in the Sahel linked to stronger evaporation in this region. This results in changes of the mean zonal circulation through the thermal wind relationship and of position and strength of key elements of the general circulation over the West Africa (Hadley and Walker circulations, African Easterly Jet).