Attribution and prediction of West African monsoon modulations at the decadal time scale

Abstract

In West Africa, rainfall during the summer period (July, August and September) has shown strong modulations on decadal time scales during the 20th century. In particular, there was a very wet period in the 1960s followed by a drought in the 1980s and a recovery of rainfall in the 2000s. These modulations have major regional socio-economic consequences. Considered as one of the most important signals of climate change in Africa, they have been the subject of numerous studies in order to understand their origin(s) and to anticipate future changes. These modulations have long been associated with the internal variability of the climate system and in particular with the role of the surface temperature of the Atlantic Ocean. However, recent studies show a significant contribution of external forcings, notably greenhouse gases and anthropogenic aerosols, in particular during the second half of the 20th century, either via Atlantic Ocean surface temperatures or via direct radiative forcing. In all cases, the link with ocean surface temperatures offers prospects for predicting these precipitation modulations. Decadal climate forecasts, i.e. with a 5-30 year time horizon, have been developed to exploit this source of predictability. They are potentially very important for economic and structural planning in West Africa. In this thesis, we propose to attribute, first, the decadal modulations of Sahel rainfall over the time period of 1901-2014. Secondly, we propose to evaluate the prediction of rainfall in West Africa on the decadal time scale over the period 1968-2012. All this study is carried out through the diagnosis of climate simulations carried out from models contributing to the phase 6 of the coupled models intercomparison project. Since coupled models commonly underestimate the maximum rainfall in the Sahel region during the summer period, we first proposed an adaptive rainfall domain for the Sahel. Our results from the first part of this thesis show th