Master thesis defense by Paula Aschenbrenner

Title: Climate Model Performance and Projections over West Africa based on ISI-MIP Data


Future projections of the response of the West African Monsoon (WAM) to global warming differ substantially across models. This introduces high uncertainties in assessments of future climate in West Africa and increases the risk of maladaption. The present study assesses the regional uncertainties of climate projections of the four global climate models that have been used in the ISI-MIP (Inter-Sectoral Impact Model Intercomparison Project) models. The analysis is done for West Africa with a particular focus on Ghana. Analyzing uncertainties includes three parts: examining the influence of the choice of the observational data set used for the bias-adjustment, analyzing the ability of the non-bias-adjusted models to simulate the current climate and the WAM processes, and evaluating the agreement between the future projections of the different models under the RCP8.5 greenhouse gas concentration scenario until the end of this century. It was shown that part of the biases in the bias-adjusted model simulations arises from errors in observational data especially in the Sahel and Sahara region. Uncertainties related to the biases in observational data are small compared to the uncertainties due to the model performance itself. Bias-adjustment of climate data hides the fact that the non-bias-adjusted models are not capable of simulating all parts of the climate in West Africa correctly. Strong biases of the four models can be found in the main region of the monsoon rainfall, the amount of summer precipitation and the temperature in the Sahara and the east Atlantic Ocean. Contrasting with the robust increasing trend in temperature projected until the end of this century, precipitation projections show high uncertainties for all regions in West Africa. For Ghana small-to-medium precipitation changes are projected, but no robust sign of the trend could be identified. For other parts of West Africa higher changes in precipitation are projected. In a warmer climate, the summer monsoon rainfall is projected to shift north-east in three of four models. This implies wetter climate in the Sahel and Sahara region east of the prime meridian and drier climate in the west at the coast north of 7\degree N. Even though not all models agree on this trend, the assessment of some of the physical processes underlying the climatic changes can largely explain the projections: Higher temperatures in the tropical Atlantic Ocean and thus higher moisture availability as well as an increasing Sahara-Atlantic Ocean temperature gradient can in the future enhance the self-amplifying moisture-advection feedback that drives the WAM. This could lead to a summer monsoon rainfall further inland of the current main monsoon region.