Steinig, Sebastian (2015) Sahel rainfall in different versions of the Kiel Climate Model. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, 76 pp.

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Abstract

The severe and widespread Sahel droughts during the 1970s and 1980s were one of the most significant climatic events of the last century and have led to dramatic consequences for the mainly rain-fed agriculture. Despite increased research there is still no consensus on the respective roles of external forcing and internal variability in driving these intraseasonal to multidecadal changes. Global sea surface temperature (SST) anomalies have been hypothesised to influence West African precipitation on a wide variety of time scales, but modelling studies differ on the associated mechanisms and pathways. This results in part from still persistent biases in current General Circulation Models (GCMs), especially in the simulation of the complex tropical climate. This thesis uses data from the Kiel Climate Model (KCM) deployed with different horizontal and vertical resolutions of the atmospheric model, coupled to the same ocean model. One of the configurations has shown to significantly reduce the summer SST warm bias in the tropical Atlantic (TA), a limitation found in nearly all current GCMs that potentially influences the simulation of the West African Monsoon (WAM). The differences in the simulated mean states and variabilities of the TA are used to assess its importance for Sahel rainfall in the KCM. Uncoupled integrations of the same atmospheric models are analysed to separate the influence of inherent model limitations from SST induced changes. The increased seasonal cooling in the TA significantly reduces the excessive precipitation in the Gulf of Guinea and leads to a more realistic further northward inland migration of the monsoonal rain band. The models differ in the strength of the wind-induced upwelling in the northeastern tropical Atlantic and show an influence of SSTs in this area on the Sahel precipitation. This sensitivity is shown to exist also in other coupled models. Total rainfall amounts are underestimated in all configurations and seem additionally limited for coarser atmospheric resolutions. A higher number of atmospheric levels is needed to reproduce the observed interannual TA variability, while the simultaneous increase of horizontal resolution is crucial to transport this signal into West Africa. Multidecadal oceanic and rainfall variabilities are underestimated in all runs. Improved simulation of the Atlantic cold tongue ( ACT) substantially influences the seasonal evolution of the WAM and greatly increases the predictability of its socioeconomically important onset.

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