The West African monsoon in the U.S. National Multi-Model Ensemble datasets

The ability of coupled climate models from the national multi-model ensemble (NMME) dataset to reproduce the interannual variability of the West African monsoon and associated teleconnections is examined. The analysis is for the period 1982-2010. The satellite based Global Precipitation Climatology Project (GPCP) data is used as proxy for observed rainfall and to validate the models. We examine rainfall patterns during MAM, June and JAS. Models are able to reproduce the northward migration of precipitation. Three of the models including the two versions of the NCEP CFS and the NASA models also have a systematic dry (wet) bias over the Sahel (Gulf of Guinea region) during the summer rainfall season, while the others show alternating wet and dry biases across West Africa. All models have spatially averaged values of standard deviation lower than that observed. Models are also able to reproduce to some extent the main features of the precipitation variability maximum, but with deficiencies in the amplitudes and locations. The areas of highest variability are generally depicted, but there are significant differences among the models. Teleconnections in the models are investigated by first conducting an EOF in the precipitation anomaly fields and then perform a regression of the first or second EOF time series onto the global SST. Focusing on JAS rainfall season, only the NASA model was able to depict the dipole pattern between the Sahel and the Gulf of Guinea rainfall. However, none of the models was able to reproduce the observed upward trend of Sahel rainfall in the last decade. The relationship to SST is also examined. The observed influence of tropical north Atlantic SST on the Sahel rainfall is only partially represented, while the NASA model inconsistently emphasizes the role of the tropical South Atlantic. A majority of the models show a partial ENSO teleconnection combined with the tropical south Atlantic mode. However, observations indicate that the influence of ENSO on northern Sub-Saharan summer rainfall has been very weak over the past 30 years. The influence of model errors on the predictions of the West African rainfall is presented. Canonical correlation analysis (CCA) is employed to correct the model simulations. A new ensemble based on models corrected forecasts is then formed and the results are presented.