Extreme precipitation indices over the Volta Basin: CMIP6 model evaluation

Climate extreme events affect people whose livelihoods are reliant on the water resources of the Volta Basin in West Africa. Therefore, decision-makers and policymakers need reliable predictions of these extremes on several time scales to develop an Early Warning System (EWS) to combat the devastating impact of extreme climatic events. In that vein, the study evaluated the performance of 41 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) in simulating six (6) extreme precipitation events developed by the Expert Team on Climate Change Detection and Indices (ETCCDI), over the Volta Basin for the period 1985–2014. The spatial biases as well as the temporal variations of the indices were analyzed using the simple bias and the Kling-Gupta Efficiency (KGE) respectively. Subsequently, the best models (models with low biases and KGE close to 1) were used to analyze the trends. The modified Mann-Kendall test showed an increasing trend in the observed heavy and very heavy rainy days (R10 mm and R20 mm), very wet and extremely wet days (R95p and R99p) but decreasing trend in the consecutive wet and dry days (CWD and CDD). Generally, models had difficulty reproducing the temporal patterns, however, the MPI-ESM1-2-LR, reproduced well the observed CDD, R10 mm and R20 mm whereas, the IITM-ESM, TaiESM1 and the CMCC-CM2-SR5 reproduced CWDs, R95p and R99p respectively. The Ensemble mean of the models showed robust performance in reproducing the observed R95p and R99p. The future evolution of these extreme poses threats to agriculture, and flood occurrence over the basin.