This research focuses on evaluating the High-Resolution Model Intercomparison Project (HighResMIP) simulations within the framework of the Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6). We used seven of its consortiums to study how CMIP6 reproduced the West African precipitation features during the 1950–2014 historical simulation periods. The rainfall event was studied for two sub-regions of West Africa, the Sahel and the Guinea Coast. Precipitation datasets from the Climate Research Unit (CRU) TS v4.03, University of Delaware (UDEL) v5.01, and Global Precipitation Climatology Centre (GPCC) were used as observational references with the aim of accounting for uncertainty. The observed annual peak during August, which is greater than 200, 25, and 100 mm/month in the Guinea Coast, the Sahel, and West Africa as a whole, respectively, appears to be slightly underestimated by some of the models and the ensemble mean, although all the models captured the general rainfall pattern. Global climate models (GCMs) and the ensemble mean reproduced the spatial daily pattern of precipitation in the monsoon season (from June to September) over West Africa, with a high correlation coefficient exceeding 0.8 for the mean field and a relatively lower correlation coefficient for extreme events. Individual models, such as IPSL and ECMWF, tend to show high performance, but the ensemble mean appears to outperform all other models in reproducing West African precipitation features. The result from this study shows that merely improving the horizontal resolution may not remove biases from CMIP6.

We analyzed 16 historical simulations from High-Resolution Model Intercomparison Project (HighResMIP) within the framework of Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6). The simulations comprise high and low-resolution simulations. The aim is to examine the effect of improved horizontal resolution models on mean and extreme precipitation over West Africa from 1985 – 2014. Observation shows southeast and southwest regions are the most active in precipitation, consistent with simulations of HighResMIP. The impact of improved horizontal resolutions is felt in the simulation of orographic-induced rainfall over the high grounds and the intensification of precipitation in most of the metrics employed. The observed highest 1-day precipitation shows most of the Guinea coast region had 1-day rainfall greater than 100 mm, which was over-estimated (under-estimated) by high (low) resolution simulations. Also, improvement in horizontal resolution seems to improve the capacity of high-resolution models to reproduce the observed pattern of heavy precipitation (R10mm) days and very heavy rainfall (R20mm) days. The spatio-temporal analysis showed the presence of uncertainty in the simulation of both the high and low resolutions simulations in reproducing the extreme precipitation over West Africa. Due to this, a bias correction approach was employed.

This research investigates the representation of present and future of mean and extreme precipitation in the High Resolution Model Intercomparison Project (HighResMIP) within the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations over West Africa. The ensemble means of eight high- and low-resolution simulations, referred to as ENSEMBLE-H and ENSEMBLE-L, were analyzed for the period 2025–2049. Future mean and extreme precipitation patterns were examined for two sub regions of West Africa: the Sahel and the Guinea Coast. Projected annual rainfall peaks at 250 mm/month in May and 150 mm/month in both August and September. The onset of precipitation is expected between the 35th and 67th day of the year for the Guinea Coast, and between the 135th and 165th day for the Sahel. Cessation days are projected between the 315th and 330th day for the Guinea Coast, and between the 250th and 280th day for the Sahel. The length of the growing season is anticipated to increase, reaching around 10 months for the Guinea Coast and 4 months for the Sahel by 2045–2049. High rainfall events are projected to occur in 2035 and 2036, exceeding normal levels. The spatio-temporal analysis of extreme precipitation shows a more significant increase in the highest one-day precipitation over the Sahel (approximately 30%) compared to the Guinea Coast (less than 20%). Additionally, heavy rainfall events (r10mm) are projected to see a positive change of around 8% over the Guinea Coast and 12% over the Sahel. These results suggest potential future re-greening of the Sahel. A greater positive change in precipitation is projected over the Sahel compared to the Guinea Coast during the 2025–2049 period.