The added value in CMIP6 models for simulating west African rainfall and its related extreme indices.

Abstract:

West African precipitation is characterized by a strong variability from intra-seasonal to climatic scales. In this region, it typically rains from March to October with differences in the annual cycle of precipitation (e.g. Sow et al., Atmosphere 2020). Studies based on the Coupled Model Inter-comparison Project (CMIP) phase 3 and 5 show that the representation of the West African Monsoon (WAM) mean state and its response to global warming, are both highly model-dependent (Roehrig et al., J. Atmos. Sci. 2013). CMIP5 simulations display large inter-model spread in both the mean precipitation climatology and higher-frequency indices by overestimating the frequency of wet days (Sow et al, 2020). Likewise, strong uncertainties are found in the simulation of the consecutive wet and dry days (CWD and CDD respectively) over west Africa.

In this study we analyse the added-value of the new generation models available from the CMIP6 archive in simulating precipitation and its extreme indices over West Africa. The same group of CMIP5 and CMIP6 models is evaluated with CRU and REGEN datasets for a long time period (more than 30 years) as well as more than four other observational datasets in a shorter time period (15 years). Results show that the inter-model spread as well as the overestimation of these indices slightly decreases in most CMIP6 historical simulations compared to their CMIP5 counterpart.

When considering experiments where sea surface temperature (SST) is prescribed over a long period (1870-2014), performed in the Global Monsoons Model Intercomparison Project (GMMIP), we find more improvement in the annual structure of precipitation over the Guinean region with less inter-model spread compared to observations. The inter-annual variability of precipitation is then relatively well captured by all CMIP6 models with significant correlations.

Almazroui et al. (2020) found that precipitation in CMIP6 models will increase in the central part and decrease in the western part of west Africa during the twenty-first century under Shared Socio-economic Pathways (SSP). In this study we further analyse how extreme precipitation indices are represented in SSP5-8.5 over West Africa.