Evolution and copula modelling of drought duration and severity over Africa using CORDEX-CORE regional climate models

This study assessed the dependence of drought severity and duration across four subregions (Southern, Western, Eastern and Central) in Africa using copula modelling. The analysis was carried out for the reference period 1991-2020 and the future period 2071-2099. Simulated daily precipitation at a horizontal resolution of 0.22 degrees were obtained from three regional climate models (RCMs) participating in the Coordinated Output for Regional Evaluations within the Coordinated Regional Downscaling Experiment (CORDEX-CORE). The RCMs were downscaled by three global climate models and validated using three high-resolution gridded daily precipitation products obtained from The Climate Hazards Group InfraRed Precipitation with Stations data (CHIRPS), Climate Prediction Center Africa Rainfall Climatology Version 2.0 (CPC-ARC2) and Tropical Applications of Meteorology using SATellite data and ground-based observations (TAMSAT). Comparison of precipitation from the RCMs with the three gridded reference observations shows relatively good performance across the different regions with median value within the range of 3-5 mm center dot day(-1) in Central Africa, 1.5-4.5 mm center dot day(-1) in Western Africa, 1.8-2.8 mm center dot day(-1) in Eastern Africa and 0.5-2.2 mm center dot day(-1) in Southern Africa. On the other hand, the correlation values of drought duration-severity from CORDEX-CORE models in the different regions of Africa exhibit predominantly strong values greater than 0.8 for both historical and projected climate. The analysis also considered two families of copulas: Archimedean (Frank, Clayton, Gumbel) and Elliptical (Gaussian, Student's t). The performance of the copula functions were estimated using the Akaike information criteria (AIC). Generally, there is a good agreement in the distribution of observed precipitation among the three observational data products across the subregion of Africa, with slight differences attributed to the different processing algorithms of the products. Across West Africa, mean precipitation values from CPC, CHIRPS and TAMSAT was 2.86, 3.16 and 3.07 mm center dot day(-1), respectively. The CCLM-NCC underestimated the mean values of precipitation reported in the observation data, while CCLM-MPI and CCLM-HAD overestimated the mean values of precipitation in the West Africa region.