Performance evaluation of CMIP6 GCMs and projection of precipitation extremes over the Jemma sub-basin, Upper Blue Nile basin of Ethiopia

Abstract

Accurate simulation and projection of precipitation extremes are critical for climate adaptation planning. This study aims to evaluate the performance of the Coupled Model Intercomparison Project Phase 6 (CMIP6) Global Climate Models (GCMs) in simulating extreme precipitation and to project future changes in the Jemma sub-basin. Daily precipitation data from seven meteorological stations in subtropical and temperate agro-ecological zones (AEZs) were used to evaluate CMIP6 GCM performance based on ten precipitation extreme indices. The top-performing GCMs were selected based on their skill assessed through the Comprehensive Rating Index (CRI). The GCMs were then used to project changes under three greenhouse gas emission scenarios (SSP2-4.5: medium emissions, SSP3-7.0: high emissions, and SSP5-8.5: very high emissions) for near (2030–2049), mid (2060–2079), and far future (2080–2099) periods relative to 1995–2014. The selected 8 GCMs demonstrated moderate skill, with CRI values ranging from 0.29 to 0.78 in the subtropical AEZ and 0.44 to 0.80 in the temperate AEZ. Indices such as Rx5day (multi-day maximum rainfall) and R10mm (heavy rainfall frequency) were captured more reliably than dry-spell metrics like consecutive dry days (CDD). Projections reveal notable variability in extreme precipitation across scenarios, periods, and AEZs. In the subtropical AEZ, R10mm and R20mm are projected to rise by 1.57 and 0.28 days/year, respectively, under SSP3-7.0 (near-term). Rx5day is expected to increase by 1.5 mm (SSP2-4.5, far-term) and 1.54 mm (SSP3-7.0, mid-term), while R95p (very wet day precipitation) rises by 7.51 mm (SSP3-7.0, near-term). In the temperate AEZ, mixed trends are observed: Rx1day increases by 0.88 mm (SSP5-8.5, far-term), Rx5day rises by 1.5 mm (SSP3-7.0, mid-term), but drops by 1.29 mm (SSP2-4.5, far-term). R95p generally trends upward, although a 5.36 mm decline occurs under SSP3-7.0 (near-term). Annual total wet-day rainfall (PRCPTOT) and SDII (simple daily intensity index) mostly increase, though occasional SDII declines suggest heightened uncertainty. The projected changes in precipitation extremes suggest increasing challenges for agricultural systems and water resource management, underscoring the need for climate-resilient planning in the Jemma sub-basin.