Climate predictions using recent and high-resolution climate models are becoming important for effective decision-making and for designing appropriate climate change adaptation and mitigation strategies. Due to highly variable climate and data scarcity of the upper Blue Nile Basin, previous studies did not detect specific unified trends. This study discusses, the past and future climate projections under CMIP6-SSPs scenarios for the basin. For the models’ validation and selection, reanalysis data were used after comparing with area-averaged ground observational data. Quantile mapping systematic bias correction and Mann–Kendall trend test were applied to evaluate the trends of selected CMIP6 models during the 21st century. Results revealed that, ERA5 for temperature and GPCC for precipitation have best agreement with the basin observational data, MRI-ESM2-0 for temperature and BCC-CSM-2MR for precipitation were selected based on their highest performance. The MRI-ESM2-0 mean annual maximum temperature for the near (long)-term period shows an increase of 1.1 (1.5) °C, 1.3 (2.2) °C, 1.2 (2.8) °C, and 1.5 (3.8) °C under the four SSPs. On the other hand, the BCC-CSM-2MR precipitation projections show slightly (statistically insignificant) increasing trend for the near (long)-term periods by 5.9 (6.1)%, 12.8 (13.7)%, 9.5 (9.1)%, and 17.1(17.7)% under four SSPs scenarios.

This study compares Climate Research Unit version 3 reference observations in the period 1980–2009 with Coupled Model Intercomparison Project Five (CMIP5)‐simulated climate patterns and trends over the Ethiopian highlands (7–14N, 36–40E) using the rcp6 scenario. Three metrics are used in a stepwise statistical evaluation: (1) mean annual cycle of area‐averaged rainfall (R) and maximum temperature (T_x), (2) spatial pattern of R and T_x over Ethiopia and (3) interannual (1.5–6 years) R and T_x variability. Those models passing the first step by simulating a unimodal annual cycle reaching 28 °C in March–May and 9 mm day⁻¹ in July–August are then evaluated for their ability to represent the cool, wet ‘island’ pattern over the highest elevation. Finally, the models' ability to follow observed area‐averaged R and T_x anomalies from 1980 to 2009 is analysed by correlation. The stepwise metrics are designed to improve confidence in the use of CMIP5 models for future resource management and development planning across the Ethiopian highlands. The models with optimal performance in the most categories include the HAD, CCSM and GFDL.

Satellite-derived rainfall and vegetation [normalized difference vegetation index (NDVI)] data for the period 1981–2000 are used to reveal spatial and temporal interrelationships via principal component analysis. The unimodal seasonal cycle peaks in austral summer over the Zambezi Valley and interior plateau. Spectral analysis indicates major cycles of intraseasonal rainfall events at ∼20 and 40 days in response to atmospheric wave forcing that is partially phase locked to the annual cycle. The spatial loadings exhibit a “center of action” along the eastern edge of the Kalahari (±23°E), extending from the western Zambezi toward central South Africa, spanning 20° of latitude. The loading pattern is consistent with tropical–temperate troughs and associated northwest (NW) cloud bands. Twenty-day rainfall oscillations occur over the Agulhas Current and Angola, hence NW cloud bands are a slower terrestrial harmonic of the faster modes at either end. NDVI also exhibits intraseasonal cycles operating near 40 days with spatial loadings collocated with the slow rainfall mode at a 10–20-day lag. It is postulated that an earlier rainfall event and subsequent “greening” results in a moisture flux that promotes the next rainfall event. This is reflected in a composite analysis of low-level velocity potential. The vegetation draws airflow toward itself in a self-sustaining way.