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The Upper Blue Nile river basin (uBN) is vital to three countries' water, food, and energy security (Ethiopia, Sudan, and Egypt). It is also a region with substantial interannual precipitation variability and the potential for significant climate change in coming decades. The El Niño–Southern Oscillation (ENSO) affects both variability and trends in uBN precipitation, with El Niño episodes historically connected with below‐average rainfall. In this context, this study seeks to: (a) examine the representation of ENSO–uBN rainfall teleconnections in simulations from the sixth Coupled Model Intercomparison Project (CMIP6), as compared to the previous generation of models (CMIP5); (b) distinguish between teleconnections during the peak of the rainy season (July–August [JA]) and the agriculturally critical end‐of‐season rains (September–October [SO]); (c) investigate evidence for Tropical Easterly Jet (TEJ) mediation of these teleconnections in CMIP6 versus CMIP5 and for both peak rains and end of the rainy season. Using a subset of high performing models, we find that CMIP6 simulations of uBN precipitation show somewhat less bias than CMIP5 for JA total rainfall, and that correlations with ENSO have become more consistent across models, but that differences between CMIP5 and CMIP6 are modest. CMIP6 simulations, like CMIP5, overestimate SO rainfall and suggest a stronger ENSO association in SO than is indicated by observations. Model representation of the TEJ and its association with both ENSO and uBN precipitation show no systematic change between CMIP5 and CMIP6. The mediating influence of the TEJ appears to be more important in JA than SO. [ABSTRACT FROM AUTHOR]

The drivers of precipitation variability in the Ethiopian Highlands are complex, and they have considerable impact on the local rural economy of the region. While interannual variability has been studied extensively, relatively less study has focused on intra-seasonal variability in precipitation, even though this variability can be critical for agricultural activities. In this study, we examine the impacts of the Boreal Summer Intra-Seasonal Oscillation (BSISO) on precipitation variability in the Upper Blue Nile Basin (UBNB) during the rainy season (June–September). In order to optimally capture Intra-seasonal variability, we developed a customized ISO index based on leading modes of the variability in the UBNB, and found that this index shares the general character of a standard BSISO index. Analyses performed with both the customized and standard BSISO indices show that a core convective region centered on the Indian Ocean propagates to the northwest, eventually exerting significant influence on Ethiopian rainfall. The primary mechanism of influence appears to be BSISO influence on both lower and upper level tropospheric wind fields over the western Indian Ocean that influence moisture flux, convergence, and convection in the study region. This includes BSISO modulation of the East Africa Low-Level Jet and the Tropical Easterly Jet. We also find that the BSISO influence on Ethiopia is consistent with a BSISO-associated Rossby wave signal, but the magnitude of the Rossby wave signal is small relative to other mechanisms of influence. [ABSTRACT FROM AUTHOR]