Evolution of intraseasonal precipitation variability in the Blue Nile River basin

The upper Blue Nile basin in Ethiopia, which contributes about 60% of the annual average discharge of the Nile River at Aswan High Dam, is subject to significant interannual precipitation variability. As this variability is crucial to local food security and water resources in downstream countries, a growing number of studies have attempted to explain and to predict Blue Nile rainy season (June-September) precipitation. Taken collectively, these studies present a wide range of large scale drivers associated with precipitation variability in the Blue Nile: El Niño/La Niña-Southern Oscillation (ENSO), the Indian summer monsoon, sea level pressure (SLP) anomalies over the Arabian Peninsula and the Gulf of Guinea, the quasi-biennial oscillation (QBO), and dynamics of the Tropical Easterly Jet (TEJ) and African Easterly Jet (AEJ) have all been emphasized to varying degrees. The purpose of the current study is to reconcile these diverse analyses by evaluating teleconnection patterns and potential mechanisms of association on sub-seasonal scale, using multiple data sets for confirmation. It is found that associations with ENSO, the Walker circulation, northwest Indian Ocean sea surface temperature (SST), and Pacific Decadal Oscillation are evident only in the late rainy season, with a maximum in September. Associations of Blue Nile precipitation with the core position and strength of TEJ and with Indian monsoon precipitation are also confined to the late rainy season, and are statistically significant only in September. Precipitation variability in August is most strongly associated with variability in westerly and southwesterly winds from the Atlantic Ocean, and thus with the strength of the South Atlantic High. As a result, August precipitation shows significant correlation with the Sahel rainfall which is not evident in other months. July precipitation, in contrast, shows significant negative correlations with the AEJ and Arabian Peninsula SLP, while June precipitation is negatively correlated with SLP over the Equatorial Atlantic. Neither June nor July precipitation shows any significant correlation with Equatorial Pacific Ocean SST. Correlations between Blue Nile precipitation and indices of the Madden-Julian Oscillation (MJO) are found in all months of the rainy season. The observed intraseasonal evolution of teleconnections across the rainy season indicates that sub-seasonal analysis is vital to advance understanding and prediction of precipitation and discharge variability in the Blue Nile basin.

Key words: Blue Nile basin, Ethiopia, ENSO, Indian monsoon, tropical easterly jet, African easterly jet