Droughts and floods over the upper catchment of the Blue Nile and their connections to the timing of El Nino and La Nina events

The Blue Nile originates from Lake Tana in the Ethiopian Highlands and contributes about 60-69% of the main Nile discharge. Previous studies investigated the relationship of sea surface temperature (SST) in the Pacific Ocean (Nino 3.4 region) to occurrence of meteorological and hydrological droughts in the Nile Basin. In this paper we focus on the dependence of occurrence of droughts and floods in the upper catchment of the Blue Nile on the timing of El Nino and La Nina events. Different events start at different times of the year and follow each other, exhibiting different patterns and sequences. Here we study the impact of these timing and temporal patterns on the Blue Nile droughts and floods. The comparison between the discharge measurements (1965-2012) at the outlet of the upper catchment of the Blue Nile and the El Nino index shows that when an El Nino event is followed by a La Nina event, there is a 67% chance for occurrence of an extreme flood. Furthermore, we also found that 83% of El Nino events starting in April-June resulted in droughts in the upper catchment of the Blue Nile. Although the current study is limited by the reduced number of samples, we propose that observations as well as global model forecasts of SST during this season could be used in seasonal forecasting of the Blue Nile flow.