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

Abstract

Abstract. 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 (Niño 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 Niño and La Niña 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 at the outlet of the upper catchment of the Blue Nile and the El Niño index shows that when an El Niño event is followed by a La Niña event, there is a 67 % chance for occurrence of an extreme flood. Furthermore, we also found that 83 % of El Niño 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.

Simulating the connections of ENSO and the rainfall regime of East Africa and the upper Blue Nile region using a climate model of the Tropics

“…The model (average of nine members) was able to reproduce the observed negative correlation between the Niño 3.4 SST and precipitation over the focus regions, even outperforming the ERA-Interim reanalysis. It also showed the highest correlation of SST and summer precipitation over the Ethiopia Highlands during AMJ (−0.62), in line with previous observational studies (Zaroug et al, 2014). This result suggests that AMJ SSTs over the Niño 3.4 region can be a useful predictor in seasonal (JJAS) precipitation forecasting over East Africa and the Blue Nile region.…”

“…Members 1, 6 and 9 were below the 95 % confidence level for all the seasons. The correlation is not significant for nearly all the members in JFM and FMA, a result in line with a recent study by Zaroug et al (2014). The average of the nine members shows a highly significant correlation when compared with each ensemble member.…”

“…The same calculation was completed for each ensemble member (not shown here) and member 7 showed the largest R 2 values. In a recent study, Zaroug et al (2014) showed that the Blue Nile discharge showed the highest correlation for drought events with the Niño 3.4 SST during AMJ, a result which appears to be in line with Fig. 10.…”

“…The average of the nine members shows a highly significant correlation when compared with each ensemble member. The confidence test for the GPCP and CRU were calculated in Zaroug et al (2014), who confirmed the strong impact of ENSO anomalies on the rainfall of the Ethiopia Highlands.…”

Future Nile river flows

“…Rainfall and river flows in the Eastern Nile Basin are strongly seasonal, peaking around late boreal summer, and exhibit a marked association with the El Niño-Southern Oscillation (ENSO), with drier conditions tending to occur during El Niño events (2). The El Niño event of 2015 was the strongest on record and associated with a consistent tele-connection pattern of drought in large parts of the Eastern Nile Basin (Figure 1).…”