Temporal and Spatial Climate Variability and Trends over Abay (Upper Blue Nile) River Basin

The spatial and temporal variabilities of climate in Abay (Blue Nile) river basin from 1979 to 2014 have been studied using both station and satellite based observations. Rotated empirical orthogonal function (REOF), regression and wavelet analyses were used to investigate the trend, frequency and intra-annual variability of climate over the Abay (Blue Nile) river basin. High variability of rainfall has occurred over the western regions during spring and southern regions during summer seasons from 1979–2014. The results have shown that, the variability over the regions is dominated by inter-decadal signals, which is similar periodic variability with large-scale circulation. Different conditions of atmospheric and oceanic anomalies are responsible for the observed negative/positive anomaly years in the two seasons. For instance, negative anomaly years during spring season are significantly associated with the negative phase of El Niño Southern Oscillation (ENSO), the weakening of the Arabian High and the subtropical westerly jet streams whereas during summer they are linked to the positive ENSO phase, a weakening of the upper level jet streams, and a lower level moisture influx from the Atlantic and Indian oceans. Un-even heating of equatorial Pacific, Atlantic and Indian Ocean significantly affects extreme anomaly years during both seasons via altering the rain producing large-scale circulations over Abay river basin.

Keywords:- Abay (Blue Nile), REOF, Wavelet, Climate variability, Large-scale circulations.