Long-term hydroclimatic variability over the semi-arid Ethiopian highlands in relation to ENSO and IOD teleconnection signals

Large-scale oceanic effects of El Nino Southern Oscillation and Indian Ocean Dipole on local rainfall and streamflow variability at the catchment scale have not been well studied. This study assesses the effects of El Nino Southern Oscillation and Indian Ocean Dipole on rainfall and streamflow variability in Geba catchment Tigray Ethiopia. Modified Mann-Kendall and Pettit's tests were used to generally understand the overall trends and expected abrupt changes in rainfall and streamflow. We also applied correlation, explained variance, and multiple regression methods to evaluate the linkages, performance, and strengths between the sea surface temperatures and station rainfall and streamflow records. The rainfall trend of the study site did not change significantly. However, the streamflow showed a significant change with the mixed increase or decrease trends. The abrupt changes in rainfall and streamflow are strongly linked with El Nino Southern Oscillation and La Nina episode events. Rainfall in the rainy season (July, August, and September) enhanced due to La Nina events and suppressed with El Nino episodes. The stream flow of the study catchment gets declined during strong El Nino years (e.g., 1982, 1983, 1987, 1997, and 2015) while also reach its peak during the strong La Nina years (1988, 1999, 2000, 2008, and 2011). Our finding ratified that 39% of rainfall and 38% of streamflow variabilities in the Geba catchment are explained by the combined effects of El Nino Southern Oscillation and Indian Ocean Dipole signals.