Interannual Variability of Seasonal Rainfall and Associated Circulations over Gambella, Ethiopia

Abstract

Ethiopia composes diversified topographic structures; undulated plateaus and mountains, raged valleys and plains. The highlands and ever-green portions of the county are fringed by the Sahara and Arabian deserts as well as East African arid climates. This study investigates the spatial and temporal seasonal (June-September [JJAS]) rainfall variability and the associated circulation patterns over Ethiopia. The study utilized ten daily station gridded rainfall, monthly SST, Outgoing Longwave Radiation (OLR) and zonal and meridional wind at various levels for 1983-2015. Statistical methods employed include EOF, composite and correlation analysis. The three leading EOF analysis revealed 57.8% that the first and second PC modes over Gambella are associated with ENSO and IOD variability. Wind and velocity potential revealed that wet (dry) years were characterized by convergence (divergence) in the lower level and divergence (convergence) at the upper level, implying rising (sinking) motion over the study area and western Indian Ocean. During normal (wet) years, maximum intensity of westerlies was generally observed at 850 hPa level, while easterlies were dominant at the upper level. There exists a significant correlation between JJAS rainfall and Nino3.4 for ENSO with r value (-0.56) but insignificant for (DMI) IOD with r value (-0.11). A cold (warm) summer SST over the central equatorial Pacific is associated with more (less) rainfall over Ethiopia. This shows that both ENSO and IOD modes have influence on rainfall variability during main rainy season. The findings of this and related studies is crucial to the development, planning and mitigation of all rainfall dependent activities.