Trend and Periodicity of Drought over Ethiopia

We analyse rainfall extreme events in Ethiopia from 1979 to 2014 using the standardized precipitation index (SPI) and the Palmer drought severity index (PDSI) derived from both station and satellite-based observation data sets. Causal mechanisms of extreme events are also discussed. Trend principal component (TPC), regression, wavelet and composite analyses are used to investigate the trend, frequency and inter/intra-annual variability of extreme events (dryness/wetness of rainfall) over Ethiopia. All methods of analysis, applied to monthly mean data, show that the north and northwest regions of Ethiopia experienced frequent and more severe drought conditions centred at the year 1983/1984, a recovery in the middle of the study period and a return to moderate dry events in recent years. For the southern and southwestern regions, drought conditions have become more frequent and intense during the study period, particularly since ∼1997. Analysis at the seasonal scale shows that the observed drying trend over the south and southwestern regions of the country is dominated by the spring season, which corresponds to the season of maximum precipitation. No observed long-term trend is found in the north, northwestern and central mountainous regions of the country. This contrast reflects differing climate sensitivities of these different portions of the country: the observed periodicity of dryness/wetness over the northern regions corresponds largely to ENSO variability in both the spring and summer rainy seasons, while the drying trend in the south and southwest is associated with Atlantic Ocean warming and sea surface temperature gradients across the western Pacific Ocean.

KEY WORDS SPI; PDSI; drought; TPC; variability; Ethiopia; wavelet