Characterization and variability of Kiremt rainy season over Ethiopia

Ethiopia has been ravaged by severe drought for many of the last 35 years, primarily due to the failure of its main (Kiremt) rainy season in boreal summer. Kiremt quality results from the timing of its onset and cessation and the frequency and duration of intervening dry-spells. To address these key aspects of Kiremt variability, we analyzed specially constructed sets of research quality, daily rainfall and rawinsonde data for the longest available periods (25-57 years). The analyses produced wide-ranging results of considerable value to Ethiopia. The long-term average spatial progression of the southwest-to-northeast Kiremt onset and its reverse cessation are documented, along with measures of their interannual variability. Treated on a similar geographical basis is the local vulnerability to Kiremt interruption by dry-spells. Rawinsonde data for central Ethiopia are analyzed to place these long-term mean surface Kiremt characteristics in the context of the annual cycles of tropospheric wind, temperature, and pressure. Investigation of the rich interannual Kiremt variation focuses on onset, cessation, and growing length (which excludes dry-spells) anomalies. The analyses begin with the compositing of indicative tropospheric profiles for sets of extremely dry and wet Kiremt seasons. This is followed by examination of 1961-99 time series of the above Kiremt parameters, which prompts case study investigations of the highly contrasting 1984 (very dry) and 1996 (much wetter) Kiremts in terms of both Ethiopian rainfall and the tropospheric circulation of the surrounding region. Finally, correlation analyses are used to investigate relations between the above key Kiremt parameters for the most drought-prone (northeastern) part of the Kiremt region and global tropical-subtropical sea surface temperature patterns, including the ENSO phenomenon.