Eyesus-Getahun, Yohannes G. (National Meteorological Services Agency, Addis Ababa, Ethiopia)
The period from October to January (of the next year) is known as "Bega" season in Ethiopia. Bega is characterizred by a southward retreating Intertropical Convergence Zone (ITCZ). As a result the rainfall activity decreases gradually from October to January. Especially during the months of December and January very few rainy days with light precipitation are experienced, normally below five days. Dry, windy & hazy days coupled with cool nights & early mornings are common features in this period. However, this dry situation is sometimes interrupted by short-living wet spells. Four major weather systems could be accounted for such type of phenomena: perturbations in the tropical easterlies, tropical disturbances which form in the Arabian Sea & the North Indian Ocean, the Rd Sea Convergence Zone (RSCZ) and interactions between tropical & extratropical weather systems.
In December 1995 & January 1996 an unusually prolonged wet spell was observed over Ethiopia. Diagnostic results of weather systems shows that frequent formation of frontal cyclones over East Mediterranean during December 1995. Also during January 1996 East Mediterranean cyclones were forming at a more southerly latitude and they were responsible for the development of slow-moving subtropical cyclones over the Red Sea & the Middle East. Moreover the value of the North Atlantic Oscillation (NAO) index dropped to -2.2 in December 1995, the first time since December 1989. Such situations enhanced low-latitude westerly flow across southern Europe and the northern Mediterranean. The overall effect of such systems' configuration had been a prolonged & frequent interaction of tropical & extratropical weather systems over the Horn of Africa & the Middle East Region. To the peculiarity of the situation the ITCZ had a southernmost position of 20░S in December 1995 & 18░S in January 1996 over East Africa.
Over the Highlands of Ethiopia the interaction has resulted in abundant rainfall amounting up to 124 mm in December 1995 & 182.5 mm in January 1996. Within a day heavy precipitation up to 51.3 mm & 91.1 mm were recorded in December 1995 & January 1996 respectively. Also the number of rainy days range between 10 to 50 days over most parts of the country. As such this kind of phenomena is particularly disastrous for the country's agricultural activity, December and January being the peak harvesting months. Eventhough economic loses due to this event could not be quantified fully, damage to standing crops had been substantial. Had there been the capacity & the knowledge on how to predict this phenomena at least 10 days ahead, it would have been possible to minimize loss of agricultural production. So as a posteriori to this type of case study, better understanding of the mechanisms of the interactions between tropical and extratropical weather systems could be achieved.