Cloudiness fluctuations over Eastern Africa

Abstract

Based on ESSA-satellite imagery for the period July 1969–June 1970, this study investigates spatial and temporal variations of East African cloudiness. The major results of this work show that the mean annual cloud amounts over East Africa are lower than those in adjacent tropical areas. One of the main reasons for this is the quasi-meridional alignement of the ITCZ over East Africa during the winter months. Within the area itself, the highest mean annual cloud amount values can generally be found in a diagonally oriented zone extending from the eastern Congo Basin to the Ethiopian Highlands. In contrast to the cloudiness north of the equator, which is dominated by oscillation periods in the range of 30–60 days, the cloudiness fluctuations encountered south of the equator show periodicities around 2 days (in the western part) and 20 days (in the eastern part), respectively. The different oscillation patterns, which are roughly separated by the Rift Valley area (longitudinally) and the equator (latitudinally), resemble the signals of the adjacent (African and Asian) monsoon regimes. However, during the winter months oscillation periods around 40 days can be found north of the equator, whereas a quasi-biweekly oscillation appears over the coastal areas in summer. Further details of the seasonal variability of East African cloudiness are discussed.