Variations of East African climate, glaciers and lakes over the past two centuries

Stefan Hastenrath

Department of Atmospheric and Oceanic Sciences
University of Wisconsin-Madison

The evidence on the climatic history of East Africa over the past two centuries comprises historical accounts of lake levels, observations and analyses of glacier variations, wind and current observations in the Indian Ocean, as well as raingauge measurements.

East Africa experiences its rainy seasons in boreal spring and autumn, centered around April-May and October-November; the spring rains being more abundant and the autumn rains more variable. Rains tend to be abundant/deficient with slow/fast westerlies (UEQ) and Eastward Equatorial Jet (EEJ) in the upper hydrosphere of the equatorial Indian Ocean. A drastic climatic dislocation took place during the last two decades of the l9th century, manifest in a drop of lake levels, onset of glacier recession, and acceleration of UEQ and EEJ. The decades immediately preceding 1880 featured high lake stands, extensive glaciation, and slow UEQ and EEJ, as compared to the 20th century. In contrast to the other two glaciated mountain regions on the Equator, glacier recession in East Africa began only in the 1880's, concomitant with an abrupt change in atmospheric circulation and decrease in cloudiness and precipitation. In East Africa increased net shortwave radiation provided the forcing for the onset of the glacier recession, while the later ice shrinkage may be ascribed to greenhouse effects. Above the mean freezing level, on the summit of Kilimanjaro, shortwave radiative forcing may continue to be important.

Stefan Hastenrath