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date: 23 April 2017

Climate of Eastern Africa

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Climate Science. Please check back later for the full article.

Eastern Africa, classically presented as a major dry climate anomaly region in the otherwise wet equatorial belt, is a transition zone between the monsoon domains of West Africa and the Indian Ocean. Its complex terrain, unequaled in the rest of Africa, results in a huge diversity of climatic conditions that steer a wide range of vegetation landscapes, biodiversity, and human occupations. Meridional rainfall gradients dominate in the west, along the Nile valley and its surroundings, where a single boreal summer peak is mostly observed. Bimodal regimes (generally peaking in April and November) prevail in the east, gradually shifting to a single austral summer peak to the south. The swift seasonal shift of the Intertropical Convergence Zone and its replacement in January through February and June through September by strong meridional, generally diverging winds (e.g., the East African low-level jet stream), account for the low rainfall. These large-scale flows interact with topography and lakes, which have their own local circulation in the form of mountain and lake breezes. This results in complex rainfall patterns, with a strong diurnal component, and a frequent asymmetry in the rainfall distribution with respect to the major relief features. Whereas highly organized, rain-producing systems are uncommon, convection is partly modulated at intra-seasonal (approximately 30–60-day) timescales. Interannual variability shows a fair level of spatial coherence in the region, at least in July through September, in the west (Ethiopia and Nile Valley), and October through December in the east along the Indian Ocean. This is associated with a strong forcing from sea-surface temperatures in the Pacific and Indian Oceans, and to a lesser extent the Atlantic Ocean. As a result, Eastern Africa shows some of the largest interannual rainfall variations in the world. Some decadal-scale variations are also found, including a drying trend, since the 1980s, of the March through May rainy season in the eastern part of the region. Eastern Africa is affected by global warming, with a mean temperature rising by 0.7 to 1 °C from 1973 to 2013, depending on the season. The strong, sometimes non-linear altitudinal gradients of temperature and moisture regimes also contribute to the climate diversity of Eastern Africa.