Nile Basin Climates

The climate of the Nile Basin is characterised by a strong latitudinal wetness gradient. Whereas the areas north of 18° N remain dry most of the year, to the south there is a gradual increase of monsoon precipitation amounts. Rainfall regimes can be divided into nine types, among which summer peak regimes dominate. In the southern half of the basin, mesoscale circulation features and associated contrasts in local precipitation patterns develop as a result of a complex interplay involving topography, lakes and swamps. Precipitation changes and variability show up as three distinct modes of variability. Drying trends since the 1950s are found in central Sudan and to some extent the Ethiopian Highlands. The equatorial lakes region is characterised by occasional very wet years (e.g. 1961, 1997). The interannual variations are strongly, but indirectly influenced by El-Nino/Southern Oscillation. Sea surface temperature variations over other ocean basins, especially the Indian and South Atlantic Oceans, also play a significant role. Projections for the late twenty-first century show a 2–4°C temperature increase over the basin, depending on the scenario, but rainfall projections are more uncertain. Most models tend to predict a rainfall increase in the equatorial regions, but there is little consistency between models over the tropical regions.