During the last 20 million years the East African Rift System developed due to plate tectonics, which led to changes in topography. The impact of these topography changes on regional climate is analysed here with a global climate model. On such long time scales other factors apart from tectonics can have significant impact on climate. One example is the change in Earth's orbital parameters, which modulate the insolation at the top of the atmosphere. This factor is also analysed with a global climate model.
Fig. 1: Simulated difference (reduced topography - present-day) of precipitation in mm/mon for winter (December to February, left) and summer (June to August, right).
For the analysis of tectonic impact, topography in Eastern and Southern Africa is strongly reduced in the model. Due to the removal of the topographical barrier, atmospheric moisture is transported from the Indian Ocean further into the continent. This leads to an increase in precipitation in Eastern Africa. Simulations with a less strong reduction of topography show very similar patterns in precipitation change, whereas the amount of change is proportionately reduced. This shows that tectonic uplift leads to a slow and continuous change in precipitation.
The insolation at the top of the atmosphere is influenced by the obliquity of the Earth's axis, the eccentricity of the orbit and the position of the perihelion. One example of a strong change in insolation is the orbital configuration of the last interglacial (the Eemian) at 125,000 years before present. During the Eemian the annual cycle of insolation was enhanced on the northern hemisphere and weakened on the southern hemisphere. This leads to a stronger warming of the northern Sahara during summer causing a strengthening of the meridional temperature gradient. Thus, more moisture is transported from the Atlantic into the continent leading to more precipitation. In winter the moisture transport from the Indian Ocean is weakened, which results in a reduction of precipitation.
Both tectonic and orbital forcing have an impact on precipitation in East Africa with a similar order of magnitude. However, tectonic forcing acts on much longer time scales than orbital forcing leading to an overlap of the results, which changes with time.
Fig. 2: Simulated difference (Eemian - present-day) of precipitation in mm/mon for winter (December to February, left) and summer (June to August, right).