Dynamic interaction of lakes, climate and vegetation over northern Africa during the mid-Holocene

Abstract. During the early to mid-Holocene, about 11,500 to 5,500 years ago, lakes expanded across the Sahel and Sahara in response to enhanced summer monsoon precipitation. To investigate the effect of these lakes on the West African summer monsoon, previous simulation studies prescribed mid-Holocene lakes from reconstructions. By prescribing mid-Holocene lakes, however, the terrestrial water balance is inconsistent with the size of the lakes. In order to close the terrestrial water cycle, we construct a dynamic endorheic lake (DEL) model and implement it into the atmosphere-land model ICON-JSBACH4. For the first time, this allows us to investigate the dynamic interaction between climate, lakes and vegetation over northern Africa. Additionally, we investigate the effect of lake depth changes on the mid-Holocene precipitation, which was neglected in previous simulation studies.

A pre-industrial control simulation shows that the DEL model realistically simulates the lake extent over northern Africa. Only in the Ahnet and Chotts Basin is the lake area slightly overestimated, which likely relates to the coarse resolution of the simulations. The mid-Holocene simulations reveal that both, the lake expansion and the vegetation expansion cause a precipitation increase over northern Africa. The sum of these individual contributions on the precipitation is however larger than the combined effect, when lake and vegetation dynamics interact. Thus, the lake-vegetation interaction causes a relative drying response over the entire Sahel. The main reason for this drying response is that the simulated vegetation expansion cools the land surface more strongly than the lake expansion, mainly the expansion of the Lake Chad area. The resulting warming response over the larger lake area causes local changes in meridional surface-temperature gradient that decelerate the inland moisture transport from the tropical Atlantic into the Sahel, which causes a drying response in the Sahel. An idealized mid-Holocene experiment shows that a similar drying response is induced when the depth of Lake Chad is decreased by about 1–5 m, without changing the horizontal lake area. By reducing the depth of Lake Chad the heat storage capacity of the lake decreases and the lakes warms faster during the summer months. Thus, in the ICON-JSBACH4 model, the lake depth significantly influences the simulated surface temperature and the simulated meridonal surface-temperature gradient between the simulated lakes and vegetation and thus, the mid-Holocene precipitation over northern Africa.