Abstract
Abstract. Enhanced summer insolation during the early and mid-Holocene drove increased precipitation and widespread expansion of vegetation across the Sahara during the African humid period (AHP). While changes in atmospheric dynamics during this time have been a major focus of palaeoclimate modelling efforts, the transient nature of the shift back to the modern desert state at the end of this period is less well understood. Reconstructions reveal a spatially and temporally complex end of the AHP, with an earlier end in the north than in the south and in the east than in the west. Some records suggest a rather abrupt end, whereas others indicate a gradual decline in moisture availability. Here we investigate the end of the AHP based on a transient simulation of the last 7850 years with the comprehensive Earth system model MPI-ESM1.2. The model largely reproduces the time-transgressive end of the AHP evident in proxy data, and it indicates that it is due to the regionally varying dynamical controls on precipitation. The impact of the main rain-bringing systems, i.e. the summer monsoon and extratropical troughs, varies spatially, leading to heterogeneous seasonal rainfall cycles that impose regionally different responses to the Holocene insolation decrease. An increase in extratropical troughs that interact with the tropical mean flow and transport moisture to the western Sahara during the mid-Holocene delays the end of the AHP in that region. Along the coast, this interaction maintains humid conditions for a longer time than further inland. Drying in this area occurs when this interaction becomes too weak to sustain precipitation. In the lower latitudes of west Africa, where the rainfall is only influenced by the summer monsoon dynamics, the end of the AHP coincides with the retreat of the monsoonal rain belt. The model results clearly demonstrate that non-monsoonal dynamics can also play an important role in forming the precipitation signal and should therefore not be neglected in analyses of north African rainfall trends.
Highlights
Periodic variations in the Earth’s orbit around the sun trigger the alternation of dry and humid phases in the north African Sahara (Kutzbach, 1981; deMenocal and Tierney, 2012)
In the lower latitudes of west Africa, where the rainfall is only influenced by the summer monsoon dynamics, the end of the African humid period (AHP) coincides with the retreat of the monsoonal rain belt
The African humid period has provided a challenging palaeoclimate modelling target due to the difficulties in reproducing the magnitude and extent of wet conditions suggested by proxy data and the evidence for nonlinear behaviour during the transitions into and out of the AHP, both of which have been ascribed to the impact of land surface feedbacks on the monsoon
Summary
Periodic variations in the Earth’s orbit around the sun trigger the alternation of dry and humid phases in the north African Sahara (Kutzbach, 1981; deMenocal and Tierney, 2012). The main driver of these changes is the precession of the equinoxes, which leads to a shift in the time of perihelion and alters the seasonal insolation. Wet conditions return roughly every 20 000 years in the Sahara (Skonieczny et al, 2019), as a consequence of increased summertime insolation. The last wet phase was established during the early- and mid-Holocene and is commonly called the “green Sahara” or “African humid period” (AHP, Claussen et al, 2017). The climate and environmental changes during that time are well-documented by diverse proxies from terrestrial and marine palaeorecords.
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