Abstract
During orbital precession minima, the Sahara was humid and more vegetated, providing potential corridors for Hominins migration. Uncertainties remain over the climatic processes controlling the initiation, demise and amplitude of these African Humid Periods. Here we study these processes using a series of transient simulations of the penultimate deglaciation and Last Interglacial period, and compare the results with a transient simulation of the last deglaciation and Holocene. We find that the strengthening of the Atlantic Meridional Overturning Circulation at the end of deglacial millennial-scale events exerts a dominant control on the abrupt initiation of African Humid Periods as the Atlantic Meridional Overturning Circulation modulates the position of the Intertropical Convergence Zone. In addition, residual Northern Hemispheric ice-sheets can delay the peak of the African Humid Period. Through its impact on Northern Hemispheric ice-sheets disintegration and thus Atlantic Meridional Overturning Circulation, the larger rate of insolation increase during the penultimate compared to the last deglaciation can explain the earlier and more abrupt onset of the African Humid Period during the Last Interglacial period. Finally, we show that the mean climate state modulates precipitation variability, with higher variability under wetter background conditions.
Highlights
During orbital precession minima, the Sahara was humid and more vegetated, providing potential corridors for Hominins migration
sea surface temperatures (SSTs) off the Iberian margin increase by up to 7.2 ∘C (Fig. 1e), in agreement with the SST estimates from marine sediment core MD01-244440. These deglacial changes are interrupted by HS11, during which a weakened AMOC29 induces a ~1∘ SST decrease in the North Atlantic compared to the penultimate glacial maximum (PGM, here taken at 140 ka)
The experiments suggest that during the LIG the Intertropical Convergence Zone (ITCZ), defined here as the latitude of maximum precipitation, was displaced northward over tropical north Africa and the West African monsoon (WAM) was stronger leading to higher precipitation rates over a larger area than during both the Holocene and PI (Figs. 3 and 6)
Summary
The Sahara was humid and more vegetated, providing potential corridors for Hominins migration. Jet (AEJ) is maintained by meridional surface temperature and soil moisture gradients over tropical north Africa[3,4], and arises from the meridional circulations controlled by the ITCZ to the south and Saharan heat low to the north[5] Both the ITCZ and the AEJ modulate present-day precipitation over the Sahel (14∘N–18∘N), with a weak and northward displaced AEJ leading to wet conditions[1,6]. As a result of the low precession and associated high boreal summer insolation prevailing during the early Holocene, the ITCZ was located further north during summer, leading to wetter conditions over the Sahel and Sahara During this African Humid Period (AHP, ~11–5.5 thousand years before present, thereafter ka) the Sahara had increased vegetation cover and biodiversity, including tropical plant species[7], and an extensive network of drainage channels and lakes[8]. During the Last Interglacial period (LIG, ~129–116 ka), the warmest interglacial period of the last 800 ka[14], summer insolation at high northern latitudes was more than 70 W/m2 higher than during the pre-industrial (PI)
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