Drivers of the evolution and amplitude of African Humid Periods

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.

Pacemakers of extreme floods during warmer and wetter climates of the “Wild Nile” stage

“…The estimated amount of rainfall brought by the monsoon during the AHP was higher by ca. 75% 10,11 and is thought to have generated a three-to fivefold increase in Nile River runoff 12 . The morphology of the Nile River itself was largely modified due to the development of a dense network of tributaries with a much larger drainage area 13,14 .…”

Accurately calibrated X-ray fluorescence core scanning (XRF-CS) record of Ti ∕ Al reveals Early Pleistocene aridity and humidity variability over North Africa and its close relationship to low-latitude insolation

“…Together with (i) the statistical differences in the relationship between Ti/Al and SITIG before and after the MPT and (ii) the strengthening of wavelengths > 100 kyr in the Ti/Al record at around the same time, this suggests an increased, although non-linear, high-latitude climate impact on North African climate at the MPT. Meltwater discharge events at glacial terminations led to a weakened monsoon system by reducing the Atlantic meridional overturning circulation (e.g., Böhm et al, 2015;Häuselmann et al, 2015;Marino et al, 2015;Menviel et al, 2021;Ziegler et al, 2010) and were probably important for driving the lagged African climate response to insolation. Such largescale meltwater events did not occur until just after the MPT (Hodell et al, 2008), explaining -at least partially -the more in-phase relationship between North African climate and (low-latitude) insolation before the MPT and the more non-linear response after.…”

Climate variability along the margin of the southern African monsoon region at the end of the African Humid Period

“…The influence of glacial boundary conditions is evident in the nature of the onset and early portions of the African Humid Period, often being marked by signals consistent with the ice-forced events (e.g. the Younger Dryas;deMenocal et al, 2000;Menviel et al, 2021;Tierney et al, 2008;Trauth et al, 2018;Weldeab et al, 2007). The termination of the African Humid Period is evidenced as having more diversity in its expression (perhaps somewhat indicative of a greater number of records from the mid-to late Holocene).…”