The end of the African humid period as seen by a transient comprehensive Earth system model simulation of the last 8000 years

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.

African Humid Period Precipitation Sustained by Robust Vegetation, Soil, and Lake Feedbacks

“…It has been suggested that the greater magnitude of precipitation inferred from leaf wax proxies would require some rainfall in the winter season (Tierney et al, 2017). Some modeling studies do show marginally increased precipitation in the winter months (Dallmeyer et al, 2020; Tierney et al, 2017). However, as for the present, most precipitation in our mid‐Holocene simulations occurs only during the summer months with very little change to the precipitation pattern in winter months (Figures 1, S13, and S14).…”

“…An alternate approach to the question of Green Sahara utilizes dynamic vegetation models (DGVMs) wherein the distribution of vegetation is dynamically simulated and responds to the state of the climate (Claussen & Gayler, 1997;Rachmayani et al, 2015). Results from DGVMs have yielded varying levels of fidelity with proxy reconstructions (Braconnot et al, 2012(Braconnot et al, , 2019Claussen et al, 2017;Hopcroft et al, 2017) although recently Dallmeyer et al (2020) have obtained improved agreement with proxies using an updated Earth system model. DGVMs are not the subject of this paper.…”

Mid‐Holocene Sahara‐Sahel Precipitation From the Vantage of Present‐Day Climate

“…Although the past decades have seen increasingly closer matches of simulated rainfall over the Sahel to that inferred from paleoclimate data, models that were tuned to match those data have continued to fall short in both precipitation amounts and latitudes of the rain belt. Coupled atmosphere‐ocean GCMs do better than those that use a slab ocean (e.g., Alder & Hostetler, 2015; Bosmans et al, 2012; Braconnot et al, 2000, 2002, 2007a, 2007b, 2019; Brown et al, 2008; Dallmeyer et al, 2018; Liu et al, 2017; Otto, Raddatz, Claussen, Brovkin, & Gayler, 2009; Otto, Raddatz, & Claussen, 2009; Shin et al, 2006). In particular, using an atmospheric GCM and 6‐ka insolation, Braconnot et al (2000) simulated the rain belt to lie ~2° north of that today, but with a coupled GCM, 3° north of it, if still short of some mid‐Holocene estimates that reach 5°.…”

Late Quaternary Climate Variability and Change from Aotearoa New Zealand Speleothems: Progress in Age Modelling, Oxygen Isotope Master Record Construction and Proxy-Model Comparisons

“…To evaluate the δ 18 O calcite relationship to temperature through time, we compared the 100-year binned Waitomo δ 18 O ivc isotope master record to time-equivalent surface temperature series compiled from several transient Holocene climate model simulations (CCSM3-TracCE-21k [66]; EC-Bilt-Clio [67]; ECHO-G [68]; MPI-ESM1.2 [69]). Annually-resolved mean monthly temperature data from the grid cell closest to Waitomo (~38 • S and~175 • E) were obtained from each model; three-month averages (e.g., June-July-August, JJA; July-August-September, JAS; August-September-October, ASO; etc.)…”