Modulation of Mid‐Holocene African Rainfall by Dust Aerosol Direct and Indirect Effects

Abstract

Climate model simulations of the mid‐Holocene (MH) consistently underestimate northern African rainfall for reasons not fully understood. While most models incorporate orbital forcing and vegetation feedbacks, they neglect dust reductions associated with greater vegetation cover. Here we simulate the MH climate response to reduced Saharan dust using CESM CAM5‐chem, which resolves direct and indirect dust aerosol effects. Direct aerosol effects increase Saharan and Sahel convective rainfall by ~16% and 8%. In contrast, indirect aerosol effects decrease stratiform rainfall, damping the dust‐induced total rainfall increase by ~13% in the Sahara and ~59% in the Sahel. Sensitivity experiments indicate the dust‐induced precipitation anomaly in the Sahara and Sahel (0.27 and 0.18 mm/day) is smaller than the anomaly from MH vegetation cover (1.19 and 1.08 mm/day). Although sensitive to dust radiative properties, sea surface temperatures, and indirect aerosol effect parameterization, our results suggest that dust reductions had competing effects on MH African rainfall.

Global vegetation patterns of the past 140,000 years

“…However, this result was not replicated by fully coupled atmosphere–ocean–vegetation models participating in PMIP2 and PMIP3, including HadCM3 with TRIFFID (Braconnot et al., 2012; Harrison et al., 2014). Recent climate model simulations suggest that changes in dust could also be important (Pausata, Messori, & Zhang, 2016), although this result may have been an artefact of that particular model (Hopcroft & Valdes, 2019; Thompson, Skinner, Poulsen, & Zhu, 2019). A lack of sensitivity of modelled vegetation to rainfall in sub‐Saharan Africa may also contribute to the problem (Hopcroft, Valdes, Harper, & Beerling, 2017).…”

Northern Hemisphere Land Monsoon Precipitation Increased by the Green Sahara During Middle Holocene

“…It should be noted that only the direct effect of dust reduction is considered in this model version, while the indirect aerosol effect (nucleation that results in the formation of more rain droplets) is not included. This may affect the results shown in this study as suggested by a recent study focusing on the indirect effect on the West African monsoon (Thompson et al, 2019).…”

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

“…Hoelzmann et al, 2000;Peyron et al, 2006;Bartlein et al, 2011;Tierney et al, 2017). Permanent lakes spread over the entire north African continent and reached at least up to 28 • N (COHMAP Members, 1988;Street-Perrott et al, 1989;Hoelzmann et al, 1998, Tierney et al, 2011Lézine et al, 2011). Sediment cores taken off the northwest African coast characterize the AHP as an interval of low atmospheric mineral dust fluxes from the Sahara, reflecting the overall increase in vegetation and lake area in this region (deMenocal et al, 2000;Adkins et al, 2006;McGee et al, 2013).…”

“…A possible influence of changes in the dust flux is controversially discussed (e.g. Pausata et al, 2016;Thompson et al, 2019). However, to date, all of these shortcomings have been discussed primarily in the context of changes in summer monsoon dynamics.…”