Single-scattering albedo drives Saharan dust forcing uncertainty and the simulated West African monsoon response in CMIP6 models

Abstract. Mineral dust affects the West African energy balance and monsoon circulation, yet climate models differ widely in the dust optical properties that set the balance between scattering and absorption. As a result, models disagree even on whether dust produces net shortwave (SW) warming or cooling over North Africa. Here we show that Saharan single-scattering albedo (SSA), an observable intensive optical property, accounts for much of the inter-model spread in the shortwave dust forcing response across climate models. The Saharan SW effective radiative forcing due to aerosol–radiation interactions (ERF_ari^SW) decreases systematically with increasing SSA, shifting from near-zero or positive values in lower-SSA, more absorbing models to negative values in higher-SSA, more scattering models. Applying an Aerosol Robotic Network (AERONET)-based observational constraint on SSA, 0.9543 ± 0.0060, gives a constrained Saharan ERF_ari^SW centered near zero at −0.66 W m⁻², with a 95 % interval of −3.87 to +2.53 W m⁻².

Grouping the models by SSA reveals two contrasting monsoon responses to the same dust-emission perturbation. Low-SSA models show stronger atmospheric heating over the Sahara, a deeper heat low, enhanced monsoon inflow, and increased Sahel rainfall, whereas High-SSA models show a weaker or opposite response and reduced rainfall. The associated cloud-radiative response forms a Sahel–tropical North Atlantic dipole. Propagating the SSA-based constraint on the ERF_ari^SW through the relationship between Saharan ERF_ari^SW and Sahel cloud-radiative response reduces its inter-model variance by about one third. Thus, an observable dust optical property constrains a significant part of the downstream monsoon and cloud-response uncertainty.