The Impact of Dust Concentration on the West African Monsoon (WAM) and Regional Climate Patterns

The West African Monsoon (WAM) is responsible for most of the annual rainfall to the Sahel and Gulf of Guinea, thus affecting the food and economic security of millions. Yet future climate projections remain uncertain, partly because global climate models incompletely represent non-CO2 forcings (e.g. mineral dust) that modulate radiation, cloud patterns, and the land-sea thermal contrast that drives the monsoon. Here we investigate the impact of atmospheric dust concentration on the West African Monsoon using a low-dust scenario derived from a climate simulation of the Miocene epoch (23.03 to 5.333 million years ago). Changes in vegetation between the Miocene and preindustrial simulations led to less dust. Thus, to isolate the role of dust in dust-monsoon coupling, we perform two fixed sea surface temperature (fSST) experiments which differ only with respect to their atmospheric dust concentrations; a pre‑industrial control experiment (modern Saharan dust) and a low‑dust experiment (Miocene conditions). Monthly diagnostics for the core monsoon season (June–September) reveal a seasonally evolving response. We find that dust reduction causes a decrease in precipitation compared to the preindustrial run over the Sahel in June and July, thereby limiting the northward expansion of the monsoon. However, we note this pattern changes in August and September, when Eastern regions of the Sahel are marked by precipitation increase, and thus higher monsoon intensity.