The Mid-Holocene West African Monsoon strength modulated by Saharan dust and vegetation

Abstract:

The West African Monsoon (WAM) is crucial for the socio-economic stability of millions of people living in the Sahel. Severe droughts have ravaged the region in the last three decades of the 20^th century, highlighting the need for a better understanding of the WAM dynamics. One of the most dramatic changes in the WAM occurred between 15,000-5,000 years BP, when increased summer precipitation led to the so-called “Green Sahara” and to a reduction in dust emissions from the region. Previous studies have shown that variations in vegetation and soil type can have major impacts on precipitation. However, model simulations are still unable to fully reproduce the intensification and geographical expansion of the African monsoon during that period, even when vegetation over the Sahara is simulated. Here, we use a fully coupled simulation for 6000 years BP in which prescribed Saharan vegetation and dust concentrations are changed in turn. A close agreement with proxy records is obtained only when both Saharan vegetation and dust decrease are taken into account (Fig. 1). The dust reduction extends the monsoon’s northern limit further than the vegetation-change case only (Fig. 2), by strengthening vegetation-albedo feedbacks and driving a deeper Saharan Heat Low. The dust reduction under vegetated Sahara conditions leads to a northward shift of the WAM extension that is about twice as large as the shift due to the changes in orbital forcing alone. We therefore conclude that accounting for changes in Saharan dust loadings is essential for improving model simulations of the MH WAM. The role of dust is also relevant when looking into the future, since Saharan dust emission may decrease owing to both direct and indirect anthropogenic impacts on land cover.