Vegetation feedbacks enhance the West African Monsoon during the Mid-Holocene

Proxy records have shown that the Mid-Holocene was a period of humid conditions across West Africa, with an enhanced West African Monsoon (WAM) reaching far into the Sahara region and with vegetation covering areas currently characterized by desert leading to conditions being referred to as the Green Sahara. However, General Circulation Models struggle with recreating this strengthened Mid-Holocene monsoon, and the results from the latest PMIP4 simulations showed a clear underestimation of the rainfall enhancement across the Sahel and Sahara region. Understanding what physical processes drive the variability of the WAM, and including those processes in our simulations, might aid in closing this gap. The vegetation-albedo feedback has long been viewed as an important process modulating the monsoon variability in West Africa, and simulations using prescribed vegetation to recreate a Green Sahara have exhibited a strengthening of the WAM and increased rainfall. However, these simulations represent an idealised vegetation cover based on proxy records found along the west coast of West Africa and do not take any environmental heterogeneity into account. Furthermore, this only represents a one-directional forcing by the vegetation on the climate, rather than the vegetation-albedo feedbacks. This might therefore over-/underestimate the changes of the WAM, as well as over-/understate the importance of the vegetation feedbacks. To address this, we have simulated the Mid-Holocene (~6 ka) climate using the high-resolution Earth System Model EC-Earth3-Veg. The results show that coupled dynamic vegetation reproduce a clear enhancement of the WAM when compared to simulations with a prescribed modern vegetation cover. This enhances warming of the Sahara region and deepens the Sahara Heat Low resulting in increased rainfall and strengthened monsoonal flow across West Africa. However, the enhancement is still below what can be viewed in proxy reconstructions which highlights the importance of investigating additional processes, such as including interactive aerosol-albedo feedbacks.