Mid-Holocene West African Monsoon Rainfall enhanced in high-resolution EC-Earth simulation with dynamic vegetation feedback

Abstract

Abstract Proxy records have shown that the Mid-Holocene was a period of humid conditions across West Africa, with an enhanced West African Monsoon (WAM) and vegetated conditions in areas currently characterized by desert, often referred to as the Green Sahara. However, General Circulation Models regularly struggle with recreating this strengthened Mid-Holocene monsoon in West Africa. The vegetation-albedo feedback has long been viewed as an essential process modulating the monsoon variability in West Africa, and simulations using prescribed vegetation to recreate a Green Sahara have shown a strengthened WAM and increased rainfall. However, these simulations represent an idealized vegetation cover 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 feedback. Using idealized vegetation cover might therefore over-/underestimate the changes of the WAM, as well as over-/understate the importance of the vegetation feedback. 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 reproduces an apparent enhancement of the WAM, with the summer rainfall in the Sahel region increasing by 15% compared to simulations with a prescribed modern vegetation cover. Vegetation feedbacks enhance the warming of the Sahara region, deepens the Sahara Heat Low, results in increased rainfall and strengthens monsoonal flow across West Africa. However, the enhancement is still below what can be viewed in proxy reconstructions, highlighting the importance of investigating other processes, such as the interactive aerosol-albedo feedback.