Role of the Biosphere in the Mid-Holocene Climate of West Africa

In previous studies, a zonally symmetric, synchronously coupled biosphere-atmosphere model ( ZonalBAM), which includes explicit representation of ecosystem dynamics, has been developed and validated based on current conditions over the region of West Africa. Here, we use ZonalBAM to study the response of the coupled biosphere-atmosphere system to changes in the Earth’s orbital forcing during the Middle Holocene (6K yrs BP) and the relative contribution of vegetation feedbacks. Simulations in which vegetation conditions were fixed to the current distribution, show that an orbitally induced increased seasonality in insolation for the Middle Holocene, by itself, results in a 1.1degrees northward shift in the location of the southern margin of the Sahara as compared to current solar forcings. When vegetation is allowed to be dynamic, a 2.4degrees northward shift is simulated. However, when dynamic vegetation is initialized to palaeovegetation, a 5.1degrees northward shift is simulated, bringing results more consistent with palaeoevidence. Based on previous studies on the role of the gradient of moist static energy on the dynamics of large-scale tropical circulations, a mechanism for the enhancement of the summer monsoon circulation has been developed. Our results suggest that multiple equilibria could have coexisted over the region of West Africa during the Middle Holocene. Furthermore, based on previous studies on the current climate over the region, we hypothesize that transitions between the different equilibria could have taken place during the Middle Holocene causing the southern desert margin to migrate between 18.1degreesN and 21.4degreesN and shaping climate variability.