The role of biosphere-atmosphere-ocean interactions in the climate of West Africa during the Middle Holocene

Abstract

In previous studies, a zonally symmetric, synchronously coupled biosphere-atmosphere model (ZonaIBAM), which includes explicit representation of ecosystem dynamics, has been developed and validated based on current forcings 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 and ocean feedbacks. Simulations in which vegetation conditions were prescribed to the current distribution, show that an orbitally-induced increased seasonality in insolation for the Middle Holocene, by itself, results in a 1.1° northward shift in the location of the southern margin of the Sahara as compared to current solar forcings. When vegetation dynamics are allowed, a 2.4° northward shift is simulated. However, when dynamic vegetation is initialized according to Hoelzmann et al. 's [1998] map of palaeovegetation, a 5.1n orthward shift is simulated, bringing results more consistent with palaeoevidence. Contrary to other studies, these results suggest that multiple climate 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 [Wang and Eltahir, 2000b], we hypothesize that transitions between these different equilibria could have taken place during the Middle Holocene causing the southern desert margin to migrate between 18.1° Nand 21.4° N and shaping low frequency climate variability. In spite of a cold bias in the simulated sea surface temperature (SSTs) for the current climate, the addition of an interactive mixed layer ocean model (MLOM) to ZonalBAM provided some insight into how changes in the SSTs in the South Eastern Tropical Atlantic (SETA) could have resulted in enhancement of the monsoon circulation during the Middle Holocene. We hypothesize that the addition of an oceanic component into Mid-Holocene simulations could result in a more northward position of the southern desert margin, bringing our simulations in much better agreement with reconstructions.