Dynamical mechanisms for African monsoon changes during the mid-Holocene

摘要

During the mid-Holocene, about 6 ka BP, the African monsoon was stronger and extended farther north than in present-day climate associated with differences in orbital parameters affecting insolation. Earlier research suggests that land surface vegetation interaction and atmosphere-ocean coupling may be important to the northward extension of the Holocene monsoon rainfall, but different climate models produce different results in terms of rainfall distribution. This study examines the dynamical mechanisms that modulate the orbital forcing effect on the African monsoon convection. A series of simulations with 6 ka BP orbital parameters, modified land surface albedo that mimics grassland extending over North Africa, and alterations of the model dynamics are used. It is found that the “ventilation mechanism” (inflow of low moist static energy air from oceanic or poleward regions) is a key process in setting the northern rainfall boundary for the African monsoon. In particular, advection and diffusion of less moist air oppose the increase of moisture that would be required to meet the increased convective instability criteria set by the warmer tropospheric air above the continent. While changes in heat fluxes into the atmospheric column favor convection in principle, the ventilation mechanism limits the poleward extent of the region that can actually convect.