Hui Su and J. David Neelin, 2005:
J. Geophys. Res., 110, D19, D19105,
doi:10.1029/2005JD005806.
Paper (PDF 951KB)
© Copyright 2005 by the American Geophysical Uniona.
Abstract. During the mid-Holocene, about 6 ka BP, the African monsoon was stronger and extended further north than in present-day climate associated with differences in orbital parameters affecting insolation. Earlier research suggests that land surface vegetation interaction can 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 mediate between orbital forcing and convection for the African monsoon. Simulations with modified land surface albedo and/or surface type are used to mimic grassland extending over all of North Africa. The albedo change favoring convection thus occurs over the largest area that vegetation feedback could affect. While the monsoon indeed extends further poleward than in experiments with only 6 ka orbital forcing, the poleward rainfall boundary is largely controlled by the ``ventilation mechanism''--- inflow of low moist static air from oceanic or poleward regions. In particular, advection and diffusion of less moist air oppose the increase of moisture that would be required to meet convective instability criteria over the warm continent. While the 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. Coupling with a slab mixed-layer ocean modestly increases precipitation over the convergence zones.