Simulated response of the African monsoon and tropical precipitation to orbital precession

This study seeks to build a mechanistic understanding of how the insolation changes associated with orbital precession impact the regional and zonal mean tropical precipitation. We isolate the role of sea surface temperature (SST) evolution in modulating the climate response to Holocene orbital precession by considering two sets of atmospheric general circulation model simulations (GFDL AM2.1): one with prescribed SSTs, and one with a 50 meter slab ocean. Model temperature, precipitation, and surface energy budget results confirm the northward migration and intensification of the African monsoon with Holocene precession. This regional response is stronger in the slab ocean experiment, though modest with respect to paleodata proxies. The summer intertropical convergence zone (ITCZ), however, shifts southward, away from the brighter hemisphere (i.e. the hemisphere which receives more sunlight), with both ocean configurations. This is contrary to what existing theories about the location of the ITCZ predict. The southward energy flux from the high precession summer hemisphere is not accomplished by a stronger Hadley mass flux in either experiment; in fact, the circulation weakens in the slab ocean case. The counterintuitive response is attributed to the increase in gross moist stability in the northern tropics in both simulations. The results demonstrate that sea surface temperatures do not fully control the gross moist stability, and that in the zonal mean direction, the tropical circulation response to precession resembles that of an aquaplanet (Merlis et al. 2012).