Response of the Monsoon to Orbital Forcing

Northern Hemisphere summer insolation during the early to mid-Holocene (Berger, 1978) resulted in increased heating over the Northern Hemisphere continents and thus intensified the thermal contrast between the land and the ocean. The increased heating over the continents resulted in the northward displacement of the intertropical convergence zone (ITCZ) and hence of the monsoon front, while the enhanced land-sea contrast increased the flux of moisture from the ocean to the continent. 

Comparison of the simulated enhancement of the African monsoon with a variety of pale o-observations shows that the PMIP simulations (in common with earlier simulations of the response to orbital forcing) consistently underestimate both the northward shift in the monsoon belt shown by paleoenvironmental data and the magnitude of the precipitation required to produce the observed lake and vegetation changes in northern Africa. Comparisons of the spatial patterns in the simulated P — E fields with lake data from northern Africa (Yu and Harrison, 1996), for example, indicate that the PMIP simulations consistently underes- 

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