A fully coupled GCM simulation of the climate of the mid-Holocene

Abstract

The response of a fully coupled ocean-atmosphere model to mid-Holocene insolation forcing is compared with results from an atmospheric model with prescribed present day sea surface temperatures and mid-Holocene insolation. The ocean response increases the amplitude of the seasonal temperature response over land, and increases its lag behind the solar forcing. The tropical oceans in the coupled model are coldest, relative to the control, in May at 6ka leading to a relative reduction in evaporation, precipitation and ascending motion over the ocean, and relative ascent and increased precipitation over equatorial Africa. By September, the tropical oceans are warmer than in the fixed SST experiment which leads to increased moisture flux from the oceans into the southeast monsoon, and a larger increase in monsoon precipitation than in the fixed SST experiment. In addition, the enhanced warming of the northern hemisphere SSTs relative to the southern subtropics leads to a northward shift of the Intertropical Convergence Zone. As a result, increased wetness of the tropical land surface in the mid Holocene experiments is more pronounced and extends further northwards when the sea surface temperatures are allowed to respond. The coupled model results are more in accord with lake level data, especially over North Africa.