Coupled simulations of the mid-Holocene and Last Glacial Maximum: new results from PMIP2

A set of coupled ocean-atmosphere simulations using state of the art climate models is now available for the Last Glacial Maximum and the mid-Holocene through the second phase of the Paleoclimate Modeling Intercomparison Project (PMIP2). This study presents the large scale features of the simulated climates and compares the new model results to those of the atmospheric models from the first phase of the PMIP, for which sea surface temperature was prescribed or computed using simple slab ocean formulations. We consider first the large scale features of the climate change, pointing out some of the major differences between the different sets of experiments. Then we quantify the latitudinal shift of the location of the ITCZ in the tropical regions during boreal summer. It is shown that this shift is limited for LGM, whereas a northward shift and an increase of precipitation are well depicted for mid-Holocene in continental regions affected by monsoon precipitation. In the last part we quantify for both periods the feedback from snow and sea-ice in mid and high latitudes. We show that it contributes for half of the cooling in the northern hemisphere for LGM, the second half being achieved by the reduced CO₂ and water vapour in the atmosphere. For mid-Holocene the snow and albedo feedbacks strengthen spring cooling and enhance boreal summer warming, whereas water vapour reinforces the late summer warming. These feedbacks are modest in the southern hemisphere. For LGM most of the surface cooling is due to CO₂ and water vapour.