SIMULATIONS OF MIDHOLOCENE CLIMATE USING AN ATMOSPHERIC GENERAL-CIRCULATION MODEL

The authors describe a first paleoclimatological application of the Ca nadian Centre for Climate Modelling and Analysis atmospheric general c irculation model (AGCM) to simulate the climate state 6000 calendar ye ars before present (6 kyr BP). Climate reconstructions for this period are performed with both fixed SSTs and with the AGCM coupled to mixed layer ocean and thermodynamic sea-ice modules. The most important dif ference between this epoch and the present involves the increased surf ace heating and cooling of the continental land masses in the Northern Hemisphere during summer and winter, respectively, which are a conseq uence of the modified orbital configuration. A comparison of a fixed S ST experiment with a calculated SST experiment, incorporating a thermo dynamic representation of oceanic response, is performed to assess the impact on the mid-Holocene climate. The results are also contrasted w ith those obtained on the basis of proxy climate reconstructions durin g this mid-Holocene optimum period. Of interest in this calculated SST experiment is the impact on the seasonal cycle of sea-ice distributio n due to the increased insolation at high latitudes during Northern He misphere summer. Also important is the fact that the mixed layer ocean in the simulation is found to further enhance the monsoon circulation beyond the enhancement found to occur due to the influence of modifie d orbital forcing alone. This increased response is found to be a cons equence of the sensitivity of tropical SST to the amplification of the seasonal cycle due to the change in insolation forcing that was chara cteristic of the mid-Holocene period.