ENTERING THE GLACIATION WITH A 2-D COUPLED CLIMATE MODEL

A 2-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea-ice, the continents, the ice sheets and their underlying lithosphere has been used to test the Milankovitch theory over the last glacial-interglacial cycle. Sensitivity tests have shown that the orbital variations can induce, in such a system, feedbacks sufficient to generate the low frequency part of the climatic variations over the last 122 ka BP. These variations at the astronomical time scale are broadly in agreement with ice volume and sea level reconstruction independently obtained from geological data. At the beginning of the integration during isotopic Substage 5e when there was no northern american nor eurasian ice sheets, June insolation at high latitudes seemed to correlate well with summer temperature at the surface of the ice sheets and with the ablation rate. The relation is more complicated when large ice sheets exist. Broadly, the June-July insolation from 60 to 70-degrees-N. leads the ice volume by roughly 5000 years over the last glacial-interglacial cycle. The simulated climate was shown to be sensitive to the ice albedo-temperature feedback, to the precipitation-altitude negative feedback over the ice sheets, to the ice sheet slope and continentality effects, and to the albedo of the ice sheets as a function of the snowfall frequency at their surface. The formation and waxing of the ice sheets are particularly sensitive to ablation, more than to snowfalls. Imperfections in the simulated climate were recognized, in particular at the last glacial maximum when the cooling was not large enough and the northern hemisphere ice sheets did not extend far enough to the south, weaknesses which are partly solved in a further experiment (Gallee et al., 1989) by using the proper CO2 atmospheric concentration given by the Vostok ice core in addition to the astronomical forcing.