ATMOSPHERIC RESPONSE TO A HYPOTHETICAL TIBETAN ICE-SHEET

The atmospheric response to a hypothetical Tibetan ice sheet was tested with the T21 Atmospheric General Circulation Model (AGCM) of the European Centre for Medium-Range Weather Forecasts (ECMWF). The model response is discussed in terms of an "autocycle" hypothesis of the ice ages proposed by Kuhle. According to this hypothesis, ice-albedo feedbacks associated with the growth and retreat of the Tibetan ice sheet are the mechanism that amplifies the variation of solar insolation on astronomical time scales, producing conditions that favor glaciation or deglaciation in North America and Eurasia. The imposed Tibetan ice sheet forcing did not increase the annual snow balance at the locations of the Laurentide and Eurasian ice sheets. Analysis of the seasonal cycle results indicated that there were small areas of locally significant temperature decreases in July (at the ice sheet locations), but no corresponding precipitation increases in January. The upper-tropospheric response to the elevated Tibetan plateau is not confined to the vicinity of the forcing, but changes in the global energetics of the atmosphere are small (less than 5%) relative to the control. The results of this experiment do not permit a conclusive decision regarding the validity of Kuhle's autocycle hypothesis. Future modeling studies need to consider ocean-atmosphere-ice sheet feedbacks and to investigate the transient response of the climate system over a complete ice age cycle.