Control of the Antarctic ice sheet by ocean–ice interaction

The Antarctic ice cap is the largest ice sheet of modern times. It is of considerable importance to predict the sea level variability due to the associated changes in ice volume. We present the results of a simple grounded ice sheet model, developed from Oerlemans Oerlemans, J., 2002. Global dynamics of the Antarctic Ice Sheet, Climate Dynamics 19, 85–93.], in which the net oceanic evaporation influences the ice cap volume in two ways, through changes in: (i) the accumulation rate, and (ii) the mean sea level. The net evaporation changes are driven by the sea surface temperature (SST) anomaly time series of Howard Howard, W.R., 1997. A warm future in the past, Nature, 388, 418–419.] for the subantarctic Southern Ocean over the period 220 kyr to the present. The effect of the waxing and waning of the northern hemisphere ice sheets is integrated into the model using an independent model, in which ice melting depends on the SST anomaly and ice calving depends on the sea level anomaly. A series of analytical expressions are derived for the related properties of the coupled ocean–ice system applicable over time scales of 100 kyr, which show, in particular, that the Antarctic ice cap volume changes are due mainly to the effects of the northern hemisphere ice sheets on sea level (which influences ice calving), rather than directly to changes in SST, and hence the ice cap volume is greatest during interglacial periods. This conclusion, which is independent of the specification of the ice melting regime for the northern hemisphere ice sheets, strongly suggests that the changes in accumulation flux estimated from the Vostok proxy temperature data and used in other studies of the Antarctic mass balance have been overestimated. A simple expression is also presented for the lag of ice cap volume to SST, and it is found that the predictions for the mean sea level variability are similar to observations for a melting flux of the northern hemisphere ice sheets about twice their accumulation flux due to the net oceanic evaporation, except during major deglaciations when these two fluxes appear to be of similar magnitude.