Evaluating an accelerated forcing approach for improving computational efficiency in coupled ice sheet-ocean modelling
Abstract. Coupled ice sheet-ocean models are increasingly being developed and applied to important questions pertaining to processes at the Greenland and Antarctic Ice Sheet margins, which play a pivotal role in ice sheet stability and sea level rise projections. One of the challenges of such coupled modelling activities is the timescale discrepancy between ice and ocean dynamics. This discrepancy, combined with the high computational cost of ocean models due to their ﬁner temporal resolution, limits the time frame that can be modeled. In this study, we introduce an “accelerated forcing” approach to address the timescale discrepancy and thus improve computational efﬁciency in a framework designed to couple evolving ice geometry to ice shelf cavity circulation. This approach is based on the assumption that the ocean adjusts faster to imposed changes than the ice sheet, with the ocean viewed as being in a slowly varying quasi-steady state over timescales of ice geometry change. By assuming that the ocean-induced ice draft change rate during one coupling interval can be reﬂected by a quasi-steady state change rate during a shortened coupling interval (equal to the regular coupling interval divided by a constant factor), we can reduce the ocean model simulation duration. We ﬁrst demonstrate that the mean cavity residence time, derived from stand-alone ocean simulations, can guide the selection of suitable scenarios for this approach. We then evaluate the accelerated forcing approach by comparing basal melting response under the accelerated forcing with that under the regular forcing based on idealized coupled ice sheetocean experiments. Our results suggest that: the accelerated approach can yield comparable melting responses to those under the regular forcing when the model is subjected to steady far-ﬁeld ocean conditions or time-varying conditions with timescales much shorter than the cavity residence time. However, it is not suitable when the timescale of the accelerated ocean conditions is not signiﬁcantly different from the cavity residence time. When used carefully, the accelerated approach can be a useful tool in coupled ice sheet-ocean modelling.
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