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 modelled. 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, so the ocean can be viewed as being in a quasi-steady state that varies slowly over timescales of ice geometry change. By assuming that the mean basal melt rate during one coupling interval can be reflected by a quasi-steady-state melt 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 standalone 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 (without the accelerated forcing) based on idealized coupled ice sheet–ocean model experiments. Our results suggest that the accelerated approach can yield comparable melting responses to those under the regular forcing approach 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 may not be suitable when the timescale of the accelerated ocean conditions is not significantly different from the cavity residence time. We have also discussed the limitations of applying the accelerated forcing approach to real-world scenarios, as it may not be applicable in coupled modelling studies addressing climate variability on sub-decadal, decadal, and mixed timescales or in fully coupled climate models with interactive ice sheets. Nevertheless, when appropriately applied, the accelerated approach can be a useful tool in process-oriented coupled ice sheet–ocean modelling or for downscaling climate simulations with a coupled ice sheet–ocean model.