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Abstract

Accurate assessment of past and future ice sheet-driven sea level variations requires continental-scale numerical models that are able to cover long response time scales of ice sheets, typically involving thousands of years. In order to allow such time scales, the Shallow Ice and Shallow Shelf approximations (SIA/SSA) are commonly introduced to simplify the Stokes equations describing the dynamical regimes of the grounded and oating ice sectors, respectively. However, the SIA is inapplicable to the fast owing regions where basal sliding operates and some of the neglected terms in the Stokes equations become important. Since these terms are included in the SSA, many recent studies have introduced heuristic \hybrid" combinations of both approximations in order to reproduce the dynamics of rapid ice ow zones. However, a side-by-side evaluation of their performance in realistic scenarios is still missing. In this study, we implement three dierent hybrid approaches into the same continental-scale numerical model of the Antarctic Ice Sheet. Our experiments involve an automated calibration of the model based on an iterative technique that uses modern observational data sets to estimate variations in the poorly constrained basal sliding parameters. For validation purposes, we compare the results with the independent observed surface velocity eld. Our results show that the automated calibration compensates for the dierences in the hybrid schemes through dierent parameter distributions, thereby producing similar ice sheet congurations. We use an averaged parameter distribution to demonstrate the in uences arising from the use of dierent hybrid approaches. Although the uncertainty in the basal conditions limits an objective evaluation of the hybrid schemes, our experiments show that the retrieved parameter distributions are not exchangeable between the schemes. This suggests that the results of the calibration and/or initialization of a specic numerical ice sheet model cannot be straightforwardly transferable to a model that uses a dierent level of approximation of the Stokes equations, which represents an important limitation for more complex and computationally expensive ice sheet models.