Sensitivity of centennial mass loss projections of the Amundsen basin to the friction law

Julien Brondex,Fabien Gillet-Chaulet,Olivier Gagliardini

doi:10.5194/tc-13-177-2019

Abstract

Abstract. Reliable projections of ice sheets' future contributions to sea-level rise require models that are able to accurately simulate grounding-line dynamics, starting from initial states consistent with observations. Here, we simulate the centennial evolution of the Amundsen Sea Embayment in response to a prescribed perturbation in order to assess the sensitivity of mass loss projections to the chosen friction law, depending on the initialisation strategy. To this end, three different model states are constructed by inferring both the initial basal shear stress and viscosity fields with various relative weights. Then, starting from each of these model states, prognostic simulations are carried out using a Weertman, a Schoof and a Budd friction law, with different parameter values. Although the sensitivity of projections to the chosen friction law tends to decrease when more weight is put on viscosity during initialisation, it remains significant for the most physically acceptable of the constructed model states. Independently of the considered model state, the Weertman law systematically predicts the lowest mass losses. In addition, because of its particular dependence on effective pressure, the Budd friction law induces significantly different grounding-line retreat patterns than the other laws and predicts significantly higher mass losses.

Highlights

The West Antarctic Ice Sheet mean annual contribution to global sea-level rise (SLR) has tripled over the last 25 years as a consequence of a growing imbalance between the mass it receives as snowfall and that which is discharged to the ocean by ice streams (Shepherd et al, 2018)
The absolute difference between modelled u and observed uobs velocities turns out to shrink when both basal shear stress and viscosity are inferred (Fig. 4a–c). This is true for the ice shelves, which do not feel any basal shear stress: the basal shear stress directly upstream of the grounding lines (GLs) do influence velocities within the downstream ice shelf, the most efficient way to obtain a better match between modelled and observed velocities in floating areas is through a local adjustment of viscosity
When the viscosity is inferred, another way for the inversion algorithm to increase the modelled velocities in areas where they would be too low otherwise is to soften the ice locally: this is the case, for example, in the higher part of PIG, in particular for the inferred state IRγ,1 for which the inversion algorithm induces a local reduction in viscosity rather than of the basal stress to increase the modelled velocities

Summary

Introduction

The West Antarctic Ice Sheet mean annual contribution to global sea-level rise (SLR) has tripled over the last 25 years as a consequence of a growing imbalance between the mass it receives as snowfall and that which is discharged to the ocean by ice streams (Shepherd et al, 2018). The most active basin of this region is the Amundsen Sea Embayment (ASE), where marine-terminating outlet glaciers draining ice to the oceans have shown sustained acceleration and thinning over the last decades, with their grounding lines (GLs), i.e. the limit between the grounded ice sheet and the floating ice shelf, retreating at rates higher than 1 km a−1 since 1992 (Mouginot et al, 2014; Rignot et al, 2014) These observations raise concerns regarding the near future of the ASE as they suggest that this sector of West Antarctica is undergoing a marine ice-sheet instability (MISI), which would imply a significant additional global SLR in the coming decades (Joughin et al, 2014; Favier et al, 2014; Cornford et al, 2015).

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Discussion

Conclusion