Grounding line stability in a regime of low driving and basal stresses

O V Sergienko,D J Wingham

doi:10.1017/jog.2019.53

O V Sergienko, D J Wingham

Open Access

https://doi.org/10.1017/jog.2019.53

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Abstract

AbstractThe dynamics of a marine ice sheet's grounding lines determine the rate of ice discharge from the grounded part of ice sheet into surrounding oceans. In many locations in West Antarctica ice flows into ice shelves through ice streams experiencing low driving stress. However, existing simple theories of marine ice sheets are developed under the assumption of high basal and driving stress. Here we analyze the grounding line behavior of marine ice streams experiencing low basal shear and driving stress. We find that in this regime, the ice flux at the grounding line is a complex function of the geometry of the ice-stream bed, net accumulation rate and gradient of the net accumulation rate. Our analysis shows that the stability of distinct steady states is determined by the same parameters, suggesting a more complex (in)stability criterion than what is commonly referred to within the context of the ‘marine ice-sheet instability hypothesis’. We also determine characteristic timescales (e-folding time) of ice-sheet configurations perturbed from their steady states. These timescales can be used to determine whether particular configurations can be considered in isolation from other components of the climate system or whether their effects and feedbacks between the ice sheet and the rest of the climate system have to be taken into account.

Highlights

Determining the location and the rate of migration of the grounding line – a transition between the grounded part of a marine ice sheet and its floating ice shelf – is a fundamental question in understanding ice-sheet interactions with the rest of the climate system
In the case of low C (Fig. 3e), the driving and basal stresses are much smaller than in the case of large C, and the divergence of the longitudinal stress is of the same order as the basal shear (Fig. 3e), in line with the conclusions of MacAyeal (1989) who found that ‘...basal drag and horizontal deviatoric stress gradients can contribute in balancing gravitational driving stress.’. These results suggest that the two regimes of high and low basal and driving stresses are very different; they suggest that the power-law sliding law is capable of producing low surface gradients and low driving stress in the vicinity of the grounding line, but through the length of the ice stream, if the sliding coefficient C is small
Motivated by the present-day Siple Coast ice streams characterized by low surface slopes and driving stress, we focused this study on the grounding line dynamics of unconfined marine ice sheets with grounded ice streams in a regime of low driving and basal stresses

Summary

Introduction

Determining the location and the rate of migration of the grounding line – a transition between the grounded part of a marine ice sheet and its floating ice shelf – is a fundamental question in understanding ice-sheet interactions with the rest of the climate system. In the regime of low or absent basal shear, ice surface slopes, and the driving stress, are small through the length of the ice stream, and in the vicinity of the grounding line.

Results

Conclusion