Marine ice sheet model performance depends on basal sliding physics and sub-shelf melting

Rupert Michael Gladstone,Olivier Gagliardini,Ralf Greve,Benjamin Keith Galton-Fenzi,Roland Charles Warner,Thomas Zwinger

doi:10.5194/tc-11-319-2017

Abstract

Abstract. Computer models are necessary for understanding and predicting marine ice sheet behaviour. However, there is uncertainty over implementation of physical processes at the ice base, both for grounded and floating glacial ice. Here we implement several sliding relations in a marine ice sheet flow-line model accounting for all stress components and demonstrate that model resolution requirements are strongly dependent on both the choice of basal sliding relation and the spatial distribution of ice shelf basal melting.Sliding relations that reduce the magnitude of the step change in basal drag from grounded ice to floating ice (where basal drag is set to zero) show reduced dependence on resolution compared to a commonly used relation, in which basal drag is purely a power law function of basal ice velocity. Sliding relations in which basal drag goes smoothly to zero as the grounding line is approached from inland (due to a physically motivated incorporation of effective pressure at the bed) provide further reduction in resolution dependence.A similar issue is found with the imposition of basal melt under the floating part of the ice shelf: melt parameterisations that reduce the abruptness of change in basal melting from grounded ice (where basal melt is set to zero) to floating ice provide improved convergence with resolution compared to parameterisations in which high melt occurs adjacent to the grounding line.Thus physical processes, such as sub-glacial outflow (which could cause high melt near the grounding line), impact on capability to simulate marine ice sheets. If there exists an abrupt change across the grounding line in either basal drag or basal melting, then high resolution will be required to solve the problem. However, the plausible combination of a physical dependency of basal drag on effective pressure, and the possibility of low ice shelf basal melt rates next to the grounding line, may mean that some marine ice sheet systems can be reliably simulated at a coarser resolution than currently thought necessary.

Highlights

Ice sheet models (ISMs) are increasingly being used in process studies, sensitivity studies and projections of marine ice sheet (MIS) future behaviour (Joughin et al, 2010; Favier et al, 2014; Gong et al, 2014), and model intercomparison projects (MIPs) to investigate the ice sheet response to ocean forced basal melting of ice shelves are currently in their design phase (Asay-Davis et al, 2016).Past ISM studies have shown inconsistent grounding line behaviour at typical resolutions (Vieli and Payne, 2005)
In the current study we do not attempt to demonstrate convergence in all cases, but instead consider the dependence on resolution across the three resolutions used (Table 2), under the premise that weaker dependence on resolution is an indicator of being closer to the converged solution
We have demonstrated that resolution requirements for marine ice sheet simulations with an evolving grounding line are highly sensitive to the physical implementation of both basal sliding and ice shelf basal melting

Summary

Introduction

Ice sheet models (ISMs) are increasingly being used in process studies, sensitivity studies and projections of marine ice sheet (MIS) future behaviour (Joughin et al, 2010; Favier et al, 2014; Gong et al, 2014), and model intercomparison projects (MIPs) to investigate the ice sheet response to ocean forced basal melting of ice shelves are currently in their design phase (Asay-Davis et al, 2016).Past ISM studies have shown inconsistent grounding line behaviour at typical resolutions (Vieli and Payne, 2005). Practical solutions have been suggested, such as parameterising the flux of ice across the grounding line as a function of ice thickness (Schoof, 2007; Pollard and DeConto, 2009), parameterising the grounding line position at sub-grid resolution (Gladstone et al, 2010b; Seroussi et al, 2014), or implementing adaptive mesh refinement to provide very high resolution at and near the grounding line (Cornford et al, 2013; Durand et al, 2009). These solutions all have limitations, and the computational cost of running a sufficiently high-resolution ISM to robustly represent grounding line motion remains high, even with adaptive refinement

Methods

Results

Discussion

Conclusion