Modeling the elastic transmission of tidal stresses to great distances inland in channelized ice streams

J Thompson,V C Tsai,M Simons

doi:10.5194/tc-8-2007-2014

Abstract

Abstract. Geodetic surveys suggest that ocean tides can modulate the motion of Antarctic ice streams, even at stations many tens of kilometers inland from the grounding line. These surveys suggest that ocean tidal stresses can perturb ice stream motion at distances about an order of magnitude farther inland than tidal flexure of the ice stream alone. Recent models exploring the role of tidal perturbations in basal shear stress are primarily one- or two-dimensional, with the impact of the ice stream margins either ignored or parameterized. Here, we use two- and three-dimensional finite-element modeling to investigate transmission of tidal stresses in ice streams and the impact of considering more realistic, three-dimensional ice stream geometries. Using Rutford Ice Stream as a real-world comparison, we demonstrate that the assumption that elastic tidal stresses in ice streams propagate large distances inland fails for channelized glaciers due to an intrinsic, exponential decay in the stress caused by resistance at the ice stream margins. This behavior is independent of basal conditions beneath the ice stream and cannot be fit to observations using either elastic or nonlinear viscoelastic rheologies without nearly complete decoupling of the ice stream from its lateral margins. Our results suggest that a mechanism external to the ice stream is necessary to explain the tidal modulation of stresses far upstream of the grounding line for narrow ice streams. We propose a hydrologic model based on time-dependent variability in till strength to explain transmission of tidal stresses inland of the grounding line. This conceptual model can reproduce observations from Rutford Ice Stream.

Highlights

1.1 Relevant observationsObservations from some Antarctic ice streams show tidally modulated surface displacements extending many tens of kilometers inland of the grounding line
Continuous GPS (CGPS) surveys on some Antarctic ice streams find surface velocities modulated at tidal frequencies (Rutford Ice Stream: Gudmundsson, 2006, 2007; Bindschadler Ice Stream: Anandakrishnan et al, 2003) or stick–slip motion correlated with extremes in Published by Copernicus Publications on behalf of the European Geosciences Union
It is convenient to define a stress decay length scale, Ltr, as the distance inland of the grounding line over which the amplitude of a tidal stress drops by an order of magnitude

Summary

Introduction

Observations from some Antarctic ice streams show tidally modulated surface displacements extending many tens of kilometers inland of the grounding line (see Fig. 1, Table 1, and associated references). Geodetic and seismic observations that probe the interaction between ocean tides and ice stream motion include surface tilt (tiltmeters), differential position (synthetic aperture radar, InSAR), absolute position (altimetric surveys and global positioning system, GPS), and basal seismicity (see Table 1). When such observations are found to fluctuate at tidal or near-tidal frequencies, they can be used to estimate the spatial extent of ocean tidal influences on the flow of ice streams (see, for example, references described below). Continuous GPS (CGPS) surveys on some Antarctic ice streams find surface velocities modulated at tidal frequencies (Rutford Ice Stream: Gudmundsson, 2006, 2007; Bindschadler Ice Stream: Anandakrishnan et al, 2003) or stick–slip motion correlated with extremes in Published by Copernicus Publications on behalf of the European Geosciences Union

Methods

Results

Discussion

Conclusion