Temporal variations in the flow of a large Antarctic ice stream controlled by tidally induced changes in the subglacial water system

S H R Rosier,J A M Green,G H Gudmundsson

doi:10.5194/tc-9-1649-2015

Abstract

Abstract. Observations show that the flow of Rutford Ice Stream (RIS) is strongly modulated by the ocean tides, with the strongest tidal response at the 14.77-day tidal period (Msf). This is striking because this period is absent in the tidal forcing. A number of mechanisms have been proposed to account for this effect, yet previous modelling studies have struggled to match the observed large amplitude and decay length scale. We use a nonlinear 3-D viscoelastic full-Stokes model of ice-stream flow to investigate this open issue. We find that the long period Msf modulation of ice-stream velocity observed in data cannot be reproduced quantitatively without including a coupling between basal sliding and tidally induced subglacial water pressure variations, transmitted through a highly conductive drainage system at low effective pressure. Furthermore, the basal sliding law requires a water pressure exponent that is strongly nonlinear with q = 10 and a nonlinear basal shear exponent of m = 3. Coupled model results show that sub-ice shelf tides result in a ∼12 % increase in mean horizontal velocity of the adjoining ice stream. Observations of tidally induced variations in flow of ice streams provide stronger constraints on basal sliding processes than provided by any other set of measurements.

Highlights

The majority of ice streams in Antarctica are forced at their boundary by ocean tides, either directly or through the motion of an adjoining ice shelf
These include stick-slip motion observed at Williams Ice Stream (Bindschadler et al, 2003a, b; Winberry et al, 2009, 2011), smooth diurnal variations observed on Kamb and Bindschadler Ice Streams (Anandakrishnan and Alley, 1997; Anandakrishnan et al, 2003), and long-periodic response found on Rutford Ice Stream (RIS) and on several other ice streams flowing into the Ronne Ice Shelf (Gudmundsson, 2006; Murray et al, 2007; Aðalgeirsdóttir et al, 2008; King et al, 2010; Marsh et al, 2013)
Most models have focused on trying to identify the mechanism responsible for the rather striking observation that the response of the ice stream is concentrated at tidal frequencies absent in the forcing

Summary

Introduction

The majority of ice streams in Antarctica are forced at their boundary by ocean tides, either directly or through the motion of an adjoining ice shelf. Several different types of tidally induced perturbations in ice flow have been observed on Antarctic ice streams These include stick-slip motion observed at Williams Ice Stream (Bindschadler et al, 2003a, b; Winberry et al, 2009, 2011), smooth diurnal variations observed on Kamb and Bindschadler Ice Streams (Anandakrishnan and Alley, 1997; Anandakrishnan et al, 2003), and long-periodic response found on RIS and on several other ice streams flowing into the Ronne Ice Shelf (Gudmundsson, 2006; Murray et al, 2007; Aðalgeirsdóttir et al, 2008; King et al, 2010; Marsh et al, 2013). Note that flow modulation at Msf frequency is not a harmonic beat of the two semidiurnal frequencies; it is a property of spectral analysis that tidal amplitudes can never arise through linear superposition of other frequencies

Methods

Results

Discussion

Conclusion