Tidally induced variations in vertical and horizontal motion on Rutford Ice Stream, West Antarctica, inferred from remotely sensed observations

Abstract

To better understand the influence of stress changes over floating ice shelves on grounded ice streams, we develop a Bayesian method for inferring time‐dependent 3‐D surface velocity fields from synthetic aperture radar (SAR) and optical remote sensing data. Our specific goal is to observe ocean tide‐induced variability in vertical ice shelf position and horizontal ice stream flow. Thus, we consider the special case where observed surface displacement at a given location can be defined by a 3‐D secular velocity vector, a family of 3‐D sinusoidal functions, and a correction to the digital elevation model used to process the SAR data. Using nearly 9 months of SAR data collected from multiple satellite viewing geometries with the COSMO‐SkyMed 4‐satellite constellation, we infer the spatiotemporal response of Rutford Ice Stream, West Antarctica, to ocean tidal forcing. Consistent with expected tidal uplift, inferred vertical motion over the ice shelf is dominated by semidiurnal and diurnal tidal constituents. Horizontal ice flow variability, on the other hand, occurs primarily at the fortnightly spring‐neap tidal period (Msf). We propose that periodic grounding of the ice shelf is the primary mechanism for translating vertical tidal motion into horizontal flow variability, causing ice flow to accelerate first and most strongly over the ice shelf. Flow variations then propagate through the grounded ice stream at a mean rate of ∼29 km/d and decay quasi‐linearly with distance over ∼85 km upstream of the grounding zone.

Tidal bending of ice shelves as a mechanism for large-scale temporal variations in ice flow

“…This mechanism is felt most strongly for a confined ice shelf, where bending occurs in the margins along the entire length of the shelf. New observations of the M sf signal reveal that it 345 is generally larger on the floating ice shelf and tends to increase in amplitude towards the ice front (Minchew et al, 2016;Rosier et al, 2017). Our modelling work shows that flexural ice-softening can replicate this amplification of the M sf signal downstream of ice stream GLs.…”

“…In some cases the ice flow responds at a different frequency to the tidal forcing, for example on the Rutford Ice Stream (RIS) the largest response is at a fortnightly (M sf ) frequency (Gudmundsson, 2006). More recent observations have shown that the M sf signal actually increases in strength on the adjoining ice shelf (Minchew et al, 2016;Rosier et al, 2017) and also exists on isolated ice shelves which do not have large ice streams feeding into them (King et al, 2011;20 A multitude of mechanisms have been proposed which could lead to a fortnightly modulation in ice flow: a nonlinear basal sliding law (Gudmundsson, 2007(Gudmundsson, , 2011Rosier et al, 2014), tidal perturbations in subglacial water pressure (Thompson et al, 2014;Rosier et al, 2015), grounding line migration (Rosier et al, 2014) and changes in the effective ice-shelf width (Minchew et al,25 2016). Understanding the root cause of the strong and widespread tidal signals observed on ice shelves and ice streams is not done for its own sake but is a means to an end.…”

“…Since many large ice shelves are confined on three sides by grounded ice, the bending stresses are generated along their entire length. This mechanism could therefore explain how the M sf signal increases in strength downstream of ice stream grounding lines, as evidenced by recent GPS and satellite observations (Minchew et al, 2016;Rosier et al, 2017). A schematic showing how vertical tidal motion can lead to a reduction in effective viscosity of ice shelf shear margins is shown in …”

“…This is possibly because it has often been assumed that the M sf signal observed on ice shelves is driven by processes occurring on neighbouring ice streams; indeed these make up the bulk of the proposed mechanisms listed above. Now that new observations show the M sf signal strengthening downstream of GLs (Minchew et al, 2016;Rosier et al, 2017) it has become clear that an alternative mechanism is needed which can generate this 40 signal, independently of anything occurring on grounded ice.…”

“…Ocean tides are known to greatly affect the horizontal flow of both ice shelves and adjoining ice streams, even far upstream of grounding lines (Doake et al, 2002;Brunt et al, 2010;Makinson et al, 2012;Legresy et al, 2004;King et al, 2011;Bindschadler et al, 2003b, a;Anandakrishnan et al, 2003;Alley, 1997;Gudmundsson, 2006;Marsh et al, 2013;Minchew et al, 2016;Rosier 15 et al, 2017). In some cases the ice flow responds at a different frequency to the tidal forcing, for example on the Rutford Ice Stream (RIS) the largest response is at a fortnightly (M sf ) frequency (Gudmundsson, 2006).…”