Inverting ice surface elevation and velocity for bed topography and slipperiness beneath Thwaites Glacier

Abstract

Abstract. There is significant uncertainty over how ice sheets and glaciers will respond to rising global temperatures. Limited knowledge of the topography and rheology of ice-bed interface is a key cause of this uncertainty, as models show that small changes in the bed can have a large influence on predicted rates of ice loss. Most of our detailed knowledge of bed topography comes from airborne and ground-penetrating radar observations. However, these direct observations are not spaced closely enough to meet the requirements of ice-sheet models, so interpolation and inversion methods are used to fill in the gaps. Here we present the results of a new inversion of surface-elevation and velocity data over Thwaites Glacier, West Antarctica, for bed topography and slipperiness (i.e. the degree of basal slip for a given level of drag). The inversion is based on a steady-state linear perturbation analysis of the shallow-ice-stream equations. The method works by identifying disturbances to surface flow which are caused by obstacles or sticky patches in the bed, and can therefore be applied wherever the shallow-ice-stream equations hold and where surface data are available, even where the ice thickness is not well known. We assess the performance of the inversion for topography with the available radar data. Although the topographic output from the inversion is less successful where the bed slopes steeply, it compares well with radar data from the central trunk of the glacier. This method could therefore be useful as either an independent test of other interpolation methods such as mass conservation and kriging, or as a complementary technique in regions where those techniques fail. We do not have data to allow us to assess the success of the slipperiness results from our inversions, but we provide maps that may guide future seismic data collection across Thwaites Glacier. The methods presented here show significant promise for using high-resolution satellite datasets, calibrated by the sparser field datasets, to generate high resolution bed topography products across the ice sheets, and therefore contribute to reduced uncertainty in predictions of future sea-level rise.

Geologic Provinces Beneath the Greenland Ice Sheet Constrained by Geophysical Data Synthesis

“…This relation is qualitatively consistent with that predicted from linearized small-perturbation theory for bed-tosurface transfer (Gudmundsson, 2003;Ng et al, 2018), that is, subglacial topographic perturbations that are sufficiently wide along-flow relative to local ice thickness commonly induce detectable surface expressions. That linearized theory is potentially exploitable to resolve subglacial topography at finer resolution (Ockenden et al, 2022(Ockenden et al, , 2023, but more physically complete modeling has demonstrated that three-dimensional effects and non-linear ice rheology further complicate its interpretation (e.g., Sergienko, 2012). Regardless, it is clear that numerous subglacial topographic features across the GrIS interior-primarily subglacial valleys-are poorly resolved by existing surveys and interpolations.…”

Alpine topography of the Gamburtsev Subglacial Mountains, Antarctica, mapped from ice sheet surface morphology

“…2b). Features oriented perpendicular to flow are more likely to be transmitted to the surface, because they present a greater obstacle to flow than those oriented parallel (Ockenden et al, 2021).…”

“…8), present insufficient obstacle to cause an expression visible on the ice surface (c.f. Ockenden et al, 2021). Since there is diversity in the orientations of LC features, it may be reasonably supposed that those that lie at greater angles to the direction of ice surface flow are more likely to represent glacial breaches.…”