Surface undulations of Antarctic ice streams tightly controlled by bedrock topography

Abstract

Abstract. Full Stokes flow-line models predict that fastflowing ice streams transmit information about their bedrock topography most efficiently to the surface for basal undulations with length scales between 1 and 20 times the mean ice thickness. This typical behaviour is independent of the precise values of the flow law and sliding law exponents, and should be universally observable. However, no experimental evidence for this important theoretical prediction has been obtained so far, hence ignoring an important test for the physical validity of current-day ice flow models. In our work we use recently acquired airborne radar data for the Rutford Ice Stream and Evans Ice Stream, and we show that the surface response of fast-flowing ice is highly sensitive to bedrock irregularities with wavelengths of several ice thicknesses. The sensitivity depends on the slip ratio, i.e. the ratio between mean basal sliding velocity and mean deformational velocity. We find that higher values of the slip ratio generally lead to a more efficient transfer, whereas the transfer is significantly dampened for ice that attains most of its surface velocity by creep. Our findings underline the importance of bedrock topography for ice stream dynamics on spatial scales up to 20 times the mean ice thickness. Our results also suggest that local variations in the flow regime and surface topography at this spatial scale cannot be explained by variations in basal slipperiness.

Response of Surface Topography to Basal Variability Along Glacial Flowlines

“…As our aim is to transfer mesoscale perturbations ( λ / H ~ 1–10), L should be at least an order of magnitude larger than the mean ice thickness H̅ (≈1.5 units for our synthetic glaciers), and thus, in the region where the shallow‐ice approximation applies. The length scales over which background parameters vary are then consistent with our use of T sb and T sc in the method (recall G2003 derived these for parallel‐slab flow) and consistent with the formula for u d , which De Rydt et al () also found using a shallow‐ice formula in their ice stream work. We chose L = 10 for these reasons.…”

“…G2003 predicts efficient transfer of basal topographic variations whose wavelengths are a few times to tens of times of the ice thickness when ice flow occurs at high slip ratio (basal sliding velocity divided by ice deformational velocity). This prediction accords with the abundance of surface undulations on active ice streams, as has been verified using their topography and flow speed (De Rydt et al, ).…”

An iterative inverse method to estimate basal topography and initialize ice flow models

“…Perturbations of the bed height lead to a non-local surface expression with a transfer amplitude and phase-shift depending on multiple factors including slip ratio and dimensions of the basal perturbation, as shown by Gudmundsson (2003) and Raymond and Gudmundsson (2005). Observational verification for many of their theoretical findings has recently been provided by De Rydt et al (2013). When recovering a single bump in an otherwise flat bed, the surface misfit and hence the bedrock correction will initially be out-of-phase with the actual bump position (when starting from a fully flat bed).…”

Dynamic response of Antarctic ice shelves to bedrock uncertainty

“…Many Antarctic phenomena are tightly controlled by the bedrock topography, such as the surface undulation of ice streams (De Rydt et al, 2013), grounding line retreat and tidewater outlet glaciers dynamics (Enderlin et al, 2013).…”