Abstract
Abstract. Full Stokes flow-line models predict that fast-flowing 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.
Highlights
The flow regime of the Antarctic ice sheet is dominated by an extensive network of fast-flowing ice streams and outlet glaciers, which account for 90 % of the ice sheet’s discharge (Bamber et al, 2000)
The physical origin of the scaling factor f < 1 is clear, the accuracy of the fit is rather poor in some cases (e.g. R3 and E10) and much better in other cases (e.g. R5 and E12). This variability can be related to uncertainties in our estimate of the mean slip ratio C(0), but it suggests that the value of f is not unique across different profiles or across the entire range of wavelengths
We have analysed a comprehensive set of radar profiles along fast- and slow-flowing sections of the West Antarctic ice sheet and determined the response of surface elevations to variations in the bed topography
Summary
The flow regime of the Antarctic ice sheet is dominated by an extensive network of fast-flowing ice streams and outlet glaciers, which account for 90 % of the ice sheet’s discharge (Bamber et al, 2000). An important role in the regulation of ice stream flow is played by their basal conditions, i.e. bedrock topography and variations in basal slipperiness. Control method inversion uses accurate remote sensing data about the surface velocity field in combination with theoretical ice flow models to obtain information about the basal shear stress distribution of ice streams (MacAyeal et al, 1995; Vieli and Payne, 2003; Joughin et al, 2004, 2006; Arthern and Gudmundsson, 2010). It is often thought that variations in basal slipperiness are the principle cause of changing flow conditions along an ice stream, and the role of the bedrock topography is rarely addressed directly in this context.
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