Basal traction mainly dictated by hard-bed physics over grounded regions of Greenland

Abstract

Abstract. On glaciers and ice sheets, identifying the relationship between velocity and traction is critical to constrain the bed physics that control ice flow. Yet in Greenland, these relationships remain unquantified. We determine the spatial relationship between velocity and traction in all eight drainage catchments of Greenland. The basal traction is estimated using three different methods over large grid cells to minimize interpretation biases associated with unconstrained rheologic parameters used in numerical inversions. We find the relationships are consistent with our current understanding of basal physics in each catchment. We identify catchments that predominantly show Mohr-Coulomb-like behavior typical of deforming beds or significant cavitation, as well as catchments that predominantly show rate-strengthening behavior typical of Weertman-type hard-bed physics. Overall, the traction relationships suggest that the flow field and surface geometry over the grounded regions of the Greenland ice sheet is mainly dictated by Weertman-type hard-bed physics. Given the complex basal boundary across Greenland, the relationships are captured surprisingly well by simple traction laws over the entire velocity range, including regions with velocities over 1000 m/yr, which provide a parameterization that can be used to model ice dynamics at large scales. The results and analysis serve as a fundamental constraint on the physics of basal motion in Greenland and provide unique insight into future dynamics and vulnerabilities in a warming climate.

A first constraint on basal melt-water production of the Greenland ice sheet

“…Our findings are consistent with a recent study showing that the simple approach overestimates the basal stresses compared to the Full-Stokes solution 32 . In addition to the uncertainty imposed by the simplified stress-balance, other uncertainties include unknown temperatures of the basal shear layer and the uncertainties from velocity datasets (see methods for a detailed discussion of the uncertainties).…”

“…The reason for the large discrepancies is likely the inability of the simplified approach to capture the complex flow regime of the Northeast Greenland ice stream in the NE sector, and the topography of numerous small outlet glaciers in the NW sector. Our findings are consistent with a recent study showing that the simple approach overestimates the basal stresses compared to the Full-Stokes solution 32 . In addition to the uncertainty imposed by the simplified stress-balance, other uncertainties include unknown temperatures of the basal shear layer and the uncertainties from velocity datasets (see methods for a detailed discussion of the uncertainties).…”

“…Frictional heat is produced by ice sliding over the bed. We retrieve an estimate of frictional heat using the Elmer/Ice model, where the complete stress balance is solved (“Full Stokes”) 31 , and where basal sliding and shear stress are related by a linear friction law 32 . Internal ice temperatures are obtained from a paleo spin-up run 33 .…”

“…Frictional heat: Elmer/Ice model. The first estimate of frictional heat is obtained with the Elmer/Ice model, which is a Full Stokes ice-flow model resolving all stresses 31,32 . The ice-flow model uses the GIMP digital elevation model (Greenland Ice sheet Mapping Project 43 ), and ice thicknesses and bed topography from Bed-Machine v3 calculated using a mass-conservation method 44 .…”

Seasonal evolution of basal conditions within Russell sector, West Greenland, inverted from satellite observations of surface flow

“…Thus, Arthern et al (2015) found that the basal stress in Antarctica, on average, roughly agrees with a uniform value of 100 kPa, however this can change locally by order of magnitudes. Spatially aggregating inversions with models of different complexity, Maier et al (2021) found that large areas under the Greenland ice sheet broadly agree with hard-bed physics. The other possibility to constrain the friction law is to use several inversions to study the temporal changes; however this can be done only where the changes are sufficiently large.…”