Constraints on the relationship between velocity and basal traction over the grounded regions of Greenland

On glaciers and ice sheets, constraints on the bed physics which control the relationship between velocity and traction are critical for simulating ice flow. However, in Greenland the relationship between velocity and traction remains unquantified over much of the ice sheet. In this work, 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 biases associated with unconstrained rheologic parameters used in numerical inversions. We find that the velocity-traction 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 velocity-traction relationships suggest that the flow field and surface geometries over the grounded regions of the Greenland ice sheet are mainly dictated by Weertman-type physics. This data- and modeling based analysis provides a first constraint on the physics of basal motion over the grounded regions of Greenland and gives unique insight into future dynamics and vulnerabilities in a warming climate.