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 controls ice flow. Yet in Greenland, these relationships remain unquantified. We determine the spatial relationship between velocity and traction in all eight major 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 of the grounded regions in Greenland is mainly dictated by Weertman-type hard-bed physics up to velocities of approximately 450 m yr−1, except within the Northeast Greenland Ice Stream and areas near floatation. Depending on the catchment, behavior of the fastest-flowing ice (∼ 1000 m yr−1) directly inland from marine-terminating outlets exhibits Weertman-type rate strengthening, Mohr–Coulomb-like behavior, or is not confidently resolved given our methodology. Given the complex basal boundary across Greenland, the relationships are captured reasonably well by simple traction laws which provide a parameterization that can be used to model ice dynamics at large scales. The results and analysis serve as a first constraint on the physics of basal motion over the grounded regions of Greenland and provide unique insight into future dynamics and vulnerabilities in a warming climate.

Greenland ice sheet mass balance from 1840 through next week

“…The term has only been measured in a handful of places (e.g., Ryser et al (2014); Maier et al (2019)) and it is unclear how representative those measurements are on ice-sheet scales. Karlsson et al (2021) therefore estimate the frictional heating using the Full Stokes Elmer/Ice model that resolves all stresses while relating basal sliding and shear stress using a linear friction law (Gillet-Chaulet et al, 2012;Maier et al, 2021 The model is tuned to match a multi-decadal surface velocity map (Joughin et al, 2018) covering 1995-2015 and it returns an estimated basal friction heat that is used to calculate the basal melt due to friction, similar to Eq. 1:…”

Seasonal evolution of basal environment conditions of Russell sector, West Greenland, inverted from satellite observation of surface flow

“…These values are consistent with previous inversions in the same area performed by Koziol and Arnold (2017) using winter 2008-2009 velocities. Together with a typical sliding velocity of 100 m yr −1 , this falls in the velocity-traction relationship derived at the catchment scale by Maier et al (2021a), which was interpreted as indicative of hard-bed conditions. However, Maier et al (2021a) also found several patches in this catchment where the bed is weaker than the average.…”

“…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 orders of magnitude. Spatially aggregating inversions with models of different complexity, Maier et al (2021a) found that large areas under the Greenland ice sheet broadly agree with hardbed 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.…”

“…Inverting for the basal friction coefficient using the observed surface velocities is now widespread in many ice-sheet models (Jay-Allemand et al, 2011;Gillet-Chaulet et al, 2012;Larour et al, 2014;Shapero et al, 2016;Maier et al, 2021a). Here, we use the variational control inverse method implemented in Elmer/Ice (Gagliardini et al, 2013), and in the following we highlight the main steps.…”

Greenland ice sheet mass balance from 1840 through next week

“…The term has only been measured in a handful of places (e.g., Ryser et al, 2014;Maier et al, 2019), and it is unclear how representative those measurements are at ice sheet scales. Karlsson et al (2021) therefore estimate the frictional heating using the full Stokes Elmer/Ice model that resolves all stresses while relating basal sliding and shear stress using a linear friction law (Gillet-Chaulet et al, 2012;Maier et al, 2021). The model is tuned to match a multidecadal surface velocity map (Joughin et al, 2018) covering 1995-2015, and it returns an estimated basal friction heat that is used to calculate the basal melt due to friction, similarly to Eq.…”