Physical Conditions of Fast Glacier Flow: 3. Seasonally‐Evolving Ice Deformation on Store Glacier, West Greenland

Abstract

Temporal variations in ice sheet flow directly impact the internal structure within ice sheets through englacial deformation. Large‐scale changes in the vertical stratigraphy within ice sheets have been previously conducted on centennial to millennial timescales; however, intra‐annual changes in the morphology of internal layers have yet to be explored. Over a period of 2 years, we use autonomous phase‐sensitive radio‐echo sounding to track the daily displacement of internal layers on Store Glacier, West Greenland, to millimeter accuracy. At a site located ∼30 km from the calving terminus, where the ice is ∼600 m thick and flows at ∼700 m/a, we measure distinct seasonal variations in vertical velocities and vertical strain rates over a 2‐year period. Prior to the melt season (March–June), we observe increasingly nonlinear englacial deformation with negative vertical strain rates (i.e., strain thinning) in the upper half of the ice column of approximately −0.03 a−1, whereas the ice below thickens under vertical strain reaching up to +0.16 a−1. Early in the melt season (June–July), vertical thinning gradually ceases as the glacier increasingly thickens. During late summer to midwinter (August–February), vertical thickening occurs linearly throughout the entire ice column, with strain rates averaging 0.016 a−1. We show that these complex variations are unrelated to topographic setting and localized basal slip and hypothesize that this seasonality is driven by far‐field perturbations in the glacier's force balance, in this case generated by variations in basal hydrology near the glacier's terminus and propagated tens of kilometers upstream through transient basal lubrication longitudinal coupling.

Coupled modelling of subglacial hydrology and calving-front melting at Store Glacier, West Greenland

“…5b), with the exception of the number of channels and mean channel flux, which both show small increases over the final SummerAverage17 values even at the end of the SummerDaily17 run. These results for 2017 also agree well with the observations of a high-pressure distributed drainage system 30 km inland in 2014-15 reported in and Young et al (2019), towards the centre of the model domain (see Fig. 3), with channels not reaching this far inland in this part of the glacier in a low-surface-melt summer.…”

“…We present the first coupled hydrology-plume model applied to a tidewater glacier in Greenland, allowing us to investigate aspects of the subglacial hydrology of Store Glacier critical to ice dynamics and calving-front melting that are poorly constrained by existing observations and models. We demonstrate that the implementation of the GlaDS hydrological model within the Elmer/Ice modelling suite recreates well the observed behaviour of the subglacial drainage system of Store (Chauché, 2016;Doyle et al, 2018;Young et al, 2019), giving us greater confidence in its use as a predictive tool.…”

“…In that study, a persistently high basal water pressure of 93-95% of ice overburden indicates a largely inefficient basal water system 30 km inland from the calving margin at the fast-flowing and heavily-crevassed Store Glacier (Store) (Doyle et al, 2018). Yet, observed seasonal velocity fluctuations on the same glacier are consistent with the development of a channelised basal drainage system closer to the margin (Young et al, 2019), which calls for a physical model to spatially and temporally constrain the formation of different types of basal drainage system. Hydrological work on marine-terminating glaciers has so far focused on the subglacial discharge that drives convective plumes in the marine terminus environment, and how this process can promote calving by undercutting the glacier through submarine melting and fjord circulation (Carroll et al, 2015;Cowton et al, 2015;Fried et al, 2015;Jackson et al, 2017;Jouvet et al, 2018;Slater et al, 2018).…”

Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide deep ice core site

“…A follow-up study by Jordan et al (2020b) demonstrated the validity of this framework, albeit with a coarse azimuthal resolution via the azimuthal rotational setup, from measurements obtained using an autonomous phase-sensitive radio-echo sounder (ApRES). The ApRES is a phase-sensitive frequency-modulated continuous-wave (FMCW) ground-based radar system (Brennan et al, 2014) that has been gaining traction over the last five years, not only to investigate englacial polarimetry (Brisbourne et al, 2019;Jordan et al, 2020b), but also in wider radioglaciological investigations involving englacial deformation (Gillet-Chaulet et al, 2011;Kingslake et al, 2014Kingslake et al, , 2016Nicholls et al, 2015;Young et al, 2019), englacial meltwater content (Kendrick et al, 2018;Vaňková et al, 2018), basal melting (Corr et al, 2002;Davis et al, 2018;Jenkins 2 https://doi.org/10.5194/tc-2020-264 Preprint. Discussion started: 28 September 2020 c Author(s) 2020.…”

Future Evolution of Greenland's Marine‐Terminating Outlet Glaciers

“…Further, outlet glaciers are likely to be underlain with subglacial sediments (Christianson et al, ; Hofstede et al, ; Walter et al, ), with complex spatial patterns of basal slipperiness and suggesting that basal conditions for outlet glaciers may vary over small ( 100 m) scales. Finally, because glaciers terminate in the ocean with water depths of 10s–100s of meters, high water pressures exist year round and seasonal changes in flow at glacier termini can result in ice thickness changes that are felt far inland (Young et al, ).…”