Physical Conditions of Fast Glacier Flow: 3.Seasonally-Evolving Ice Deformation onStore Glacier, West Greenland

Temporal variations in ice sheet flow directly impact the internal structure within ice sheetsthrough englacial deformation. Large-scale changes in the vertical stratigraphy within ice sheets havebeen previously conducted on centennial to millennial timescales; however, intra-annual changes inthe morphology of internal layers have yet to be explored. Over a period of 2 years, we use autonomousphase-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 iceis∼600 m thick and flows at∼700 m/a, we measure distinct seasonal variations in vertical velocities andvertical strain rates over a 2-year period. Prior to the melt season (March–June), we observe increasinglynonlinear englacial deformation with negative vertical strain rates (i.e., strain thinning) in the upper half ofthe ice column of approximately−0.03 a−1, whereas the ice below thickens under vertical strain reachingup to+0.16 a−1. Early in the melt season (June–July), vertical thinning gradually ceases as the glacierincreasingly thickens. During late summer to midwinter (August–February), vertical thickening occurslinearly throughout the entire ice column, with strain rates averaging 0.016 a−1. We show that thesecomplex variations are unrelated to topographic setting and localized basal slip and hypothesize that thisseasonality is driven by far-field perturbations in the glacier's force balance, in this case generated byvariations in basal hydrology near the glacier's terminus and propagated tens of kilometers upstreamthrough transient basal lubrication longitudinal couplin