A Consistent Framework for Coupling Basal Friction With Subglacial Hydrology on Hard-Bedded Glaciers
Predicting the sliding speed of glaciers and ice sheets is challenged by the difficulties of assessing the water pressure at the glacier base. Here, we improve the coupling between existing theories about basal friction and subglacial hydrology by introducing a consistent description of roughness and hydraulic transmissivity. Our work breaks with the common view on the subglacial environment that water pressure drives the sliding speed by modulating friction at the glacier base. Instead, our findings show that at multi-day and longer timescales sliding speed and water pressure are imposed by the water discharge along the glacier base that needs to be accommodated. Our results open new perspectives for understanding contemporary glacier and ice sheet sliding and predicting its future behavior under changing climate.
Read more: Gilbert A., F. Gimbert, K. Thøgersen, T. V. Schuler and A. Kääb, 2022. A Consistent Framework for Coupling Basal Friction With Subglacial Hydrology on Hard-Bedded Glaciers, Geophysical Research Letters, 49, e2021GL097507, doi:10.1029/2021GL097507
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undergone a phase of significant fragmentation during the last decade. Observations as well as simulations support the theory that the glacier's acceleration increased the damage in the floating part of the ice by backstress imposed from its pinning points. HiDEM (Helsinki Discrete Element Model) simulations primed with basal friction coefficients obtained by means of Elmer/Ice simulations using data assimilation indicate a significant zone of shear, upstream of the main pinning point, seeding damage on the shelf. Subsequently, basal melting and positive feedback between damage and strain rates weakened TEIS, allowing damage to accumulate. Despite a diminishing backstress caused by a shrinking pinning point, accumulation of damage has ensured that the ice in TEIS in the shear zone remained the weakest link in the system. Besides hydro-fracturing and detachment from pinning points, this study suggest a third mechanism for ice-shelf instability: backstress triggered failure. 
Significant change in the dynamics of tidewater glaciers after a calving event are mainly tied to a sudden loss of resistive stresses. This article investigates how such a stress perturbation affects the whole glacier upstream. Simulations (using Elmer/Ice) and perturbation theory revealed that calving events and subsequent terminus readvance produce quasi-periodic, sawtooth oscillations in stress that originate at the terminus and propagate upstream with speeds significantly exceeding ice velocities. In laterally resisted glaciers, these signals decay within an upstream distance equivalent to a few ice thicknesses. Terminus fluctuations caused by individual calving events tend to be much higher frequency than climate variations. Thus, individual calving events have little direct impact on the viscous delivery of ice to the terminus. This suggests that the primary mechanism by which calving events can trigger instability is by causing fluctuations in stress that weaken the ice and lead to additional calving and sustained terminus retreat. Our results further demonstrate a stronger response to calving events in simulations that include the full stress tensor, highlighting the importance of accounting for higher order stresses when developing calving parameterizations for tidewater glaciers.
