The dynamic response of the Greenland Ice Sheet (GrIS) depends on feedbacks between surface meltwater delivery to the subglacial environment and ice flow. Recent work has highlighted an important role of hydrological processes in regulating the ice flow, but models have so far overlooked the mechanical effect of soft basal sediment. Here we use a threedimensional model to investigate hydrological controls on a GrIS soft-bedded region. Our results demonstrate that weakening and strengthening of subglacial sediment, associated with the seasonal delivery of surface meltwater to the bed, modulates ice flow consistent with observations. We propose that sedimentary control on ice flow is a viable alternative to existing models of evolving hydrological systems, and find a strong link between the annual flow stability, and the frequency of high meltwater discharge events. Consequently, the observed GrIS resilience to enhanced melt could be compromised if runoff variability increases further with future climate warming.

Marine‐terminating outlet glaciers of the Greenland Ice Sheet make significant contributions to global sea level rise, yet the conditions that facilitate their fast flow remain poorly constrained owing to a paucity of data. We drilled and instrumented seven boreholes on Store Glacier, Greenland, to monitor subglacial water pressure, temperature, electrical conductivity, and turbidity along with englacial ice temperature and deformation. These observations were supplemented by surface velocity and meteorological measurements to gain insight into the conditions and mechanisms of fast glacier flow. Located 30 km from the calving front, each borehole drained rapidly on attaining ∼600 m depth indicating a direct connection with an active subglacial hydrological system. Persistently high subglacial water pressures indicate low effective pressure (180–280 kPa), with small‐amplitude variations correlated with notable peaks in surface velocity driven by the diurnal melt cycle and longer periods of melt and rainfall. The englacial deformation profile determined from borehole tilt measurements indicates that 63–71% of total ice motion occurred at the bed, with the remaining 29–37% predominantly attributed to enhanced deformation in the lowermost 50–100 m of the ice column. We interpret this lowermost 100 m to be formed of warmer, pre‐Holocene ice overlying a thin (0–8 m) layer of temperate basal ice. Our observations are consistent with a spatially extensive and persistently inefficient subglacial drainage system that we hypothesize comprises drainage both at the ice‐sediment interface and through subglacial sediments. This configuration has similarities to that interpreted beneath dynamically analogous Antarctic ice streams, Alaskan tidewater glaciers, and glaciers in surge.

[1] A surface mass balance model aimed at being coupled to a Global Circulation Model (GCM) for future climate prediction is described and tested for the Greenland Ice Sheet. The model builds on previous modeling designed to be forced by automatic weather station data, and includes surface energy balance as well as processes occurring near the surface such as water percolation and refreezing. Surface albedo is calculated with a new scheme that differentiates the timescale for aging of wet and dry snow and incorporates the effect of a thin layer of water and/or fresh snow at the surface. The model was driven with automatic weather station data from two sites located in the ablation zone in the Kangerlussuaq area (West Greenland), and calculated reasonable annual mass balance values (within 10% in seven out of eight cases) for four individual and consecutive years (1998)(1999)(2000)(2001), using both measured and calculated albedo. This implies that the albedo parameterization is adequate and climate feedbacks affecting the mass balance are well captured. The model was then applied to a distributed 20-km-resolution grid covering the whole ice sheet, and forced with 10 years of the European Centre for Medium-range Weather Forecast (ECMWF) reanalysis (ERA-40) data. With the aim of coupling the model to a GCM, this study focuses on the ability to model the interannual variability in mass balance rather than to assess the present state of balance of the ice sheet. Modeled spatial and temporal wet zone extent compares well with information derived from passive microwave satellite data.

[1] Predicting ice sheet mass balance is challenging because of the complex flow of ice streams. To address this issue, we have coupled a three-dimensional higher-order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress. The model was tested for its sensitivity to regional water availability. First, with an assumed undrained bed, the ice stream oscillates between active and stagnant phases, solely as a result of thermodynamic feedbacks occurring at the ice-till interface. However, the velocity amplitude decreases over time, as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing "ice sheet mode." Second, we assume that the till is able to assimilate water from a hypothetical regional hydrological system. This leads to significantly different long-term behavior, as a continuously oscillating "ice stream mode" is maintained. The extra water incorporated in the till leads to higher velocities, triggering stronger thermodynamic feedbacks between the ice and till layer. Results also suggest that fast-flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase. Similarly, till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase. Our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system, underscoring the need to accurately describe the coupling between ice dynamics, basal conditions and regional subglacial hydrology in ice sheet models.

Satellite observations have revealed active hydrologic systems beneath Antarctic ice streams, but sources and sinks of water within these systems are uncertain. Here we use numerical simulations of ice streams to estimate the generation, flux, and budget of water beneath five ice streams on the Siple Coast. We estimate that 47% of the total hydrologic input (0.98 km 3 yr À1 ) to Whillans (WIS), Mercer (MIS), and Kamb (KIS) ice streams comes from the ice sheet interior and that only 8% forms by local basal melting. The remaining 45% comes from a groundwater reservoir, an overlooked source in which depletion significantly exceeds recharge. Of the total input to Bindschadler (BIS) and MacAyeal (MacIS) ice streams (0.56 km 3 yr À1), 72% comes from the interior, 19% from groundwater, and 9% from local melting. This contrasting hydrologic setting modulates the ice streams flow and has important implications for the search for life in subglacial lakes.

Supraglacial lakes on the Greenland Ice Sheet are expanding inland, but the impact on ice flow is equivocal because interior surface conditions may preclude the transfer of surface water to the bed. Here we use a well-constrained 3D model to demonstrate that supraglacial lakes in Greenland drain when tensile-stress perturbations propagate fractures in areas where fractures are normally absent or closed. These melt-induced perturbations escalate when lakes as far as 80 km apart form expansive networks and drain in rapid succession. The result is a tensile shock that establishes new surface-to-bed hydraulic pathways in areas where crevasses transiently open. We show evidence for open crevasses 135 km inland from the ice margin, which is much farther inland than previously considered possible. We hypothesise that inland expansion of lakes will deliver water and heat to isolated regions of the ice sheet’s interior where the impact on ice flow is potentially large.

[1] Ross ice streams supply over 90% of the ice volume flowing out of the Ross sector of the West Antarctic ice sheet (WAIS). Stoppage of Ice Stream C (ISC) ca. 150 years ago appears to have pushed this sector of WAIS from negative into positive mass balance [Joughin and Tulaczyk, 2002]. We propose an explanation for the unsteady behavior of ISC using a new numerical ice-stream model, which includes an explicit treatment of a subglacial till layer. When constrained by initial conditions emulating prestoppage geometry, dynamics, and mass balance of ISC, the model yields a rapid ($100 years) stoppage of the main ice-stream trunk. The stoppage is triggered by basal freeze-on, which consolidates and strengthens the subglacial till. Our numerical simulations produce results consistent with a number of existing observations, for example, continuing activity of the two tributaries of ISC. The model always yields rapid stoppage unless we specify icestream width that is smaller than its prestoppage values (maximum of $80 km). We conjecture that if ISC was active for at least a few thousand years before slowdown, its width was significantly smaller than today to sustain the long active phase. Ice-stream width is a key control that helps determine whether ice-stream flow is sustainable over a long term. Our work indicates that the recent stoppage of Ice Stream C could have been part of inherent ice-stream cyclicity, and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns.

[1] Long-term predictions of sea level rise from increased Greenland ice sheet melting have been derived using Positive Degree Day models only. It is, however, unknown precisely what uncertainties are associated with applying this simple surface melt parameterization for future climate. We compare the behavior of a Positive Degree Day and Energy Balance/Snowpack model for estimating the surface mass balance of the Greenland ice sheet under a warming climate. Both models were first tuned to give similar values for present-day mass balance using 10 years of ERA-40 climatology and were then run for 300 years, forced with the output of a GCM in which atmospheric CO 2 increased to 4 times preindustrial levels. Results indicate that the Positive Degree Day model is more sensitive to climate warming than the Energy Balance model, generating annual runoff rates almost twice as large for a fixed ice sheet geometry. Roughly half of this difference was due to differences in the volume of melt generated and half was due to differences in refreezing rates in the snowpack. Our results indicate that the modeled snowpack properties evolve on a multidecadal timescale to changing climate, with a potentially large impact on the mass balance of the ice sheet; an evolution that was absent from the Positive Degree Day model. Citation: Bougamont, M.,