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The flow speed of the Greenland Ice Sheet changes dramatically in inland regions when surface meltwater drains to the bed. But ice-sheet discharge to the ocean is dominated by fast-flowing outlet glaciers, where the effect of increasing surface melt on annual discharge is unknown. Observations of a supraglacial lake drainage at Helheim Glacier, and a consequent velocity pulse propagating down-glacier, provide a natural experiment for assessing the impact of changes in injected meltwater, and allow us to interrogate the subglacial hydrological system. We find a highly efficient subglacial drainage system, such that summertime lake drainage has little net effect on ice discharge. Our results question the validity of common remote-sensing approaches for inferring subglacial conditions, knowledge of which is needed for improved projections of sea-level rise.

 

Abstract Surface meltwater reaching the base of the Greenland Ice Sheet transits through drainage networks, modulating the flow of the ice sheet. Dye and gas-tracing studies conducted in the western margin sector of the ice sheet have directly observed drainage efficiency to evolve seasonally along the drainage pathway. However, the local evolution of drainage systems further inland, where ice thicknesses exceed 1000 m, remains largely unknown. Here, we infer drainage system transmissivity based on surface uplift relaxation following rapid lake drainage events. Combining field observations of five lake drainage events with a mathematical model and laboratory experiments, we show that the surface uplift decreases exponentially with time, as the water in the blister formed beneath the drained lake permeates through the subglacial drainage system. This deflation obeys a universal relaxation law with a timescale that reveals hydraulic transmissivity and indicates a two-order-of-magnitude increase in subglacial transmissivity (from 0.8 ± 0.3  $${\rm{m}}{{\rm{m}}}^{3}$$ m m 3 to 215 ± 90.2  $${\rm{m}}{{\rm{m}}}^{3}$$ m m 3 ) as the melt season progresses, suggesting significant changes in basal hydrology beneath the lakes driven by seasonal meltwater input. 

Abstract We develop a two-dimensional, plan-view formulation of ice-shelf flow and viscoelastic ice-shelf flexure. This formulation combines, for the first time, the shallow-shelf approximation for horizontal ice-shelf flow (and shallow-stream approximation for flow on lubricated beds such as where ice rises and rumples form), with the treatment of a thin-plate flexure. We demonstrate the treatment by performing two finite-element simulations: one of the relict pedestalled lake features that exist on some debris-covered ice shelves due to strong heterogeneity in surface ablation, and the other of ice rumpling in the grounding zone of an ice rise. The proposed treatment opens new venues to investigate physical processes that require coupling between the longitudinal deformation and vertical flexure, for instance, the effects of surface melting and supraglacial lakes on ice shelves, interactions with the sea swell, and many others. 

Surface meltwater reaching the base of the Greenland Ice Sheet transits through drainage networks, modulating the flow of the ice sheet. Dye and gas-tracing studies conducted in the western margin sector of the ice sheet have directly observed drainage efficiency to evolve seasonally along the drainage pathway. However, the local evolution of drainage systems further inland, where ice thicknesses exceed 1000 m, remains largely unknown. Here, we infer drainage system transmissivity based on surface uplift relaxation following rapid lake drainage events. Combining field observations of five lake drainage events with a mathematical model and laboratory experiments, we show that the surface uplift decreases exponentially with time, as the water in the blister formed beneath the drained lake permeates through the subglacial drainage system. This deflation obeys a universal relaxation law with a timescale that reveals hydraulic transmissivity and indicates a two-order-of- magnitude increase in subglacial transmissivity (from 0.8 ± 0.3 mm3 to 215 ± 90.2 mm3) as the melt season progresses, suggesting significant changes in basal hydrology beneath the lakes driven by seasonal meltwater input. 

Abstract. In the 2019/2020 austral summer, the surface melt duration andextent on the northern George VI Ice Shelf (GVIIS) was exceptional comparedto the 31 previous summers of distinctly lower melt. This finding is basedon analysis of near-continuous 41-year satellite microwave radiometer andscatterometer data, which are sensitive to meltwater on the ice shelfsurface and in the near-surface snow. Using optical satellite imagery fromLandsat 8 (2013 to 2020) and Sentinel-2 (2017 to 2020), record volumes ofsurface meltwater ponding were also observed on the northern GVIIS in2019/2020, with 23 % of the surface area covered by 0.62 km3 of ponded meltwater on 19 January. These exceptional melt andsurface ponding conditions in 2019/2020 were driven by sustained airtemperatures ≥0 ∘C for anomalously long periods (55 to 90 h)from late November onwards, which limited meltwater refreezing.The sustained warm periods were likely driven by warm, low-speed (≤7.5 m s−1) northwesterly and northeasterly winds and not by foehn windconditions, which were only present for 9 h total in the 2019/2020 meltseason. Increased surface ponding on ice shelves may threaten theirstability through increased potential for hydrofracture initiation; a riskthat may increase due to firn air content depletion in response tonear-surface melting. 

Abstract. The frontal flux balance of a medium-sized tidewater glacier in westernGreenland in the summer is assessed by quantifying the individual components(ice flux, retreat, calving, and submarine melting) through a combination ofdata and models. Ice flux and retreat are obtained from satellite data.Submarine melting is derived using a high-resolution ocean model informed bynear-ice observations, and calving is estimated using a record of calvingevents along the ice front. All terms exhibit large spatial variability alongthe ∼5 km wide ice front. It is found that submarine melting accountsfor much of the frontal ablation in small regions where two subglacialdischarge plumes emerge at the ice front. Away from the subglacial plumes,the estimated melting accounts for a small fraction of frontal ablation.Glacier-wide, these estimates suggest that mass loss is largely controlled bycalving. This result, however, is at odds with the limited presence oficebergs at this calving front – suggesting that melt rates in regionsoutside of the subglacial plumes may be underestimated. Finally, we arguethat localized melt incisions into the glacier front can be significantdrivers of calving. Our results suggest a complex interplay of melting andcalving marked by high spatial variability along the glacier front.