Abstract. We present detailed records of lake discharge, ice motion and passive seismicity capturing the behaviour and processes preceding, during and following the rapid drainage of a ∼ 4 km 2 supraglacial lake through 1.1-km-thick ice on the western margin of the Greenland Ice Sheet. Peak discharge of 3300 m 3 s −1 coincident with maximal rates of vertical uplift indicates that surface water accessed the ice-bed interface causing widespread hydraulic separation and enhanced basal motion. The differential motion of four global positioning system (GPS) receivers located around the lake record the opening and closure of the fractures through which the lake drained. We hypothesise that the majority of discharge occurred through a ∼ 3-km-long fracture with a peak width averaged across its wetted length of ∼ 0.4 m. We argue that the fracture's kilometre-scale length allowed rapid discharge to be achieved by combining reasonable water velocities with sub-metre fracture widths. These observations add to the currently limited knowledge of in situ supraglacial lake drainage events, which rapidly deliver large volumes of water to the ice-bed interface.

Abstract. Supraglacial lakes represent an ephemeral storage buffer for meltwater runoff and lead to significant, yet shortlived, episodes of ice-flow acceleration by decanting large meltwater and energy fluxes into the ice sheet's hydrological system. Here, a methodology for calculating lake volume is used to quantify storage and drainage across Russell Glacier, West Greenland, between 2002 and 2012. Using 502 MODIS scenes, water volume at ∼ 200 seasonally occurring lakes was derived using a depth-reflectance relationship, which was independently calibrated and field validated against lake bathymetry. The inland expansion of lakes is strongly correlated with air temperature: during the record melt years of 2010 and 2012, lakes formed and drained earlier, attaining their maximum volume 38 and 20 days earlier than the 11 yr mean, as well as occupying a greater area and forming at higher elevations (> 1800 m) than previously. Despite occupying under 2 % of the study area, lakes delay the transmission of up to 7-13 % of the bulk meltwater discharged. Although the results are subject to an observational bias caused by periods of cloud cover, we estimate that across Russell Glacier, 28 % of supraglacial lakes drain rapidly (< 4 days). Clustering of such events in space and time suggests a synoptic trigger mechanism. Further, we find no evidence to support a unifying critical size or depth-dependent drainage threshold.

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.

Abstract. The concept of a positive feedback between ice flow and enhanced melt rates in a warmer climate fuelled the debate regarding the temporal and spatial controls on seasonal ice acceleration. Here we combine melt, basal water pressure and ice velocity data. Using 20 years of data covering the whole ablation area, we show that there is not a strong positive correlation between annual ice velocities and melt rates. Annual velocities even slightly decreased with increasing melt. Results also indicate that melt variations are most important for velocity variations in the upper ablation zone up to the equilibrium line altitude. During the extreme melt in 2012, a large velocity response near the equilibrium line was observed, highlighting the possibility of meltwater to have an impact even high on the ice sheet. This may lead to an increase of the annual ice velocity in the region above S9 and requires further monitoring.

We present surface velocity measurements from a high-elevation site located 140 km from the western margin of the Greenland ice sheet, and~50 km into its accumulation area. Annual velocity increased each year from 51.78 ± 0.01 m yr À1 in 2009 to 52.92 ± 0.01 m yr À1 in 2012-a net increase of 2.2%. These data also reveal a strong seasonal velocity cycle of up to 8.1% above the winter mean, driven by seasonal melt and supraglacial lake drainage. recently argued that ice motion in the ablation area is mediated by reduced winter flow following the development of efficient subglacial drainage during warmer, faster, summers. Our data extend this analysis and reveal a year-on-year increase in annual velocity above the equilibrium line altitude, where despite surface melt increasing, it is still sufficiently low to hinder the development of efficient drainage under thick ice.

Meltwater drainage across the surface of the Greenland ice sheet (GrIS) is well constrained by measurements and modeling, yet despite its critical role, knowledge of its transit through the subglacial environment remains limited. Here we present a subglacial hydrological analysis of a land-terminating sector of the GrIS at unprecedented resolution that predicts the routing of surface-derived meltwater once it has entered the basal drainage system. Our analysis indicates the probable existence of small subglacial lakes that remain undetectable by methods using surface elevation change or radar techniques. Furthermore, the analysis suggests transient behavior with rapid switching of subglacial drainage between competing catchments driven by seasonal changes in the basal water pressure. Our findings provide a cautionary note that should be considered in studies that attempt to relate and infer future response from surface temperature, melt, and runoff from point measurements and/or modeling with measurements of proglacial discharge and ice dynamics. Hydraulic potential analysis enables subglacial drainage to be estimated from ice thickness and basal topography on spatial scales where the main control of the drainage is the geometry of the ice sheet [Shreve, 1972]. This method has been applied in Greenland [e.g.

Intense rainfall events significantly affect Alpine and Alaskan glaciers through enhanced melting, ice-flow acceleration and subglacial sediment erosion, yet their impact on the Greenland ice sheet has not been assessed. Here we present measurements of ice velocity, subglacial water pressure and meteorological variables from the western margin of the Greenland ice sheet during a week of warm, wet cyclonic weather in late August and early September 2011. We find that extreme surface runoff from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the ice-sheet interior. We suggest that the late-season timing was critical in promoting rapid runoff across an extensive bare ice surface that overwhelmed a subglacial hydrological system in transition to a less-efficient winter mode. Reanalysis data reveal that similar cyclonic weather conditions prevailed across southern and western Greenland during this time, and we observe a corresponding ice-flow response at all land- and marine-terminating glaciers in these regions for which data are available. Given that the advection of warm, moist air masses and rainfall over Greenland is expected to become more frequent in the coming decades, our findings portend a previously unforeseen vulnerability of the Greenland ice sheet to climate change.authorsversionPeer reviewe

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.