Abstract. Increased surface melt over the Greenland Ice Sheet (GrIS) is now estimated to account for half or more of the ice sheet's total mass loss. Here, we show that some meltwater is stored, over winter, in buried supraglacial lakes. We use airborne radar from Operation IceBridge between 2009 and 2012 to detect buried supraglacial lakes, and we find that they were distributed extensively around the GrIS margin through that period. Buried supraglacial lakes can persist through multiple winters and are, on average, ∼ 1.9 + 0.2 m below the surface. Most buried supraglacial lakes exist with no surface expression of their occurrence in visible imagery. The few buried supraglacial lakes that do exhibit surface expression have a unique visible signature associated with a darker blue color where subsurface water is located. The volume of retained water in the buried supraglacial lakes is likely insignificant compared to the total mass loss from the GrIS, but the water may have important implications locally for the development of the englacial hydrologic system and ice temperatures. Buried supraglacial lakes represent a small but year-round source of meltwater in the GrIS hydrologic system.

Abstract. Surface melt over the Greenland Ice Sheet (GrIS) is increasing and estimated to account for half or more of the total mass loss. Little, however, is known about the hydrologic pathways that route surface melt within the ice sheet. In this study, we present over-winter storage of water in buried supraglacial lakes as one hydrologic pathway for surface melt, referred to as buried lakes. Airborne radar echograms are used to detect the buried lakes that are distributed extensively around the margin of the GrIS. The subsurface water can persist through multiple winters and is, on average, ~4.2 + 0.4 m below the surface. The few buried lakes that are visible at the surface of the GrIS have a~unique visible signature associated with a darker blue color where subsurface water is located. The volume of retained water in the buried lakes is likely insignificant compared to the total mass loss from the GrIS but the water will have important implications locally for the development of the englacial hydrologic network, ice temperature profiles and glacial dynamics. The buried lakes represent a small but year-round source of meltwater in the GrIS hydrologic system.

Supraglacial lakes over the Greenland Ice Sheet can demonstrate multi-model drainage states. Lakes can demonstrate incomplete drainage, where residual melt can become buried under ice and snow and survive throughout the winter. We evaluate atmospheric factors that influence the propensity for the formation of buried lakes over the ice sheet. We examine the spatial and temporal occurrence and behavior of buried lakes over the Jakobshavn Isbrae and Zachariae Isstrøm outlet basins and assess the magnitude of insolation necessary to preserve melt water using a numerical lake model from 2009 to 2012. Buried lakes tend to occur at higher elevations within the ablation zone and those present at elevations > 1000 m tend to reoccur over several seasons. Lakes without buried water are relatively small (∼1 km2), whereas lakes with buried water are larger (∼6–10 km2). Lake area is correlated with the number of seasons sub-surface water persists. Buried lakes are relatively deep and associated with complex supraglacial channel networks. Winter stored water could be a precursor to the formation of supraglacial channels. Simulations of the insulation potential of accumulated snow and ice on the surface of lakes indicate substantial regional differences and inter-annual variability. With the possibility of inland migration of supraglacial lakes, buried lakes could be important in the evolution of ablation/percolation zone hydrology.

The impact of melt water injection into ice streams over the Greenland Ice Sheet is not well understood. Water-filled crevasses along the shear margins of Jakobshavn Isbrae are known to fill and drain, resulting in weakening of the shear margins due to reduced basal friction. Seasonal variability in the hydrologic dynamics of these features has not been 10 quantified. In this work, we characterize the spatial and temporal variability in the hydrological state (filled or drained) of these water-filled crevasse systems. A fusion of multi-sensor optical satellite imagery was used to examine hydrologic states from 2000 to 2015. The monthly distribution of crevasse systems observed as water filled is unimodal with peak number of filled days during the month of July at 329 days, while May has the least at 15. Over the study period the occurrence of drainage within a given season increases. Inter-seasonal drain frequencies over these systems ranged from 0 to 5. The 15 frequency of multi-drainage events are correlated with warmer seasons and large strain rates. Over the study period, summer temperatures averaged from -1 and 2 ⁰C and tensile strain rates have increased to as high as ~ 1.2 s -1 . Intermittent melt water input during hydrofracture drainage responsible for transporting surface water to the bed is largely facilitated by high local tensile stresses. Drainage due to fracture propagation may be increasingly modulated by ocean-induced calving dynamics for the lower elevation ponds. Water-filled crevasses could expand in extent and volume as temperatures increase resulting in 20 regional amplification of ice mass flux into the ice stream system. Motivation and Prior WorkThe Greenland Ice Sheet (GrIS) has experienced considerable mass loss over the last few decades (Krabill et al., 2004;Joughin et al., 2004;Alley et al., 2005aAlley et al., , 2005bLuthcke et al., 2006;Hanna et al., 2008) resulting in negative mass balance and substantive contribution to sea level rise (Rignot et al., 2008;van den Broeke et al., 2009; Shepherd et al., 2012). 25Commensurate with these changes has been the documented impact of surface meltwater on ice sheet velocity during the summer within the ablation zone (Zwally et al., 2002;Joughin et al., 2008; van de Wal et al., 2008;Shepherd et al., 2009;Bartholomew et al., 2010;Sundal et al., 2011;Palmer et al., 2011;Hoffman et al., 2011), via supraglacial lakes, channels, The Cryosphere Discuss., doi:10.5194/tc-2017Discuss., doi:10.5194/tc- -86, 2017 Manuscript under review for journal The Cryosphere Discussion started: 24 May 2017 c Author(s) 2017. CC-BY 3.0 License. 2and moulins largely beyond regions of fast flow (Echelmeyer et. al., 1991;Box and Ski, 2007; McMillan et al., 2007;Sneed and Hamilton, 2007;Das et al., 2008, Sundal et al, 2009Lampkin, 2011;Selmes et al., 2011; Tedesco and Steiner, 2011;Howat et al., 2013; Koenig et al., 2015). The presence of ponded water within regions of fast flow has received little attention. Lampkin et al. (2013) evaluated the spatial and tempora...

The Greenland Ice Sheet has experienced accelerated mass loss over the last couple decades, in part due to destabilization of marine-terminating outlet glaciers. Retreat and acceleration of outlet glaciers coincides with atmospheric and oceanic warming resulting in a significant contribution to sea-level rise. The relative role of surface meltwater production, runoff and infiltration on the dynamics of these systems is not well-understood. To assess how surface meltwater impacts shear margin dynamics and regional ice flow of outlet glaciers, we investigate the impact of basal lubrication of Jakobshavn Isbrae shear margins due to drainage from water-filled crevasses. We map the areal extent of inundated crevasses during summer (May-August) from 2000 to 2012 using satellite imagery and determined an increasing trend in the total areal extent over this time interval. We use a numerical ice flow model to quantify the potential impact of weakened shear margins due to surface melt derived basal lubrication on regional flow velocities. Ice flow velocities 10 km from the lateral margins of Jakobshavn were amplified by as much as 20%, resulting in an increase of ∼0.6 Gt yr −1 in ice-mass discharge through the shear margins into the ice stream. Under future warming scenarios with increased surface melt ponding, simulations indicate up to a 30% increase in extra-marginal ice flow. We conclude that surface meltwater will likely play an important role in the evolving dynamics of glacier shear margins and the future mass flux through Greenland's major marine-terminating outlet glaciers.