Radar Evidence for Widespread Englacial Over-Winter Water Storage in Greenland’s Ablation Zone

In the western Greenland ablation zone, most meltwater is thought to drain to the bed of the ice sheet through moulins or hydrofractures, leading to surface mass loss and seasonal ice velocity variations. However, there is a growing body of work on slow and partial depth hydrofracture, which could store meltwater englacially for longer periods of time. If widespread, this process would reduce total surface mass loss from the ablation zone, delay or reduce meltwater delivery to the subglacial system, and warm the ice through latent heat release, thus modulating all aspects of glacier mass balance.Here, we investigate a spatially extensive, non-conformal englacial volume scattering horizon observed in Operation IceBridge ice-penetrating radar data collected in the springs of 2011-2019 in the western Greenland ablation zone. The depth of this horizon coincides with thermal anomalies in borehole temperature profiles, suggesting that it may be evidence of englacial liquid water pockets. We test this hypothesis in the Sermeq Avannarleq catchment using a Mie scattering model and show that the radar reflectivity and attenuation of this horizon are most consistent with scattering from sparse, meter-scale water inclusions in a layer of macro-porous ice ~60-80 m thick. These inversion results suggest that around 0.8 m/m2 of liquid water are stored over winter in the bottoms of surface crevasses at this site. At this same site, we also show that interannual variability in the attenuation anomaly from the scattering horizon is highly correlated with the preceding summer’s melt volume, providing further evidence linking this structure to water storage. Finally, we map the extent of this scattering horizon across the western Greenland ablation zone and find extensive spatial coverage in almost every glacier catchment from 60°-77° N. Our results show that englacial water storage is likely ubiquitous in the western Greenland ablation zone and therefore may play a more important role in modulating englacial temperature, surface mass balance, and subglacial drainage than previously assumed.