| Summary: | The impact of summer surface melt on the dynamics of the Greenland Ice Sheet is modulated by the state of the subglacial hydrologic system. Studies of ice motion indicate that efficiency of the subglacial system increases over the melt season, decreasing the sensitivity of ice motion to surface melt. However, these inferences are based on limited indirect observations of the subglacial hydrologic system that leave many factors poorly constrained, particularly the presence and stability of subglacial channels. Here we use observations from 11 GPS stations, from which we derive ice velocity, longitudinal strain rates, and basal uplift, alongside observations of surface ablation and supraglacial lake drainage events, to explore the coevolution of ice motion and the subglacial hydrologic system in the Pakitsoq region of western Greenland during the 2011 melt season. We observe ice acceleration after the onset of local surface melting, followed by gradual ice deceleration, consistent with the pattern expected from increased subglacial drainage efficiency. Supraglacial lake drainages appear to precipitate ice deceleration and increased basal traction, suggesting that lake drainages effectively reorganize the local subglacial hydrologic system into a more efficient state that persists through the remainder of the melt season. At high elevations, ice velocity and inferred basal uplift suggest that continued cavity growth or sediment behavior, not subglacial channelization, drive the apparent increase in subglacial efficiency. Our results further indicate that these transient perturbations are critical in the seasonal evolution of ice motion.
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