Seasonal Evolution of the Subglacial Hydrologic System Modified by Supraglacial Lake Drainage in Western Greenland

The impact of summer surface melt on Greenland Ice Sheet dynamics 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 inputs. However, the behavior of the subglacial hydrologic system is complex and some characteristics are still poorly constrained. Here we investigate the coevolution of subglacial hydrology and ice motion in the Pakitsoq region of western Greenland during the 2011 melt season. We analyze measurements from 11 Global Positioning System stations, from which we derive ice velocity, longitudinal strain rates, and basal uplift, alongside observations of surface ablation and supraglacial lake drainages. We observe ice acceleration after the onset of local surface melting, followed by gradual ice deceleration, consistent with increasing subglacial efficiency. In the study area, supraglacial lake drainages cooccur with a change in regional strain rate patterns and ice deceleration, suggesting that lake drainages contribute to rapid subglacial reorganization. At lower ice surface elevations (below 900m above sea level), ice motion is correlated with both total basal uplift and its rate of change, while at higher elevations (900-1,100m above sea level), ice motion correlated only with the basal uplift rate. This pattern suggests that continued cavity growth or subglacial sediment dynamics may be important in the apparent increase in subglacial drainage efficiency at higher elevations in the ablation zone. Our results further suggest that transient subglacial behavior is important in the seasonal evolution of ice motion. Plain Language Summary Each summer, the margins of the Greenland Ice Sheet experience intense surface melting. This meltwater is routed over the surface in supraglacial streams and stored in supraglacial lakes, but eventually reaches the bed of the ice sheet via crevasses and moulins. The interaction between this meltwater and the overlying ice causes changes to the subglacial hydrologic system, which subsequently causes changes in ice motion. Here we use measurements from 11 Global Positioning System stations, alongside observations of surface melt rates and supraglacial lake drainages, to improve our understanding of the subglacial hydrologic system. In our study area, supraglacial lake drainages tend to cooccur with slowdowns in ice motion, suggesting that the rapid drainage of these large volumes of water can alter the subglacial hydrologic system, allowing it to more readily transmit meltwater. Our observations also indicate that at high elevations, the seasonal pattern of ice motion is controlled by small changes over large regions, either in sediments or in pockets of water on the downstream side of bedrock bumps, not necessarily by the formation of large subglacial channels. These findings suggest that current models of the subglacial system need modifications to include the physics associated with supraglacial lake drainages and small-scale processes.