Search

Surface‐derived meltwater can access the bed of the Greenland ice sheet, causing seasonal velocity variations. The magnitude, timing, and net impact on annual average ice flow of these seasonal perturbations depend on the hydraulic efficiency of the subglacial drainage system. We examine the relationships between drainage system efficiency and ice velocity, at three contrasting tidewater glaciers in southwest Greenland during 2014–2019, using high‐resolution remotely sensed ice velocities, modeled surface melting, subglacial discharge at the terminus, and results from buoyant plume modeling. All glaciers underwent a seasonal speed‐up, which usually coincided with surface melt onset, and subsequent slow‐down, which usually followed inferred subglacial channelization. The amplitude and timing of these speed variations differed between glaciers, with the speed‐up being larger and more prolonged at our fastest study glacier. At all glaciers, however, the seasonal variations in ice flow are consistent with inferred changes in hydraulic efficiency of the subglacial drainage system and qualitatively indicative of a flow regime in which annually averaged ice velocity is relatively insensitive to interannual variations in meltwater supply—so‐called "ice flow self‐regulation." These findings suggest that subglacial channel formation may exert a strong control on seasonal ice flow variations, even at fast‐flowing tidewater glaciers. Plain Language Summary: Each summer, meltwater produced at the surface of the Greenland ice sheet reaches and lubricates its base, causing the overlying ice to accelerate. Continual water flow during the summer months melts hydraulically efficient drainage pathways (conduits) into the basal ice, enabling rapid evacuation of water and causing the overlying ice to decelerate. At fast‐flowing glaciers, like those studied here, basal conduit formation is thought to be disrupted, thereby negating its braking effect. We test this idea by examining ice flow and meltwater discharge at three ocean‐terminating glaciers, of varying velocities, over 5 years. Every year, each glacier initially accelerated in response to surface melting, then decelerated following inferred basal conduit formation. The acceleration was greater, and deceleration smaller, at glaciers that were faster flowing on average. At all our studied glaciers, however, we found that the formation of basal conduits caused ice flow deceleration. This suggests that, even at very fast glaciers, basal conduits can form and exert a strong control on glacier velocity at seasonal timescales. Key Points: We use high‐resolution ice velocity estimates and plume modeling and observations to investigate drivers of seasonal ice flow variabilityWe observe large seasonal ice flow variations; the amplitude, pattern, and longevity of which varied between glaciersSeasonal subglacial channel evolution can explain these flow variations, which result in minimal interannual differences in ice flow [ABSTRACT FROM AUTHOR]