Quantifying the influence of melt on velocity variations at a large Greenland outlet glacier

The flow speed of Greenland outlet glaciers is governed by many factors, some of which are poorly understood. One such factor is surface-generated melt water, which has been shown to have a significant effect by enhancing basal lubrication. Previously, we have demonstrated a correlation between variations in glacier flow speed and meltwater input at Helheim Glacier, East Greenland. Here, we analyze local and across-glacier melt water estimates along with daily GPS-derived mean surface velocities from the same area to investigate spatial and temporal variations in glacier response to melt-water input. We perform linear least-squares fits of velocity to melt and thereby invert for a sensitivity value with which to produce predicted velocity records. We also investigate the seasonal variability of melt influence in 2008. We track the residual misfits for a sliding time window using steps of one day while performing the inversion for each step. We detect decreasing values of sensitivity with distance along the flowline from the calving front. Moreover, we observe that misfits drop gradually as the season progresses, accompanied by a steep increase in modeled sensitivity. Toward the end of the season, a very strong correlation between melt and surface displacement occurs and is sustained for ~6 days. We interpret this change as reflecting a gradual change in subglacial hydraulic routing from a tunnel-dominated to a linked-cavity dominated system and back. Investigations of melt-water influence on fast outlet glacier flow may be key in understanding solid mass loss from the Greenland Ice Sheet, which has been shown to comprise half of the current negative mass budget.