Spatial patterns in seasonal coupling between ice sheet hydrology and motion in west Greenland

Ice motion within the land-terminating ablation zone of west Greenland typically follows a seasonal pattern with fastest flow in the spring as surface meltwater first accesses the hydraulically inefficient subglacial drainage system, followed by a gradual reduction in ice motion over the summer as efficient subglacial channels evacuate water stored over wide swathes of the subglacial environment. Minimum speeds occur in autumn, when surface meltwater delivery to the bed ceases, then, over winter, ice flow gradually recovers before the cycle repeats once again. This understanding is principally derived from high temporal resolution field observations with limited spatial coverage. It is now, however, possible to measure these seasonal patterns in detail over large spatial scales, enabling basin-scale comparisons between meltwater supply, subglacial hydrology and ice motion.

We present near continuous time-series of ice motion for a ~60,000 km² portion of predominantly the land-terminating western margin of the Greenland Ice Sheet (with coverage extending up to 150 km from the margin in winter and 25 km in summer) between 2016 and 2022 at up to 12-day temporal resolution derived from Sentinel-1, Sentinel-2 and Landsat 8 imagery. We compare ice motion with surface melt magnitude and timing (derived from meteorological observations and a positive degree day model), theoretical subglacial water routing, and glaciological context (e.g. ice thickness).

Our results reveal clear spatial patterns in seasonal coupling between ice sheet hydrology and motion. Overall, the amplitude of the seasonal cycle of ice motion is generally greater closer to the ice margin as has been shown previously by field observations. There is, however, distinct variability between subglacial hydraulic catchments. Subglacial catchments whose principal drainage pathway has a high hydraulic potential gradient typically experience smaller peaks in summer ice motion, but more prominent autumn ice flow minima. Subglacial water flow in such catchments has more potential to create hydraulically efficient channels, which can both accommodate spikes in meltwater delivery with muted ice flow acceleration, and also reduce basal water pressure across a wider area of the ice bed as surface meltwater delivery declines.

As ice sheet surface mass balance becomes more negative and marginal ice thins fastest (where the ice is land-terminating), subglacial hydraulic gradients and subglacial discharge will both increase. Based on our findings, these changes may lead to smaller summer ice flow accelerations and bigger winter slow-downs, driving an overall decrease in mean annual ice motion. Our results also highlight the importance of careful selection of field sites for measuring ice motion and caution against scaling up from spatially limited observations.