Meltwater Drainage in Troughs Controls end-of-summer Regional Ice Flow Deceleration in Western Greenland

The subglacial drainage system of the Greenland Ice Sheet plays a central role in modulating basal sliding and ice flow dynamics. While the seasonal evolution of this system has been widely studied during the melt season onset, its late and post-melt season behavior remains poorly constrained due to limited spatial and temporal observational coverage. Here, we use Sentinel-1 double-difference interferometric SAR data to document spatially continuous patterns of vertical and horizontal ice sheet motion across the late melt season in a predominantly land-terminating region of western Greenland. Our observations reveal recurrent, localized vertical subsidence features, generally coinciding with subglacial troughs, that persist several weeks into the post-melt season (late August to October) after surface melt inputs have largely ceased. The localized subsidence signals are accompanied by ice flow deceleration at a regional scale, extending more than 100 km inland, to ice thicknesses above 1300 m. We interpret this pattern as a dynamic response to the gradual drainage of water stored in weakly-connected cavities or, alternatively, englacial or sedimentary components, which drives a large-scale water pressure decrease and hence widespread flow deceleration. Although prior studies have suggested that weakly-connected cavities are drained following de-pressurization of efficient channels, the multi-week time scale and far-inland extent of the observed dynamic response suggest that channels may not be the sole driving mechanism. The magnitude of both subsidence and slow-down scales with melt season intensity, suggesting a mechanistic link between meltwater drainage efficiency and late-season ice dynamics, in line with previous observations of ice flow self-regulation. Our findings offer new insights into the seasonal evolution of Greenland's basal hydrology with continuous spatial coverage, highlighting how localized corridors of meltwater evacuation affect ice motion over much larger scales.