Connectivity between primary and secondary subglacial drainage subsystems beneath a land‐terminating outlet glacier of the Greenland Ice Sheet

The evolution and connectivity of subglacial drainage systems control basal sliding and therefore modulate ice motion, yet direct observations of these systems remain limited. Here, we investigate hydraulic connectivity and its influence on ice motion at Isunnguata Sermia, a large land-terminating outlet glacier of the Greenland Ice Sheet. We use “Cryoegg” wireless sensors to obtain moulin water pressure and electrical conductivity, in conjunction with passive seismics to measure glacio-hydraulic tremor and GNSS-derived measurements of ice motion. We identify rapid switching (<24 hr) of subglacial hydraulic connectivity between distinct subsystems: a large, primary drainage axis located in a deep trough, and secondary subglacial channels. When surface melt inputs are high, the secondary subsystem fed by the instrumented moulins connects with the efficient primary drainage axis and exhibits smoothed diurnal variability and synchronization with regional ice motion and seismic tremor. When surface melt decreases, hydraulic connectivity is reduced, and the secondary subsystem becomes disconnected and responds sensitively to variations in local melt inputs, increasing local meltwater residence time. Regional ice motion is controlled by the characteristics of the primary drainage axis and is insensitive to local inputs into the secondary subsystem.