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

The evolution and connectivity of subglacial drainage systems controls basal sliding and therefore modulates ice flow, 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 hours) of subglacial hydraulic connectivity between a large, primary subglacial drainage axis located in a deep trough, and subsidiary secondary subglacial channels. When surface melt inputs are high, the secondary channels fed by the instrumented moulins couple to the efficient primary subglacial drainage axis, exhibiting smoothed diurnal variability and synchronization with regional ice motion and seismic tremor. When melt inputs decrease, the secondary drainage system becomes isolated and responds sensitively to variations in local melt inputs, causing a reduction in hydraulic connectivity and an increase in local meltwater residence time. Regional ice motion appears completely controlled by the characteristics of the primary subglacial drainage axis and is insensitive to local inputs to secondary channels.