The Greenland Ice Sheet speeds up every summer as melt from the surface penetrates km-thick ice through moulins, but observations show that continued meltwater inputs lead to a decay in the speedup and late summer velocities that can be lower than the winter speed. While this decreasing ice dynamic response to meltwater forcing has often been attributed to channelization of the subglacial drainage system, recent observations of water pressure in moulins at one location indicate that the efficiency of the moulin-channel system does not increase during the second half of summer, failing to explain the concurrent drop in ice velocity. Simultaneous observations of lowering water pressure in boreholes sampling poorly-connected regions of the bed suggest that changes in isolated regions of the bed over summer modulate the overall basal traction of the ice sheet.

We explore the role of isolated drainage using models of subglacial hydrology and ice dynamics. The subglacial hydrology model includes components for distributed linked-cavity drainage, channelized drainage, and a new component for weakly-connected isolated drainage. Passive cavity opening in the isolated drainage region is capable of reproducing observed diurnal variations in borehole water pressure. Within the model, gradual drainage of water from the isolated cavities leads to a seasonal reduction in water pressure there and an associated decrease in ice speed, similar to the observations.