Modelling seasonal meltwater forcing of the velocity of the Greenland Ice Sheet

Abstract. Surface runoff at the margin of the Greenland Ice Sheet drains to the ice-sheet bed leading to enhanced summer ice flow. Ice velocities show a pattern of early summer acceleration followed by mid-summer deceleration, due to evolution of the subglacial hydrology system in response to meltwater forcing. Modelling the integrated hydrological – ice dynamics system to reproduce measured velocities at the ice margin remains a key challenge for validating the present understanding of the system, and constraining the impact of increasing surface runoff rates on dynamic ice mass loss from the GrIS. Here we show that a multi-component model incorporating supraglacial, subglacial, and ice dynamic components applied to a land-terminating catchment in western Greenland produces modeled velocities which are in good agreement with those observed in GPS records for three melt seasons of varying melt intensities. This provides support for the hypothesis that the subglacial system develops analogously to alpine glaciers, and supports recent model formulations capturing the transition between distributed and channelized states. The model shows development of efficient conduit drainage up-glacier from the ice sheet margin which develops more extensively, and further inland, as melt intensity increases. This suggests current trends of decadal timescale slow-down in the ablation zone will continue in the near future, although the strong summer velocity scaling in our results could begin to offset potential future fall and winter velocity decreases for very high melt rates which are predicted for the end of the 21st century.