Tidewater-glacier response to supraglacial lake drainage

The flow speed of the Greenland Ice Sheet changes dramatically in inland regions when surface meltwater drains to the bed. But ice-sheet discharge to the ocean is dominated by fast-flowing outlet glaciers, where the effect of increasing surface melt on annual discharge is unknown. Observations of a supraglacial lake drainage at Helheim Glacier, and a consequent velocity pulse propagating down-glacier, provide a natural experiment for assessing the impact of changes in injected meltwater, and allow us to interrogate the subglacial hydrological system. We find a highly efficient subglacial drainage system, such that summertime lake drainage has little net effect on ice advection as observed by a network of Global Positioning System (GPS) receivers. To test our interpretation that the velocity pulse results from a pressure pulse in the subglacial drainage system, and that a well-developed drainage system is necessary for transporting this pulse down-glacier at the speeds observed in the GPS data, we simulate supraglacial lake drainage in a numerical model of subglacial hydrology at Helheim Glacier. Our GPS observations and model results suggest that, in regions of emerging surface melt, tidewater glacier flow response to individual melt events will decrease as more melt occurs and accesses the glacier bed. Moreover, our observations suggest that near-terminus seasonal velocity patterns at glaciers of this type may not be well correlated with drainage system efficiency, or the corresponding capacity to accommodate meltwater input with minimal impact on glacier discharge. At fast-flowing tidewater glaciers, where a marine margin and high surface melt rates keep subglacial water pressure high, observations of seasonal velocity patterns may be misinterpreted to indicate inefficient drainage, where, in fact, an efficient system exists and is capable of exporting large volumes of meltwater rapidly. The widely used binary framework for interpreting subglacial drainage system efficiency from seasonal velocity patterns, though observationally justified for inland ice-sheet regions, does not explain the full range of behaviors observed at lightly grounded, fast-flowing tidewater glaciers with high meltwater throughput.