Cascading supraglacial drainage observed to cause large-scale acceleration and uplift during winter in Greenland

On land-terminating regions of ice sheets, large and transient changes in surface motion are not expected outside of summer due to the lack of concurrent melt forcing.  Here, we document the dynamic response to a cascading lake drainage that occurred during winter in Greenland using a high-resolution DInSAR timeseries (6-day) acquired from Sentinel-1 and optical imagery from Landsat-8 and Sentinel-2. A total of fifteen supraglacial lakes and several smaller supraglacial water features were identified to have drained during the event resulting in a velocity wave that propagates from the inland regions to the margin. Along the wave path, speeds increase up to three times pre-drainage velocities as the wave passes. Bifurcation of the velocity wave during the event implies at least two distinct subglacial flood pathways develop which drain from the margin over 100 km apart. By tracking the wavefront, we estimate the wave velocity through the event which we infer to be similar to the drainage velocity. Wave speeds of between 0.04 and 0.28 m s^-1 suggests the subglacial flood propagates mainly through an inefficient drainage system. Using temporally overlapping portions of two DInSAR velocity maps which have an accuracy of greater than 0.1 m yr^-1, we decompose the signal and demonstrate some of the motion is a result of surface uplift which constrains locations of likely flow pathways. Overall, our results demonstrate a sustained dynamic response to melt forcing can occur in the absence of surface melt. This indicates melt-induced ice motion changes are not limited to summer and transient winter dynamics might commonly occur as stored meltwater is released.