Drivers of extreme 2012 melt and basin-scale runoff from three catchments along the West Greenland ice sheet

Greenland ice sheet surface melt was extraordinary in summer 2012 - affecting 98.6% of its area. This study provides insights into the intricacies of in situ river discharge and runoff response during the record-setting melt year at three basins distributed from north-to-south along the western margin of the ice sheet. We investigate the river discharge response of North River (Thule), Watson River (Kangerlussuaq), and Naujat Kuat River (Nuuk) during two distinct melt episodes: July 8-15 and July 27-August 6. Moreover, we identify potential mechanisms contributing to exceptional runoff using automatic weather station and reanalysis data. Our results demonstrate that river discharge response in summer 2012 was remarkable at all three basins with a considerable increase in total discharge volume relative to the second-highest melt year of available data. Peak discharge pulses corresponded to the two major melt episodes, with large intra-annual variability in daily discharge up to ± 2.4 standard deviations difference. High geopotential height anomalies at 500 hPa during summer 2012 were centered along and off the coast of West Greenland. The greatest covariability with weather station observations and daily discharge is linked to surface temperature. With continued atmospheric warming, high melt episodes are expected to become more frequent in the future, with runoff contributions dominating mass loss. In this context, characterizing current and future exceptional melt years is crucial for improving our understanding of Greenland's complex hydrologic system and its parameterization in regional/global climate models.