The drainage and freezing behaviour of lakes from all parts of the Greenland Ice Sheet (Invited)

The rapid drainage of supraglacial lakes is capable of causing accelerations in ice-sheet flow even when efficient subglacial drainage prevents slower meltwater inputs from affecting ice velocity. In addition, the fractures generated during drainage are a major source of connections between the supraglacial and subglacial hydrological systems. However, a previous study has shown that only 13 % of lakes drain in this manner. It has been suggested that lakes can have one of three fates: sudden drainage to the ice bed, slower supraglacial drainage through incision of a drainage channel, or freezing at the end of the melt season. These processes have never been quantified at the ice-sheet scale, and little is known about their distribution. We took a combination of MODIS visible imagery, MODIS land surface temperature data, and a pre-existing database of lake-area time-series, and used these data to classify 2600 lakes from all parts of the ice sheet over the period 2005-2009 into the three proposed lake-types: fast-draining, slow-draining, and freezing. Of the lakes studied, 34 % slow-drained, presumably supraglacially, and 47 % froze over at the end of the melt season. Previous studies have found a lack of moulins and reduced melt/velocity feedback in the higher part of the lake-forming zone. We find that lakes in this area are very unlikely to drain to the bed of the ice sheet, and instead lakes are far more likely to freeze, despite apparently having sufficient size and lifespan to drain. We suggest that this is evidence that lake drainage may be inhibited at higher elevations. At lower elevations fast and slow-draining lakes dominate, and frequently switch drainage-types interannually. Fast-draining, slow-draining, and freezing lakes are easily confused if infrequent observations are used, a fact worth considering for future remote-sensing studies to avoid overestimating the amount and rate of meltwater delivery to the ice-bed.