Satellite mapping of the varying moulin distribution on the southwest Greenland ice sheet

Moulins are vertical conduits on the Greenland ice sheet (GrIS) formed by hydrofracturing of crevasses and supraglacial lakes. They determine where, when, and how much surface meltwater can be drained to ice sheet bed, thereby controlling subglacial water pressure, developing subglacial drainage system, and eventually affecting ice motion. Therefore, obtaining moulin distribution over extended time period is essential for understanding variability in surface-to-bed meltwater connections. Using 10 m Sentinel-2 satellite imagery, we map interannual moulin distributions on the southwest GrIS during late summer from 2016 to 2021, as well as the seasonal moulin evolution throughout the warm 2019 summer. We find that widespread moulins drain the majority of meltwater into the ice sheet during 2016-2021. They are even denser in warmer years with larger proportions of meltwater drainage. Notably, a considerable number of moulins recur in multiple years and act as stable meltwater connections. During the warm 2019 summer, moulins first increase substantially at low elevations in June, then expand to high elevations in July, and remain relatively stable in August. As a result, surface-to-bed meltwater connections are spatially localized early in the melt season and become more discrete as the melt season progresses.