The control of short-term ice melange weakening episodes oncalving activity at major Greenland outlet glaciers

The dense mixture of iceberg of various sizes and sea ice observed in many of Greenland's fjords, called ice melange (sikussak in Greenlandic), has been shown to have a significant impact on the dynamics of several Greenland tidewater glaciers, mainly through the seasonal support it provides to the glacier terminus in winter. However, a clear understanding of shorter-term ice melange dynamics is still lacking, mainly due to the high complexity and variability of the processes at play at the ice-ocean boundary. In this study, we use a combination of Sentinel-1 radar and Sentinel-2 optical satellite imagery to investigate in detail intra-seasonal ice melange dynamics and its link to calving activity at three major outlet glaciers: Kangerdlugssuaq Glacier, Helheim Glacier and Sermeq Kujalleq in Kangia (Jakobshavn Isbr & UAELIG;). In those fjords, we identified recurrent ice melange weakening (IMW) episodes consisting of the up-fjord propagation of a discontinuity between jam-packed and weaker ice melange towards the glacier terminus. At a late stage, i.e., when the IMW front approaches the glacier terminus, these episodes were often correlated with the occurrence of large-scale calving events. The IMW process is particularly visible at the front of Kangerdlugssuaq Glacier and presents a cyclic behavior, such that we further analyzed IMW dynamics during the June-November period from 2018 to 2021 at this location. Throughout this period, we detected 30 IMW episodes with a recurrence time of 24 d, propagating over a median distance of 5.9 km and for 17 d, resulting in a median propagation speed of 400 m d(-1). We found that 87 % of the IMW episodes occurred prior to a calving event visible in spaceborne observations and that similar to 75 % of all detected calving events were preceded by an IMW episode. These results therefore present the IMW process as a clear control on the calving activity of Kangerdlugssuaq Glacier. Finally, using a simple numerical model for ice melange motion, we showed that a slightly biased random motion of ice floes without fluctuating external forcing can reproduce IMW events and their cyclic influence and explain observed propagation speeds. These results further support our observations in characterizing the IMW process as self-sustained through the existence of an IMW-calving feedback. This study therefore highlights the importance of short-term ice melange dynamics in the longer-term evolution of Greenland outlet glaciers.

Automated summary

Ice melange, a dense mixture of icebergs and sea ice in Greenland's fjords, has been shown to significantly affect the dynamics of Greenland tidewater glaciers. However, the dynamics of ice melange in the shorter term are still not well understood due to its complexity and variability at the ice-ocean boundary.