Intra-annual grounding line migration and retreat: insights from high-resolution satellite and in-situ observations over Milne Glacier in the Canadian High Arctic

Abstract. Changes at the boundary where marine terminating glaciers transition from grounded to floating, known as the grounding line, are critical for glacier stability and ice discharge prediction. We explore changes in the ice flexure zone (FZ), which spans from the inland limit of tidal flexure, known as the hinge line, across the actual grounding line, to where the ice tongue becomes freely floating. FZ movements on Milne Glacier, Nunavut, Canada from 2023 to 2024 were measured at a spatial resolution of 10 m using 4-day RADARSAT Constellation Mission (RCM) repeat acquisitions from four different orbits. The Double Difference Interferometric Synthetic Aperture Radar (DDInSAR) technique was employed to delineate 116 FZ positions, which were then examined with respect to tides, glacier velocity, surface and bed topography. Tides modulated by atmospheric pressure were the major driver of changes in the FZ with the correlation coefficient between DDInSAR and sea surface height displacements ranging from 0.81 to 0.94 for different RCM orbits. The lowest error (<0.1 cm in the SAR line-of-sight) occurred during the ‘stable regime’, when changes in glacier velocity were minimal, with ascending RCM tracks at incidence angles below 30°. During the ‘unstable regime’, the glacier velocity became more variable, and the error increased by up to 4 times. In summer, ice in the FZ flowed ~2.7-fold faster than the grounded section upglacier. We discovered a new ~2.4 km grounding line retreat associated with a pinning point at the eastern margin of the glacier. This rapid change is likely due to water intrusion and subglacial melt near the pinning point and shows how these bed features alter the grounding line short-term migration and its long-term retreat.