Overview: For this study, we used a pair of Gamma Remote Sensing Terrestrial Radar Interferometers (TRIs) to measure the 2D flow of icebergs within the proglacial ice mélange at Jakobshavn Isbræ - Greenland’s most dynamic tidewater glacier. The Gamma TRIs are capable of measuring millimeter scale surface deformation at distances up to 16 km from the instrument. Our results show that between calving events, individual, “granular” icebergs within the ice mélange flow in unison and behave as a cohesive unit, which inhibits iceberg calving. However, small downfjord expansions of the ice mélange, due likely to interactions between subglacial meltwater discharge and ocean tidal fluctuations in the fjord, are sufficient to facilitate rotations of individual icebergs. This consequently increases stain rates in the mélange, which leads to granular decoherence and ultimately iceberg calving at the glacier terminus. This study provided the first evidence of summer, granular ice melange influence on the timing of tidewater glacier calving. Our results were published in the peer-reviewed journal Nature Geoscience.
https://doi.org/10.1038/s41561-021-00754-9
Ice Mélange Variability from Calving and Tides
Time-lapse of the TRI backscatter images - the magnitude of the radar’s reflected signal. Each image represents a radar scan acquired every 3 minutes over an ~2 week period; it has been sped up to emphasize the dynamic activity of the ice mélange.
An important feature of the video are surface waves that propagate downfjord after each calving event, evidence of the disruptive energy of iceberg calving on the proglacial fjord and ice mélange. A second important observation is the repeating downfjord motion of the ice mélange away from the calving front. This is most likely related to ocean tidal modulations, subglacial discharge of meltwater, or some complex interaction between these two forces.
Two-dimensional ice mélange velocities from TRI.
Granular Ice Mélange Impact on Glacier Calving.
The key finding of our study is that local strain rates (blue/green points in b-e) within the ice mélange increase ~1 hour before calving. This enhances the bulk strain (black points) within the mélange, which leads to calving events (vertical dashed lines set at 0 minutes from calving). The bulk strain remains elevated for another hour, while the mélange re-adjusts to the disturbance and returns to some background strain rate, This is true for most events unless the original calving event causes the entire mélange to reset by forcing it downfjord, such as the 8/2 or 8/5 events, then a second calving event occurs shortly thereafter (dashed vertical lines 1-2 hours after the initial calving event). We observed this phenomena nearly a dozen times over our two week observation period. This was the first conclusive evidence of ice mélange impacts on the timing of calving along a major tidewater glacier.