Tidally influenced, short term grounding line variations measured with Sentinel-1, RADARSAT-2, and Cosmo Skymed

The grounding line of a glacier or ice stream is the boundary between ice in contact with the bed and floating ice, either an ice shelf or a floating ice tongue. It represents the zone where ice flow shifts from basal drag controlled to drag-free, side shear controlled and also represents the ideal location to estimate ice flux into the ocean. The location of the grounding line is not static and, in combination with the slope of the bed underneath the ice, represents crucial information in determining the vulnerability of a glacier. While inherently difficult to measure in-situ, several methods using satellite data have been developed. Here, we use the current state of the art method, differential SAR interferometry, to delineate grounding lines for the entire Antarctic ice sheet. Thanks to a vast increase in C-band spaceborne SAR observations in recent years for coastal Antarctica (Sentinel-1) and to some degree in relevant regions Central Antarctica (RADARSAT-2) that are suitable for grounding line measurements, we moved from a handful of measurements per decade to a measurement per year based on a best-of data approach. More recently we started to utilize all available measurements to generate multiple grounding lines per year. We observe short term grounding line migration in the km range for a number of glaciers and analyze the tidal impact on this short-term grounding line migration, which exceeds predictions from interaction of ice with an undeformable, hard bed. Instead, what we find suggests complex interactions with a deformable bed, which implies that the GZ (where GL migrates during a tidal cycle) is nearly one order of magnitude larger than predicted by the hard bed model. This has important consequences for glacier modeling at the grounding line and for understanding ice-ocean interaction in that critical part of the glacier. We use the available C-band SAR data to identify areas around Antarctica that are vulnerable to this phenomenon, evaluate the suitability of available data for the task and identify critical areas where additional data acquisitions with missions best suitable for the task (Cosmo Skymed, 1 day repeat data) are needed for a comprehensive evaluation of short and long term grounding line migration in Antarctica.

This work is funded through a NASA MEaSUREs project