|Paper title||Considerations on short- and long-term grounding line migration assessment in Antarctica|
|Form of presentation||Poster|
The grounding line location (GLL) is a geophysical product of the Antarctic Ice Sheet Climate Change Initiative (AIS_cci) project and has been derived over major ice streams and glaciers around the continent through the InSAR technique. Currently the AIS_cci GLLs stretch over the period 1994 – 2020 from ERS-1/2 era to Sentinel-1 A/B.
The position of the grounding line is not constant in time. There are different processes in the grounding zone causing shifts:
• at short time scale GLL moves back and forth with the vertical movement of the floating ice induced by ocean tides. The tide amplitude depends on location and atmospheric conditions
• at long term scale GLL migration in one direction can occur. Usually a GLL retreat is expected due to ice thinning. This phenomenon is a climate change indicator.
The multitemporal AIS_cci GLLs from the ERS tandem and Sentinel-1 epochs show the short as well as the long-term migration of the grounding line. These two effects must be separated before the interpretation of grounding line retreat observed over long time periods.
The regularly Sentinel-1 acquisitions over Antarctica’s margins allow quantification of the short term GLL migration at locations with preserved coherence. Time series of individual GLLs from Sentinel-1 SAR triplets acquired at various dates within the ocean tide cycle can be processed. The associated tide levels are given by models (e.g. CATS2008) at points on the ice shelf. The short-term displacements of the grounding line need a reference to which they are calculated. We built a concave hull around the GLLs and with the support of a medial axis and lines normal to it we defined the position of the points belonging to the averaged GLL. The displacement of the individual GLLs from the average is quantified by a polygon comparison procedure. We are using a buffer around the reference GLL increased until the individual GLL is completely contained within the buffer. The overlap and histogram statistics give the final distance.
The resulting short-term horizontal displacements give interesting insights on the possible range of the tidal-induced grounding line migration and the site-specific factors influencing its magnitude over one tidal cycle. Short time series computed over entire year could reveal seasonal GLL variations due to the influx of ocean water under the ice shelf.
The averaging of the GLLs over one short period can be further used to investigate the long-term changes of the grounding line. The averaging is mainly feasible in the Sentinel-1 epoch because dense GLL time series can be derived and is less appropriate in earlier times when only single GLLs could be derived during a period short enough to exclude additional effects of ice thinning or acceleration. From ERS single and Sentinel-1 averaged GLLs we want to investigate possible grounding line retreats in the last 2.5 decades at key areas around Antarctica as signs of ice shelf instability. The surface slope, subglacial topography, ice velocity and thickness are additional parameters considered to explain why large migration occurs. The average over a short period and long-term grounding line retreat are valuable measurements contributing in the estimation of ice shelf area and area change parameters within ESA’s Polar+ Ice Shelf project.