Grounding line migration through the calving season at Jakobshavn Isbræ, Greenland, observed with terrestrial radar interferometry

Abstract

Abstract. Ice velocity variations near the terminus of Jakobshavn Isbræ, Greenland, were observed with a terrestrial radar interferometer (TRI) during three summer campaigns in 2012, 2015, and 2016. We estimate a ∼ 1 km wide floating zone near the calving front in early summer of 2015 and 2016, where ice moves in phase with ocean tides. Digital elevation models (DEMs) generated by the TRI show that the glacier front here was much thinner (within 1 km of the glacier front, average ice surface is ∼ 100 and ∼ 110 m above local sea level in 2015 and 2016, respectively) than ice upstream (average ice surface is > 150 m above local sea level at 2–3 km to the glacier front in 2015 and 2016). However, in late summer 2012, there is no evidence of a floating ice tongue in the TRI observations. Average ice surface elevation near the glacier front was also higher, ∼ 125 m above local sea level within 1 km of the glacier front. We hypothesize that during Jakobshavn Isbræ's recent calving seasons the ice front advances ∼ 3 km from winter to spring, forming a > 1 km long floating ice tongue. During the subsequent calving season in mid- and late summer, the glacier retreats by losing its floating portion through a sequence of calving events. By late summer, the entire glacier is likely grounded. In addition to ice velocity variation driven by tides, we also observed a velocity variation in the mélange and floating ice front that is non-parallel to long-term ice flow motion. This cross-flow-line signal is in phase with the first time derivative of tidal height and is likely associated with tidal currents or bed topography.

Constraints on Terminus Dynamics at Greenland Glaciers From Small Glacial Earthquakes

“…Blue dashed lines show 3.6 km/s velocity moveout. (b) Image from Xie et al () showing calving front on 2 August 2012. Solid red line indicates terminus position at 23:00, before calving activity began.…”

“…Columns give date and time of small event; azimuth of centroid‐single‐force (CSF) vector in degrees east of north; plunge of CSF vector in degrees below horizontal; CSF amplitude M CSF , in units of kilogram meter, to be scaled by 10 13 . Observing periods of Xie et al () were 31 July to 12 August, 2012; 6 June to 10 June, 2015; and 7 June to 20 June, 2016. Type‐B small events are listed in bold; all other small events listed are type A.…”

“…A small seismic event on 2 August 2012 at 23:04 (Figure ) preceded a glacial earthquake at Jakobshavn Isbræ by 6 min. The events were captured by time‐lapse imagery during a field campaign that observed the terminus of the glacier using terrestrial radar interferometry (Xie et al, ). During the period of the observing campaign surrounding the glacial earthquake, images of the calving front were captured every 6 min.…”

“…The first additional time period we consider is defined by available imagery. Using the same template waveform for Jakobshavn Isbræ as previously, we perform cross‐correlation analysis on vertical‐component seismic data from station ILULI for the full time period during which Xie et al () collected radar‐interferometry observations. Their observations were made at the terminus of Jakobshavn Isbræ for a total of 32 days over the summers of 2012, 2015, and 2016.…”

Quasi‐Static Granular Flow of Ice Mélange

“…Published observations of ice mélange motion, which can be used to validate or negate potential constitutive relations, are limited to GPS surveys of individual icebergs (Amundson et al, ; Sutherland et al, ), short terrestrial radar surveys (Burton et al, ; Peters et al, ; Xie et al, ), and a handful of satellite‐derived velocity fields (Foga et al, ; Joughin et al, ). Here we apply particle image velocimetry (PIV) to Landsat 8 imagery to produce ice mélange velocity fields at Greenland's three most productive glaciers (e.g., Rignot & Kanagaratnam, ): Jakobshavn Isbræ, Helheim Glacier, and Kangerdlugssuaq Glacier.…”