Ice velocity of Jakobshavn Isbræ, Petermann Glacier, Nioghalvfjerdsfjorden and Zachariæ Isstrøm, 2015–2017, from Sentinel 1-a/b SAR imagery

Abstract

Systematically monitoring Greenland's outlet glaciers is central to understanding the timescales over which their flow and sea level 10 contributions evolve. In this study we use data from the new Sentinel-1a/b satellite constellation to generate 187 velocity maps, covering 4 key outlet glaciers in Greenland; Jakobshavn Isbrae, Petermann Glacier, Nioghalvfjerdsfjorden and Zachariae Isstrøm.These data provide a new high temporal resolution record of each glacier's evolution since 2014, and resolve recent seasonal and inter-annual changes in Greenland outlet glacier speed with an estimated certainty of 10 %. We find that since 2012, Jakobshavn Isbrae has been decelerating, and now flows approximately 1250 m yr -1 (10 %) slower than 5 years previously, thus reversing an 15 increasing trend in ice velocity that has persisted during the last decade. Despite this, we show that seasonal variability in ice velocity remains significant; up to 750 m yr -1 (14 %) at a distance of 12 km inland of the terminus. We also use our new dataset to demonstrate a strong relationship between ice front position and ice flow at Jakobshavn Isbrae, with increases in speed of ~1800 m yr -1 in response to 1 km of retreat. Elsewhere, we record significant seasonal changes in flow of up to 25 % and 18 % at Petermann Glacier and Zachariae Isstrøm, respectively. This study provides a first demonstration of the capacity of a new era of operational radar s atellites 20 to provide frequent, and timely, monitoring of ice sheet flow, and to better resolve t he timescales over which glacier dynamics evolve. Introduction 25Between 1992 and 2011, the Greenland Ice Sheet lost mass at an average rate of 142±49 Gt yr -1 [Shepherd et al., 2012], increasing to 269±51 Gt yr -1 between 2011 [McMillan et al., 2016. Ice sheet mass balance is determined from the ratio of surface mass input and ice discharge exported from the ice sheet [van den Broeke et al., 2009]. In 2005, dynamic imbalance was responsible for roughly two-thirds of Greenland's total mass balance, ma king an important contribution to freshwater input into the ocean and 0.34 mm yr -1 to the global sea level rise at that time [Rignot and Kanagaratnam, 2006]. Despite the anomalous atmospheric warmin g 30 events, especially during 2012 [Tedesco et al., 2013], and therefore more spatially extensive and longer lasting surface melt during this period, marine-terminating outlet glaciers in Greenland still contributes roughly 30 % (2000-2012) of total mass loss [Enderlin et al., 2014]. The observed acceleration of many marine-based glaciers in the western and northern regions of Greenland over the last years may have been driven by rises in air and adjacent ocean temperatures, which enhanced the surface melting and erosion of their termini [Holland et al., 2008; Moon et al, 2014; Moon et al., 2015]. The associated increases in basal sliding and calving of 35 their ice fronts in turn produce enhanced discharge, leading to dynamical imbalance and additional ice loss [Joughin et al., 2010; Jough...

Proper orthogonal decomposition of ice velocity identifies drivers of flow variability at Sermeq Kujalleq (Jakobshavn Isbræ)

“…2) and an analysis period of summer 2012 (1 June) to end of winter 2016 (1 March). The inter-annual velocity variability of SK-JI is strongly related to its termi- nus position (Lemos et al, 2018;Bondzio et al, 2017), and this period corresponds to a time of relatively stable frontal position with only a minor multi-year slowdown, contrasting to after 2016 when the rate of slowdown increases markedly (Lemos et al, 2018). Our study area contrasts to previous studies which have investigated the velocity of SK-JI with high temporal resolution at the near-terminus (Cassotto et al, 2015;Sundal et al, 2013).…”

“…Prising apart the influence of the physical processes responsible for ice dynamic patterns has been a key aim in glaciology for decades and remains challenging for researchers. Technological advances and a growing appreciation of the need to resolve a wider range of glacier behaviour has led to satellite-derived ice velocity images being acquired with increasing regularity (Joughin et al, 2020a;Vijay et al, 2019;Lemos et al, 2018;Fahnestock et al, 2016), which, in turn, has allowed for the development of new analytical techniques and treatments of datasets (Riel et al, 2021;Greene et al, 2020).…”

Increased variability in Greenland Ice Sheet runoff from satellite observations

“…These areas represent just 5 % (20,525 km 2 ) of the runoff zone (Fig. 1a ) and coincide with fast-flowing marine-terminating glaciers that are known to be in a state of imbalance 33 , 34 , 43 , 44 , 50 , 51 , including areas that exhibit seasonal changes (e.g., 44 , 52 , 53 ) as well as dynamic thinning of up to 2 m/yr. Across the remaining area, we adjust the altimeter summer height changes within each 5 × 5 km grid cell to remove the elevation signal associated with the steady-state divergence of ice and thereby isolate the contribution due to SMB anomalies alone (see Methods).…”

MELT: Monitoring Iceberg Calving using Synthetic-Aperture Radar

“…Petermann Glacier (PG) is one of the major outlet glaciers in northern Greenland and drains roughly 4% of the Greenland ice sheet [13]. It has the second largest floating ice tongue in Greenland and so loses most of its mass (80%) by bottom melting [14].…”