A decade of variability on Jakobshavn Isbræ: ocean temperatures pace speed through influence on mélange rigidity

Abstract

Abstract. The speed of Greenland's fastest glacier, Jakobshavn Isbræ, has varied substantially since its speed-up in the late 1990s. Here we present observations of surface velocity, mélange rigidity, and surface elevation to examine its behaviour over the last decade. Consistent with earlier results, we find a pronounced cycle of summer speed-up and thinning followed by winter slowdown and thickening. There were extended periods of rigid mélange in the winters of 2016–2017 and 2017–2018, concurrent with terminus advances ∼6 km farther than in the several winters prior. These terminus advances to shallower depths caused slowdowns, leading to substantial thickening, as has been noted elsewhere. The extended periods of rigid mélange coincide well with a period of cooler waters in Disko Bay. Thus, along with the relative timing of the seasonal slowdown, our results suggest that the ocean's dominant influence on Jakobshavn Isbræ is through its effect on winter mélange rigidity, rather than summer submarine melting. The elevation time series also reveals that in summers when the area upstream of the terminus approaches flotation, large surface depressions can form, which eventually become the detachment points for major calving events. It appears that as elevations approach flotation, basal crevasses can form, which initiates a necking process that forms the depressions. The elevation data also show that steep cliffs often evolve into short floating extensions, rather than collapsing catastrophically due to brittle failure. Finally, summer 2019 speeds were slightly faster than the prior two summers, leaving it unclear whether the slowdown is ending.

Glacier Forecast: Jakobshavn Isbrae Primed for Thinning and Acceleration

“…After the arrival of cold water in 2016, Jakobshavn Isbrae slowed, advanced and thickened (Khazendar et al, 2019;Joughin et al, 2020). During the 2019 summer melt season, however, Jakobshavn accelerated and thinned enough that yearly elevation change became negative for the rst time in 4 years.…”

“…These melting rates depend on both fjord temperature and the volume of meltwater discharge that ows along the glacier's bed during summer (Jenkins, 2011). In contrast, Joughin et al (2020) suggested that fjord water temperatures affected the glacier by controlling the stiffness of the ice mélange that lls the fjord. This hypothesis suggests that during the winter, ice mélange stiffens and exerts increased back-stress against the glacier front (Amundson et al, 2010;Cassotto et al, 2015).…”

Formation, flow and break-up of ephemeral ice mélange at LeConte Glacier and Bay, Alaska

“…However, ice mélange can also exist without substantial sea ice coverage if iceberg productivity is sufficiently high, such as at Jakobshavn Isbræ, Helheim Glacier and Kangerdlugssuaq Glacier, Greenland. At these glaciers, ice mélange is held together by iceberg–iceberg and iceberg–bedrock contact forces (Joughin and others, 2008; Amundson and others, 2010; Seale and others, 2011; Jakobsson and others, 2012; Amundson and Burton, 2018), although sea ice growth in winter likely provides additional rigidity (Cassotto and others, 2015; Robel, 2017; Bevan and others, 2019; Joughin and others, 2020).…”

Observing traveling waves in glaciers with remote sensing: New flexible time series methods and application to Sermeq Kujalleq (Jakobshavn Isbræ), Greenland

“…PANGAEA.897066 (Zhang et al, 2019). The post-2017 calving fronts delineated from TerraSAR-X data are available at the University of Washington Research Works Archive at https://doi.org/10.6069/XQS7-CD47 (Joughin et al, 2020). Landsat-8 imagery are available from EarthExplorer (https://earthexplorer.usgs.gov, last access: June 2020).…”