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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/tc-2019-197
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2019-197
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 04 Sep 2019

Submitted as: research article | 04 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal The Cryosphere (TC).

A Decade of Variability on Jakobshavn Isbrae: Ocean Temperatures Pace Speed Through Influence on Mélange Rigidity

Ian Joughin1, David E. Shean2, Benjamin E. Smith1, and Dana Floricioiu3 Ian Joughin et al.
  • 1Applied Physics Laboratory, University of Washington, Seattle, 98105, USA
  • 2Department of Civil and Environmental Engineering, University of Washington, Seattle, 98185, USA
  • 3German Aerospace Center (DLR), Remote Sensing Technology Institute, SAR Signal Processing, Muenchenerstr. 20, 82230 Wessling, Germany

Abstract. The speed of Greenland’s fastest glacier, Jakobshavn Isbrae, has varied substantially since its speedup 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 speedup and thinning followed by winter slowdown and thickening. There were extended periods of rigid mélange in the winters of 2016–17 and 2017–18, 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 Isbrae 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 near 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 are slightly faster than the prior two summers, leaving it unclear whether the slowdown is ending.

Ian Joughin et al.
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Short summary
Jakobshavn Isbrae, considered to be Greenland’s fastest glacier, has varied its speed and thinned dramatically since the 1990s. Here we examine the glacier’s behavior over the last decade to better understand this behavior. We find that when the floating ice (mélange) in front of the glacier freezes in place during the winter, it can control the glaciers speed and thinning rate. A recently colder ocean has strengthened this mélange, allowing the glacier to recoup some of its previous losses.
Jakobshavn Isbrae, considered to be Greenland’s fastest glacier, has varied its speed and...
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