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 were slightly faster than the prior two summers, leaving it unclear whether the slowdown is ending.
Highlights
Except for a few brief intervals, Jakobshavn Isbræ has been the fastest and largest glacier in Greenland over the last several decades (Mouginot et al, 2019)
The increased extension during the summer speed-up produced near-terminus dynamic thinning of ∼ 30 m yr−1, which was partially offset by ∼ 15 m yr−1 of thickening as the glacier slowed over the winter (Joughin et al, 2012b)
As described throughout this section, we have assembled a comprehensive set of terminus position, velocity, and elevation data that provide a detailed view of Jakobshavn Isbræ’s behaviour over the last decade
Summary
Except for a few brief intervals, Jakobshavn Isbræ (see location in Fig. 1) has been the fastest and largest (by discharge volume) glacier in Greenland over the last several decades (Mouginot et al, 2019). In the mid-1990s, it thickened slightly after slowing down relative to the 1980s (Joughin et al, 2004; Thomas et al, 2003). The increased extension during the summer speed-up produced near-terminus dynamic thinning of ∼ 30 m yr−1, which was partially offset by ∼ 15 m yr−1 of thickening as the glacier slowed over the winter (Joughin et al, 2012b). Over the winter of 2016– 2017, the terminus began to slow (Lemos et al, 2018), leading to a peak speed during the summer of 2017 that was comparable to the minimum speeds for several of the previous winters (Joughin et al, 2018)
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