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