Elmer/Ice News

The role of subtemperate slip in thermally driven ice stream margin migration

haseloff2018The amount of ice discharged by an ice stream depends on its width, and the widths of unconfined ice streams such as the Siple Coast ice streams in West Antarctica have been observed to evolve on decadal to centennial timescales. Thermally driven widening of ice streams provides a mechanism for this observed variability through melting of the frozen beds of adjacent ice ridges. This widening is driven by the heat dissipation in the ice stream margin, where strain rates are high, and at the bed of the ice ridge, where subtemperate sliding is possible. The inflow of cold ice from the neighboring ice ridges impedes ice stream widening. Determining the migration rate of the margin requires resolving conductive and advective heat transfer processes on very small scales in the ice stream margin, and these processes cannot be resolved by large-scale ice sheet models. Here, we exploit the thermal boundary layer structure in the ice stream margin to investigate how the migration rate depends on these different processes. We derive a parameterization of the migration rate in terms of parameters that can be estimated from observations or large-scale model outputs, including the lateral shear stress in the ice stream margin, the ice thickness of the stream, the influx of ice from the ridge, and the bed temperature of the ice ridge. This parameterization will allow the incorporation of ice stream margin migration into large-scale ice sheet models.

Stretching and melting at Totten ice shelf

The Totten Ice Shelf (IS) has a large drainage basin, much of which is grounded below sea level, with a large potential of future sea level rise contribution. Small retreats of the calving front from its current position are predicted to cause a change in flow at the grounding line, which makes understanding the processes behind calving on the Totten IS essential to predict its future dynamics. Using Helsinki Discrete Element Model (HiDEM) this article shows that beside local crevasse production (prominently at pinning-points), the advection of crevasses along the fast outlet is an essential feature of the observed fracture pattern. Cook TottenTo address this aspect, Elmer/Ice was used to better understand the evolution of advected crevasses, which, alongside with another model accounting for melt-induced change of crevasse shape, showed that strain as well as ocean melt have the potential to deform narrow fractures into the broad basal features observed near the calving front. This multiple factors influencing calving at Totten IS will be a challenge for predicting its future mass loss.

Coincidentally, this is the 100th peer reviewed publication with a contribution from Elmer/Ice.

Read more: Cook, S., J. Åström, T. Zwinger, B.K. Galton-Fenzi, J.S. Greenbaum, and R. Coleman, 2018. Modelled fracture and calving on the Totten Ice Shelf, The Cryosphere, 12, 2401-2411,doi: 10.5194/tc-12-2401-2018

Never underestimate smaller Greenland glaciers

The Greenland Ice Sheet is losing mass at an accelerating rate and consequently sea-level contributions have also risen. Hitherto, the main contributors to this increase were believed to be outlet glaciers in the west and southeast of Greenland. However, we show that significant changes are also occurring in at least one of the major outlet glaciers (79Deg glacier) in the northeast of the ice sheet. While the floating tongue’s area of 79Deg glacier has been unchanged over the last century, we show through a combination of observations and ice-sheet modelling that the tongue has been thinning since 2001. 

An increased ocean heat flux is the most plausible cause of the observed thinning. If similar conditions continue to persist over the coming decades, our modelling shows that large parts of the floating tongue will be lost, reducing the buttressing onto the grounded ice sheet, and resulting in thinning upstream of the grounding line.

Read more: Mayer, C., J. Schaffer, T. Hattermann, D. Floricioiu, L. Krieger, P.A. Dodd, T. Kanzow, C. Licciulli, and C. Schannwell, 2018. Large ice loss variability at Nioghalvfjerdsfjorden Glacier, Northeast-Greenland, Nature Communications 9 (1), 2768, doi: 10.1038/s41467-018-05180-x

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