Terminus Geometry as Main Control on Outlet Glacier Velocity

Ice flow velocities close to the terminus of major outlet glaciers of the Greenland Ice Sheet can vary on the time scale of years to hours. Such flow speed variations can be explained as the reaction to changes in terminus geometry with help of a 3D full-Stokes ice flow model. Starting from an initial steady state geometry, parts of an initially 7 km long floating terminus are removed. Flow velocity increases everywhere up to 4 km upstream of the grounding line, and complete removal of the floating terminus leads to a doubling of flow speed. The model results conclusively show that the observed velocity variations of outlet glaciers is dominated by the terminus geometry. Since terminus geometry is mainly controlled by calving processes and melting under the floating portion, changing ocean conditions most probably have triggered the recent geometry and velocity variations of Greenland outlet glaciers.

💡 Research Summary

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The paper investigates how changes in the geometry of a glacier’s floating terminus control the flow velocity of Greenland outlet glaciers, using a three‑dimensional full‑Stokes ice‑flow model (FISMO). Starting from a steady‑state configuration with a 7 km long floating terminus that mimics Jakobshavn Isbræ, the authors systematically remove portions of the terminus (reducing its length to 2 km, 1 km, and finally 0 km). For each geometry the model solves the incompressible Stokes equations coupled with Glen’s flow law (n = 3, A = 2 × 10⁻¹⁵ Pa⁻³ s⁻¹) using Q2Q1 isoparametric Taylor‑Hood finite elements on a hexahedral mesh. Boundary conditions include zero velocity at the bed, stress‑free air contact, hydrostatic water pressure on the ice‑ocean interface, and a far‑field normal stress that reproduces an infinite inclined slab.

The results show a clear, monotonic increase in flow speed as the floating terminus shortens. Velocity increases are observed up to 4 km upstream of the grounding line, and when the floating terminus is completely removed the grounding‑line speed roughly doubles. The authors attribute this acceleration to a redistribution of stresses: with a floating terminus present, water pressure exerts a horizontal compressive force that is spread over the entire cross‑section at the grounding line, suppressing longitudinal extension. When the terminus disappears, water pressure acts only below sea level, creating a strong extensional (deviatoric) stress concentrated near the water line, which in turn drives faster ice flow.

The study also examines the mean stress (deviation from overburden pressure) and longitudinal deviatoric stress along the flowline. In the presence of a floating terminus, the longitudinal stress is compressive at the grounding line; after removal it becomes tensile, confirming the stress‑re‑distribution mechanism. These model outcomes reproduce the step‑wise velocity jumps recorded during calving events at Jakobshavn Isbræ and Helheim Glacier, where field measurements show velocity spikes within 15 minutes of a calving episode. The model, however, does not capture the observed velocity increase extending 40 km upstream because surface readjustment (draw‑down) after terminus loss was not allowed to evolve; in reality, the surface depression propagates inland, extending the stress perturbation.

A key implication concerns the concept of ice‑shelf buttressing. Traditional interpretations emphasize friction at pinning points or lateral drag as the source of buttressing. The present work demonstrates that, in the absence of basal friction, buttressing is essentially a geometric effect: the presence of a floating ice tongue modifies the distribution of water‑pressure forces and thus the stress field at the grounding line. Consequently, changes in terminus geometry—driven by oceanic melting or calving—are the primary drivers of short‑term velocity variability in Greenland outlet glaciers.

The authors conclude that terminus geometry dominates the velocity field near grounding lines, and that rapid changes such as calving, melt‑induced thinning, or the formation of embayments can instantly alter glacier dynamics. Monitoring terminus shape and its evolution, together with ocean temperature and melt‑rate observations, is therefore essential for improving predictions of glacier contribution to sea‑level rise. The study underscores the need to incorporate realistic terminus geometry and ocean‑ice interaction in ice‑sheet models to capture the observed rapid accelerations of Greenland’s major outlet glaciers.