Decomposing Seasonal Ice-flow Variability in Western Greenland using Modelling and Observations

Outlet glaciers of the Greenland Ice Sheet typically undergo a seasonal cycle in ice flow, yet the magnitude and timing of peak annual velocities vary substantially among glacier systems, across years, and with distance away from the terminus. At tidewater glaciers, this variability reflects mainly the competing influences of surface meltwater-driven basal lubrication and flexural perturbations associated with calving-front dynamics. Because observations alone cannot readily separate these processes, we develop a physics-based framework that integrates ice-flow simulations with high-resolution surface velocity observations to decompose seasonal ice motion into basal and frontal components.

We apply this approach to 61 tidewater glacier basins in western Greenland and show that seasonal velocity variations are primarily controlled by evolving basal hydrologic conditions. Frontal perturbations nonetheless exert a secondary but persistent influence on seasonal ice flow. Near glacier termini, mixed basal–frontal control occurs 49.6–62.3% of the time, and although the influence of frontal forcing generally diminishes inland, it can extend to elevations of up to 2000 metres above sea level at fast-flowing glaciers such as Sermeq Kujalleq (Jakobshavn Isbræ). Our method further isolates signals that are subdued in raw velocity observations and closer aligned with expected patterns of seasonal basal drainage development. Importantly, results from three independent transient model configurations demonstrate that our conclusions are robust to the choice of sliding law, with consistent identification of the dominant controlling process in 97.1% of cases. We therefore propose that this framework provides a reliable basis for process-level interpretation of seasonal ice-flow variability across Greenland.