Temporal ice-stream dynamics resulting from subtemperate sliding instabilities

Ice streams are fast-flowing “rivers” of ice within an ice sheet, and are responsible for the majority of mass loss from continental ice sheets. The onset region of these ice streams is especially interesting, as that is where ice transitions from slow interior flow to fast, sliding-dominated ice-stream flow. Subtemperate sliding, i.e., sliding below the melting point, is thought to be important in enabling this transition. Previous theoretical work has shown that the subtemperate region is subject to a host of temporal instabilities. Yet, the role of these instabilities in driving the temporal dynamics of ice streams remains unclear. In this work, we use a thermomechanically-coupled Stokes flow model of an idealised, 2D ice-sheet flowline in Elmer/Ice to investigate how these temporal linear instabilities play out in the full nonlinear evolution of the ice sheet. Using a combination of numerical simulations and theory allows us to investigate the physical mechanisms behind sliding onset, and to gain insight into what controls the observed switching “on and off” of ice streams over time. We also explore details of a thermodynamically consistent numerical implementation in Elmer/Ice of frozen-temperate boundaries at the bed.