Kjetil Thøgersen , Adrien Gilbert, Coline Bouchayer, Thomas Vikhamar Schuler

The fast flow of glaciers and ice sheets is largely influenced by friction at the ice- bedrock interface, and our imperfect understanding of subglacial friction accounts for one of the largest uncertainties in predictions of future sea-level rise. Glacier motion ranges from slow creep to cyclic surge instabilities and devastating glacier collapse as well as continuously fast-flowing ice-streams. Glaciers dynamics also exhibits seasonal velocity variations, up-glacier and down-glacier propagation of surges, interrupted surges, as well as short-duration speed up events that do not develop into surges. Several aspects of this wide range of glacier dynamical behavior remain elusive. This knowledge gap highlights the crucial need of developing improved descriptions of the physical processes that occur at the glacier bed as well as their couplings. Here, we show that this wide range of sliding behavior can be understood from the transient evolution of subglacial cavities and till porosity which results from a feedback loop between subglacial drainage efficiency and friction. We find potential for surging behavior at glaciers that exhibit low hydraulic conductivity at the base, together with a weak increase in hydraulic conductivity with sliding, and where the frictional response contains a transition to velocity weakening friction. This potential materializes if the local topography and surface mass balance create sufficiently thick glaciers to shut down the conduit drainage system, and where the water input to the glacier base is sufficiently high. Accurately accounting for feedback loops between friction and drainage has the potential to improve surge understanding and future assessments of hazard potential of glaciers.