Glacier Surges Controlled by the Close Interplay Between Subglacial Friction and Drainage

The flow of glaciers and ice sheets is largely influenced by friction at the ice-bed interface that can trigger rapid changes in glacier motion ranging from seasonal velocity variations to cyclic surge instabilities or even devastating glacier collapse. This wide range of transient glacier dynamics is currently not captured by models, and its implications for long-term glacier evolution are uncertain. This highlights the need of developing improved descriptions for processes that occur at the glacier bed. Here, we present a model that describes the evolution of basal friction inspired by a “rate and state” approach, coupled to models of subglacial drainage and glacier flow, and investigate how these couplings affect the dynamics of glaciers. We show that a wide range of sliding behavior results from a feedback loop between subglacial drainage efficiency and friction which depends on the evolution of a frictional state that can be interpreted as the degree of cavitation or till porosity for hard and soft beds, respectively. In our simulations, we find that glaciers are susceptible to surging if they exhibit a transition to velocity weakening friction associated with a poor sensitivity of the drainage capacity to the frictional state. This potential materializes if the local topography and mass balance create the conditions for high water pressure to build up in an area sufficiently large to exceed a critical length. We advocate accounting for feedback loops between friction and drainage as a promising avenue for better understanding dynamical instabilities of glaciers and ice sheets.