Subglacial Bed Topography and Ice Sheet Surges

Bed topography at the scale of the ice thickness can lead to multiple flow states for an ice sheet. By analogy with classical hard-bed sliding, ice flow over this type of bed obstacle, which occurs commonly in the form of drumlins and similar bedforms, leads to the generation of drag on the ice, particularly when ice flow is rapid. In this presentation, we investigate the effect of this

drag on the bulk dynamics of the overlying ice sheet. A crucial difference with classical sliding is that the upper, free surface of the ice has a significant effect on the sliding process through the formation of a standing wave. The shape of this wave not only depends on bed topography and sliding velocity, but also affects the stress field in the ice and hence the sliding velocity. The consequent nonlinearity in the sliding process leads to the possibility of multiple flow states, that is, of multiple surface wave configurations corresponding to different possible sliding velocities for the same large-scale ice sheet geometry. Using a theoretical model, we demonstrate that such multiple flow states are indeed possible, and that they lead to oscillatory behaviour in the ice sheet which can be interpreted as ice sheet surges.