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>Here we report on a cyclic, physical ice-discharge instability in the\udParallel Ice Sheet Model, simulating the flow of a three-dimensional,\udinherently buttressed ice-sheet-shelf system which periodically surges on a\udmillennial timescale. The thermomechanically coupled model on 1 km\udhorizontal resolution includes an enthalpy-based formulation of the\udthermodynamics, a nonlinear stress-balance-based sliding law and a very\udsimple subglacial hydrology. The simulated unforced surging is characterized\udby rapid ice streaming through a bed trough, resulting in abrupt discharge of\udice across the grounding line which is eventually calved into the ocean. We\udvisualize the central feedbacks that dominate the subsequent phases of ice\udbuildup, surge and stabilization which emerge from the interaction between\udice dynamics, thermodynamics and the subglacial till layer. Results from the\udvariation of surface mass balance and basal roughness suggest that ice sheets\udof medium thickness may be more susceptible to surging than relatively thin\udor thick ones for which the surge feedback loop is damped. We also\udinvestigate the influence of different basal sliding laws (ranging from\udpurely plastic to nonlinear to linear) on possible surging. The presented\udmechanisms underlying our simulations of self-maintained, periodic ice growth\udand destabilization may play a role in large-scale ice-sheet surging, such as\udthe surging of the Laurentide Ice Sheet, which is associated with Heinrich\udevents, and ice-stream shutdown and reactivation, such as observed in the\udSiple Coast region of West Antarctica

From cyclic ice streaming to Heinrich-like events: the grow-and-surge instability in the Parallel Ice Sheet Model