Modulation of glacier surge cycles on decadal to centennial timescales by intrinsic thermal-structure variability

Abstract. Glacier surges occur across a range of geographical and environmental settings, but are favored by a climate envelope in which polythermal glaciers are common. Thermal structure is known to play a regulatory role in surge propagation, but whether surging has a long-term influence on thermal structure has been comparatively under-investigated. In this study, we analyze the intrinsic variability of glacier thermal structure associated with the surge cycle, and its potential impact on surging behavior itself, in the presence of a stationary climate. We apply a thermo-mechanically coupled Stokes ice-flow model to an idealized two-dimensional flowband glacier geometry intended to represent a class of small polythermal glaciers in Yukon, Canada, where changes in surge character have been observed and changes in thermal structure projected. The idealized glacier is largely temperate in the accumulation area and cold in the ablation area. Surge-like events are induced in the model by prescribing abrupt reductions in basal friction at 15–60-year recurrence intervals. Surging is found to produce glaciers that are, on average, both smaller and colder than their non-surge-type counterparts. When surge events are sufficiently vigorous, a secondary, thermally-driven, internal oscillation emerges that dramatically alters the magnitude and spatial extent of surge events. This oscillation reflects variations in the length of the temperate (thawed) portion of the glacier bed, induced by imbalances between horizontal advection of temperate ice and vertical conductive cooling. Its amplitude and period are a function of surge vigor, with periods ranging from ~150–450 years. While the existence, let alone detectability, of such a secondary oscillation in a non-stationary climate may be questioned, this result motivates a more thorough attribution study of the evolution of surge character in polythermal glaciers.