Thermomechanically coupled modeling of polythermal glacier response to surge-like perturbations

While glacier surges occur across a range of geographical and environmental settings, they favor a climate envelope in which polythermal glaciers are common. Polythermal structure has been correlated with surging in some regions and is known to play a regulatory role in surge propagation. Whether surging, in turn, has a long-term influence on thermal structure has not been thoroughly investigated. A thermo-mechanically coupled Stokes ice-flow model(Elmer/Ice), applied to a 2-D flowband (x-z) glacier geometry was employed to evaluate the potential for surge events to alter glacier thermal structures on timescales much longer than the active phase. The synthetic geometry resembles a small polythermal glacier in Yukon, Canada, where changes in surge character have been observed and changes in thermal structure projected. Consistent with previous work, surging was found to produce smaller glaciers, but surging was also found to produce glacier that are colder than their non-surge-type counterparts. When surge events are sufficiently vigorous, a new ∼300 a secondary internal oscillation emerges that dramatically alters the magnitude (e.g., 50 % variation in mean surface velocity) 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. A threshold for the emergence and persistence of this oscillation is empirically identified and depends on the advective ice flux and ice-thickness change at the equilibrium line. All simulations were conducted under stationary climate forcing; the existence, let alone detectability, of the secondary oscillation in a non-stationary climate therefore remains an open question. Nonetheless, this work should motivate re-evaluation of observed changes in polythermal glacier surge characteristics to assess what role, if any, thermodynamics might play.