This study utilises the results of bi-daily surface ice velocity surveys and simultaneous dye-tracer experiments from two melt-seasons (2000, 2001) to investigate at a high spatial and temporal resolution the dynamic response of the ablation zone of the mostly-cold polythermal John Evans Glacier to seasonal changes in its subglacial drainage system. The thermal regime of the glacier is characterised by a thick mantle of cold ice overlying a thin area of warm ice at, or immediately above, the glacier bed in the ablation zone - critically enabling the occurrence of subglacial drainage. Previous studies from John Evans Glacier, and other High Arctic glaciers with similar polythermal regimes (e.g. White Glacier, McCall Glacier, Finsterwalderbreen, Midre LovÃ©nbreen), have uncovered significant intra-seasonal variations in glacier dynamics, particularly in the warm-based ablation zone, but it is not known whether these anomalies propagate temporally or spatially, nor how they interact with variations in subglacial hydrology. Using a network comprising 33 stakes drilled and frozen into the surface across the lower 5 km of John Evans Glacier, we examine the occurrence and rates of propagation of hydrologically-induced flow anomalies at this site. The results demonstrate that the seasonal establishment of supraglacial-subglacial connections through cold ice in the mid-ablation zone induces a downglacier propagation both of high glacier surface velocities and vertical surface uplift over the lower ablation zone lasting ~ 2-4 days. This propagation presumably reflects the downglacier propagation of high subglacial water pressures, hence enhanced basal motion, as large volumes of supraglacially-derived meltwaters pass through the distributed subglacial drainage system en route to the subglacial outflow. After this 'spring event', horizontal velocities fall (although typically remain higher than overwinter levels until the end of summer) and vertical velocities change sign. Further transient periods of high horizontal surface velocities are observed when high meltwater inputs resume after periods of cold weather, or when unusually high meltwater inputs exceed the capacity of the channelised subglacial drainage system. The magnitude of the hydrology-dynamics coupling at John Evans Glacier is such that it may significantly affect the net mass balance, leading to more rapid transfer of ice to lower, warmer elevations, where melting is enhanced, than might otherwise be the case. If the processes are typical of High Arctic glaciers, then High Arctic glaciers may respond more rapidly to future climatic warming than current models (which neglect this coupling) suggest.
|Translated title of the contribution||Spatio-temporal propagation of high-velocity events at polythermal John Evans Glacier|
|Title of host publication||International Glaciological Society, British Branch Meeting, Queen's University Belfast|
|Publication status||Published - 2003|