Abstract
Abstract. The presence of strong diurnal cycling in basal water pressure records obtained during the melt season is well established for many glaciers. The behaviour of the drainage system outside the melt season is less well understood. Here we present borehole observations from a surge-type valley glacier in the St Elias Mountains, Yukon Territory, Canada. Our data indicate the onset of strongly correlated multi-day oscillations in water pressure in multiple boreholes straddling a main drainage axis, starting several weeks after the disappearance of a dominant diurnal mode in August 2011 and persisting until at least January 2012, when multiple data loggers suffered power failure. Jökulhlaups provide a template for understanding spontaneous water pressure oscillations not driven by external supply variability. Using a subglacial drainage model, we show that water pressure oscillations can also be driven on a much smaller scale by the interaction between conduit growth and distributed water storage in smaller water pockets, basal crevasses and moulins, and that oscillations can be triggered when water supply drops below a critical value. We suggest this in combination with a steady background supply of water from ground water or englacial drainage as a possible explanation for the observed wintertime pressure oscillations.
Highlights
The drainage of melt water along the glacier bed can have a significant effect on ice flow velocities by modulating basal sliding
The presence of strong diurnal cycles in water input is a key characteristic of many subglacial drainage systems during summer, leading to corresponding diurnal cycles in basal water pressure (Hubbard et al, 1995; Fudge et al, 2008; Shepherd et al, 2009) and possibly playing a role in ice flow speed-up (Schoof, 2010; Hewitt, 2013)
We address the observed doublet structure in diurnal pressure peaks using the same model. We demonstrate that such doublets, with a two-day period, can result from the nonlinear coupling between water storage at or near the bed and a surface water input that varies purely diurnally
Summary
The drainage of melt water along the glacier bed can have a significant effect on ice flow velocities by modulating basal sliding. The presence of strong diurnal cycles in water input is a key characteristic of many subglacial drainage systems during summer, leading to corresponding diurnal cycles in basal water pressure (Hubbard et al, 1995; Fudge et al, 2008; Shepherd et al, 2009) and possibly playing a role in ice flow speed-up (Schoof, 2010; Hewitt, 2013). The termination of this strong diurnal signal often occurs while there is still measurable surface melting We use our model to show that water storage may cause this behaviour, indicating that storage capacity may play a prominent role in modulating drainage year-round
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