Finite-element Modelling of Haupapa/Tasman Glacier's Basal Sliding Events
The rate of ice loss from glaciers and ice caps is a major source of uncertainty in predicting sea level rise out to 2100. Improving the predictive capability of ice flow models will, in part, require a more robust coupling of climate to long-term and short-term variability in glacial discharge. An ongoing concern is the role that surface melting and rainfall plays in accelerating glacier flow. Rapid drainage of surface water to the base of a glacier or ice sheet is thought to elevate basal water pressure, reduce basal friction, and thereby increases sliding speed. Here, we present several rain-induced speed-ups of Haupapa/Tasman Glacier, South Island, New Zealand, recorded by GNSS (Global Navigation Satellite System) instruments. Observed speed-up events involve large vertical offsets (up to ~53 cm) and large horizontal accelerations of up to twenty-four times background velocity. Due to it's pronounced sliding events, Haupapa/Tasman Glacier offers a useful case study for investigating the processes that govern the sliding behaviour of large glaciers prone to increasing meltwater variability as a cause of enhanced mass loss in a warming climate. The observed correspondence of vertical displacement and horizontal acceleration in this study suggests that the rapid growth of water-filled cavities at the bed controls basal motion during speed-ups. However, sliding laws that relate changes in basal velocity to changes in water pressure do not account for cavity growth. To investigate the processes governing a typical speed-up event, we use a finite-element modelling approach combined with a commonly-used sliding law to recreate internal deformation and basal sliding of Haupapa/Tasman Glacier during rain-induced acceleration. In general, we find peak velocities can only be achieved when basal water pressure exceeds ice overburden and velocity at the glacier sides is allowed to exceed that observed by a GNSS unit situated near the margins. The sliding law requires a more complete treatment of cavity growth under rapid water pressure changes to better capture basal acceleration observed at Haupapa/Tasman Glacier.