Glacier velocity variability due to rain-induced sliding and cavity formation

Abstract

The largest accelerations of glaciers and ice sheets are caused by changes in basal slip. Here we examine glacier speed and rain-induced accelerations using a near-continuous 26-month-long GNSS time series from a large maritime glacier (Tasman Glacier, New Zealand). During periods of high rain-rate we observe short-term increases in 24-hour speeds to up to 15-times background speed. Speeds calculated over 3-hour intervals increase to up to 36-times background speed. Acceleration events correspond with times when bed separation also increases rapidly indicating that the acceleration is associated with the growth of water-filled cavities at the bed. Glacier speeds then decrease prior to the reduction in bed separation, indicating cavity growth, not cavity extent, controls the acceleration. The short-term accelerations are superimposed on longer-term periods of enhanced velocity that persist for days to weeks and decay at similar rates to bed separation estimates and proglacial lake levels. A power-law relationship between observed rain-rate and speed exists at the glacier front and exhibits no apparent upper bound. Overall, we estimate that rain-induced accelerations account for 11-14% of Tasman

Friction at the bed does not control fast glacier flow

“…Direct observations in the ablation zone of the GrIS demonstrate the importance of spatiotemporal variability in water pressure in the subglacial drainage system (23). Similarly, observations of Tasman Glacier in New Zealand show that velocity variations are linked to rain-induced variations in subglacial water pressure (24). However, a simple relation between sliding speed and subglacial water pressure may not exist because expansion and contraction of subglacial cavities is driven by fluctuations in water pressure, rather than by water pressure itself (21,22).…”

Seismic observations of crevasse growth following rain-induced glacier acceleration, Haupapa/Tasman Glacier, New Zealand

“…We consider both the periods during which rainfall precedes an increase in all other time series (Fig. 4) as rain-induced glacier accelerations, and in both cases we observe 24-hour smoothed rain rates exceeding the 3.2 mm hr −1 threshold defined by Horgan and others (2015).…”

“…4). The relationship between rainfall, glacier elevation, and glacier velocity at Haupapa/Tasman Glacier is explored in detail by Horgan and others (2015). These authors found that rain rates above 3.2 mm hr −1 (when represented as 24-hour smoothed values) trigger glacier accelerations at Haupapa/Tasman Glacier.…”

“…GNSS antennas were positioned on tripods extending 0.9 m (TG3) and 1.6 m (TG4) above the debris cover and recorded at a 15 seconds sampling interval. The GNSS data were processed using the method described by Horgan and others (2015). Receiver positions were calculated every 30 seconds using kinematic differential carrier wave positioning (Chen and others, 1998) with loosely constrained site motion.…”

“…The lower glacier is debris-covered (Purdie and Fitzharris, 1999), and receives ~4 m of rainfall annually (Henderson and Thompson, 1999). Horgan and others (2015) found that above a certain rain rate Haupapa/Tasman Glacier experiences brief accelerations of up to 15 times its daily background speed, and that such acceleration events occur on average every 1–2 months, making Haupapa/Tasman Glacier an ideal setting to study crevasse growth and its relationship to rainfall and strain rate change.

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