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
The Tasman Glacier is the largest glacier in New Zealand. Although 20th century warming caused down-wastage, it remained at its Little Ice Age terminus until the late 20th century. Since then, rapid calving retreat (Ur) has occurred, allowing a large (5.96 ¥ 10 6 m 2 ) proglacial lake to form (maximum depth~240 m). From sequential satellite image analysis and echo sounding of Tasman Lake, we document (Ur) from 2000 to 2008. Ur varies temporally, with mean Ur of 54 m/a from 2000 to 2006 and a mean Ur of 144 m/a from 2007 to 2008. Consistent with global data sets, calving rate appears closely associated with lake depth at the calving terminus.
the controlling process of ice loss at the terminus was iceberg calving resulting from thermal undercutting. In contrast, the retreat between 2006 and 2008 was probably controlled by buoyancy-driven iceberg calving caused by decreased overburden pressure as a result of supraglacial pond growth. As a result, the surface area of Tasman Lake has increased by 86% over the period 2000-2008, with lake volume increasing by 284% between 1995 and 2008. Currently, the volume of Tasman Lake is 510 × 10 6 m 3. It will increase dramatically in near future as the lake expands into deeper water.
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