Abstract
Abstract. Glacier surges often initiate in winter, but the mechanism remains unclear in contrast to the well-known summer speed-up at normal glaciers. To better understand the mechanism, we used radar images to examine spatial-temporal changes in the ice velocity of surge-type glaciers near the border of Alaska and the Yukon, focusing on their quiescent phase. We found significant accelerations in the upstream region from autumn to winter, regardless of surging episodes. Moreover, the winter speed-up propagated from upstream to downstream. Given the absence of surface meltwater input in winter, we suggest the presence of water storage near the base that does not directly connect to the surface, yet can promote basal sliding through increased water pressure. Our findings have implications for the modelling of glacial hydrology in winter, which may help us better understand glacier dynamics.
Highlights
Ice flow on mountain glaciers and ice sheets typically shows greatest acceleration from spring to early summer, followed by deceleration in midsummer to autumn (e.g. Iken and Bindschadler, 1986; MacGregor et al, 2005; Sundal et al, 2011). These speed changes are attributed to subglacial slip associated with water pressure changes, and these changes arise from seasonal variability of meltwater input and the evolution of the subglacial hydraulic system (Schoof, 2010; Bartholomaus et al, 2011; Hewitt, 2013; Werder et al, 2013)
We focus on winter speed-up signals at surge-type glaciers that were in their quiescent phase during the analysis period
Glacier (Fig. 2e), the winter speed is more than 50 % greater than the autumn speed
Summary
Ice flow on mountain glaciers and ice sheets typically shows greatest acceleration from spring to early summer, followed by deceleration in midsummer to autumn (e.g. Iken and Bindschadler, 1986; MacGregor et al, 2005; Sundal et al., 2011). To examine the spatial and temporal changes, we first set a flow line at each glacier, and averaged the velocity pixel data over the ∼ 350 × 350 m2 area with its centre at the flow line. In contrast to the propagation toward the upstream region of the summer speed-up observed in Greenland outlet glacier (Bartholomew et al, 2010), the higher-velocity area expands from upstream in autumn, to the downstream section in winter. This propagation toward downstream is most clearly observed at Anderson Glacier (Fig. 2a). The much smaller tributary in the upper reach of Malaspina Glacier (Fig. 1) exhibits greater velocities in winter, suggesting that the winter speedup mechanism is independent of the glacier’s size
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