Seasonal speed-up of two outlet glaciers of Austfonna, Svalbard, inferred from continuous GPS measurements

Abstract

Abstract.A large part of the ice discharge from ice caps and ice sheets occurs through spatially limited flow units that may operate in a mode of steady flow or cyclic surge behaviour. Changes in the dynamics of distinct flow units play a key role in the mass balance of Austfonna, the largest ice cap on Svalbard. The recent net mass loss of Austfonna was dominated by calving from marine terminating outlet glaciers. Previous ice-surface velocity maps of the ice cap were derived by satellite radar interferometry (InSAR) and rely on data acquired in the mid-1990s with limited information concerning the temporal variability. Here, we present continuous Global Positioning System (GPS) observations along the central flowlines of two fast flowing outlet glaciers over [2008][2009][2010]. The data show prominent summer speedups with ice-surface velocities as high as 240 % of the presummer mean. Acceleration follows the onset of the summer melt period, indicating enhanced basal motion due to input of surface meltwater into the subglacial drainage system. In 2008, multiple velocity peaks coincide with successive melt periods. In 2009, the major melt was of higher amplitude than in 2008. Flow velocities appear unaffected by subsequent melt periods, suggesting a transition towards a hydraulically more efficient drainage system. The observed annual mean velocities of Duvebreen and Basin-3 exceed those from the mid-1990s by factors two and four, respectively, implying increased ice discharge at the calving front. Measured summer velocities up to 2 m d −1 for Basin-3 are close to those of Kronebreen, often referred to as the fastest glacier on Svalbard.

Surface speed and frontal ablation of Kronebreen and Kongsbreen, NW Svalbard, from SAR offset tracking

“…This behavior has been observed at many glaciers in different regions in the world, e.g., the Alps (Iken and Bindschadler, 1986), Svalbard (Dunse et al, 2012), Canada (Copland et al, 2003) and Alaska (Burgess et al, 2013). In each year, secondary speed peaks were detected after the main melt induced summer speedup which are caused by rain events; the subglacial system is saturated for a short period again, allowing enhanced basal sliding.…”

Seasonal velocities of eight major marine-terminating outlet glaciers of the Greenland ice sheet from continuous in situ GPS instruments

“…2). The GPS receiver as well as the subsequent data processing particular to this system has previously been described in detail in Den Ouden et al (2010) and further in Dunse et al (2012). The GPS receiver is a single-frequency (L1 band) system that is designed for extended operation in harsh conditions with no maintenance at a low instrument cost.…”

“…Press et al (1992), which has an exponent of 2. Dunse et al (2012) similarly used a 7-day running average, yielding robust results by using a square window. Average positions are calculated if more than 95 % of the records within a given time window are present, except for Upernavik Glacier, where only 50 % are required due to numerous data gaps.…”

Multi-year surface velocities and sea-level rise contribution of the Basin-3 and Basin-2 surges, Austfonna, Svalbard

“…Originally 'five GPS receivers were deployed along the mid-1990s central flowline, 5 to 21 5 km upglacier from the calving front (Dunse et al, 2012). We used GPS single-frequency code receivers (L1 band, C/A code only).…”

“…The Global Positioning System (GPS) velocity time series starting in 2008 from Dunse et al (2012) and Dunse et al (2015) was prolonged until July 2016. Originally 'five GPS receivers were deployed along the mid-1990s central flowline, 5 to 21 5 km upglacier from the calving front (Dunse et al, 2012).…”