On the verge of a surge; Kongsvegen, northwestern Svalbard.

Surge-type glaciers switch regularly between slow and fast modes of flow, with surge-phase velocities that are typically 2-3 orders of magnitude higher than those in the longer quiescent phase. Surging arguably represents an extreme case of a glacier dynamic instability.

Kongsvegen is a surge-type polythermal glacier in Svalbard whose last surge occurred around 1948, and which has been steadily building up mass in its 'reservoir zone' since then.

Long-term GPS measurements of mass balance stakes on Kongsvegen, which started in 2004, show that the glacier began to accelerate after 2014. Centerline velocities at the equilibrium line averaged 6 m/yr in the period 2004-2013, then doubled to 12 m/yr in 2016. In 2017, winter velocities were higher than summer velocities for the first time in the record. Acceleration has continued up to the latest measurements, reaching over 18 m/yr. A full surge of Kongsvegen therefore appears imminent, presenting an exceptional opportunity to test new instruments and concepts for measuring and understanding key processes governing the transition from quiescence to full surge. The early detection of an incipient surge of a polythermal glacier is exceptionally rare, because surge cycles are infrequent and highly variable, and surge onset cannot be predicted accurately.

In Spring 2018 we initiated a series of measurements, including the following: (1) Five continuously-logging GNSS receivers installed at ca. 3 km intervals along the glacier centerline; (2) Regular acquisition of high-resolution satellite SAR (e.g. TerraSAR, Radarsat-2) imagery, supplementing the frequent coverage of Sentinel-1 SAR imagery; (3) Ice-penetrating radar surveys conducted to complete existing bed topography maps and to document changes in the thermal structure since earlier surveys in the 1990s; (4) A 330-m long borehole drilled to the glacier bed and instrumented with a thermistor string and basal water pressure sensor. We present results from these unique measurements from a glacier, in the earliest stage of a surge.