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

Abstract. Basin-3, the largest outlet basin of the Austfonna ice cap, started to surge in autumn 2012. A maximum velocity of 18.8 m d^-1 was found in December 2012 / January 2013. Here we present a time series of area wide velocity fields from synthetic aperture radar (SAR) offset tracking and Global Positioning System (GPS) data in the aftermath of the velocity maximum, extending the previously published data from May 2013 to July 2016. We find that terminus velocity slowed down by ~ 50 % until spring 2014, whereas the upper parts of the basin continued to speed-up and reached their maximum only in summer 2014. Until the date of writing (July 2016), Basin-3 maintained high velocity with maxima between 8.9–11.4 m d^-1. Summer speed-ups were superimposed even on the otherwise fast surge motion. The total frontal ablation A_f over the period 19 April 2012 to 26 July 2016 was calculated to 22.2 ± 8.1 Gt (5.2 ± 1.9 Gt yr^-1) from the ice mass flux q_fg = 33.2 ± 11.5 Gt (7.8 ± 2.7 Gt yr^-1) and the terminus mass change q_t = 11.0 ± 3.4 Gt (2.6 ± 0.8 Gt yr^-1). Additional advance of the terminus led to a total sea-level rise equivalent of 31.3 ± 11.2 Gt (7.3 ± 2.6 Gt yr^-1).

This rate of frontal ablation roughly equals previous estimates of both the calving flux and total mass loss from the entire archipelago, resulting in a doubling of the current ice-mass loss from Svalbard. In vicinity of Basin-3, we also observe a terminus advance and a speed-up of the northern part of Basin-2 starting in autumn 2014, with surface velocity reaching 8.71 m d^-1 in August 2015. The related ice mass loss of Basin-2 between 20 June 2015 and 26 July 2016 amounts to 0.8 Gt (min: 0.3 Gt, max: 1.6 Gt). Accounting also for the replacement of ocean water, we find a total sea-level rise equivalent of 1.1 Gt (min: 0.5 Gt, max: 2.1 Gt).