Pause in Dynamic Thinning on Jakobshavn Isbrae Triggered by Reduced Surface Melt and a Cool Ocean

Jackobshavn Isbrae is a major outlet glacier on Western Greenland, and the single largest contributor to ice sheet mass loss. However, after 20 years of speedup and thinning, observations now show that since 2014 Jakobshavn Isbrae has entered a phase of dynamic slowdown and thickening. We use intensity feature tracking of TerraSAR-X and Sentintel-1 SAR data to measure ice velocity, and swath mode processing of CryoSat-2 data to measure change in the elevation of the ice sheet surface. Our results show that since 2013 satellites observed a slowdown in ice speed, along with a reversal of the ice thinning trend that has dominated the ice stream for the last decade. The fast-flowing trunk is now thickening at a rate of 20 m/yr, a reversal of the previous rate of thinning at -20 m/yr. This change in speed and thickness has coincided with an advance in the glacier calving front. We use auxiliary ocean and surface melt datasets to investigate the physical mechanism responsible for this change on Jackobshavn Isbrae. A decade of ocean temperature profiles collected in Disco Bay show that although ocean temperatures in 2016 and 2017 were as cool as historically cold years (e.g. 2010), the observed slowdown preceded this change in ocean heat. Anomalously low surface melt in 2013, which drives fjord overturning circulation, indicates that a series of connected changes in both atmospheric and ocean forcing may have driven the observed pause in dynamic thinning on Jakobshavn.