Sammendrag
The Arctic is warming at a greater rate than the global mean, and resulting increase in melt and net mass loss from glaciers and ice caps is contributing to global sea-level rise. Mass loss from runoff is influenced by meltwater retention in firn (compressed multi-year snow) through refreezing or storage in perennial firn aquifers. Meltwater retention in firn can buffer runoff; conversely, formation of thick near-surface ice layers can limit percolation and accelerate mass loss. Although refreezing is known to play an important role in the mass balance of Svalbard glaciers, the retention processes in frozen and liquid forms and their changes under warming climate are not well understood. Here, these processes are investigated on the Austfonna ice cap, Svalbard, with observational data and model simulations. Firn stratigraphy and density from 16 firn cores drilled near the summit of the ice cap (750-791 m a.s.l.) spanning from 1958 to 2022 are evaluated. The firn stratigraphy consists mainly of thin ice layers; thus, ice slabs (> 1 m layers) that have recently been found in Greenland, are not present on the ice cap. Firn density and ice content decreased during 2005-2012, a period of below average melt, indicating a replenishment of firn pore space. The firn temperature record at site Camp (773 m a.s.l.) from 2009 to 2022 reveals a shift in the firn thermal regime in 2014 or 2015, from typically cold firn to fully temperate firn except for seasonal cooling. Simulations using the CryoGrid community model at site Camp from 2003 to 2022 confirm the thermal shift, and results suggest a formation of a seasonal firn aquifer in 2004, 2020 and 2022. Simulated firn cold content and pore space in the upper 10 m decrease significantly 2008-2022. However, simulated refreezing is not found to be decreasing, which may partly be due to model limitations leading to inaccurate simulation of the amount of melt.