Seasonal to decadal evolution of firn properties and impacts on hydrology of the Juneau Icefield

Abstract. Alpine glaciers of Alaska are a significant contributor to global sea-level rise. Most Alaskan glaciers lose mass through surface melting due to increasing atmospheric temperatures, which may change regional glacier hydrology, including firn properties and the firn's capacity for meltwater retention. Here, we use field observations and firn modeling to investigate seasonal to decadal changes in the thermal and physical properties of firn on the Juneau Icefield, Alaska, and the impacts on the firn's capacity for meltwater retention. Firstly, we find that mean density and liquid water content generally increase up to 5 % and 71 %, respectively, through the 2024 summer season, which suggests seasonal transient retention of meltwater in the snow and firn. Second, we find that modeled firn-air content from 1980–2019 decreased between 22 % to 35 % due to decreasing firn thickness and increasing firn density. Third, modeled results show that decreasing firn cold content caused increasing meltwater runoff from the firn (63 % to 76 %), decreased meltwater refreeze (-24 to -39 %), and shift of the refreeze and runoff transition 7–18 days earlier in the season from 1980–2019 due to increasing surface melt and decreasing snow accumulation. Our results suggest that firn on the Juneau Icefield and other temperate alpine glaciers of Alaska will continue to lose long-term meltwater refreezing capacity. Further, inter-seasonal shifts in liquid water retention on these glaciers may introduce uncertainties in mass-balance calculations for sea-level rise estimates.