Seasonal and interannual variability in freshwater sources for Greenland's fjords

The magnitude, source, release location, and timing of freshwater that ends up in the numerous Greenland fjords is of special interest for ice–ocean interactions and ecosystems. In this study, we investigate intra- and interannual variability in the various freshwater sources for Greenland's fjords in seven climatologically distinct regions. For this, we use direct and statistically downscaled output from regional climate models for the mass fluxes, process-based estimates of basal melt, and observational data for solid ice discharge. For the period 1940/1958 through 2023, we separately quantify runoff from the Greenland ice sheet, peripheral ice caps and tundra regions, and precipitation directly falling in the fjords. From 2009 onwards, the available data allow us to resolve the full seasonal cycle of freshwater input. The results indicate a diverse range of relative contributions from freshwater sources between seasons and regions. Freshwater input in fjords in the wet South-East and North-West is dominated by solid ice discharge (55 % and 67 %, respectively) with a small contribution of tundra runoff, whereas in the relatively drier North, North-East, and South-West the contribution of tundra runoff is more important (20 %, 25 %, and 30 %, respectively). Precipitation in fjords and tundra runoff can represent a large fraction of the monthly total, i.e. up to 11 % and 35 %, respectively, for winter and spring. However, the relative contribution of tundra runoff has been decreasing with time as the result of rapid increases in ice sheet and ice cap runoff over the past decades following atmospheric and oceanic warming. We show that the regional glacier-integrated melt-over-accumulation (MoA) ratio is a good predictor for the relative contributions of solid ice discharge, tundra runoff, and ice sheet runoff. These findings have implications for the use of freshwater fluxes forcing in regional ocean models and fjord studies and enhance our understanding of their impact on ocean and fjord circulation and biogeochemistry.