Refreezing variability in the Greenland Ice Sheet firn zone and its response to extreme melt events

Firn refreezing over the Greenland Ice Sheet (GrIS) buffers surface mass loss by retaining meltwater. However, the spatial heterogeneity of long-term firn refreezing variability and its relationship with extreme melt events remain largely unclear. In this study, the Community Firn Model (CFM) is used to investigate refreezing processes at 13 firn sites across the GrIS, forced by automatic weather station (AWS) observations (2009‒2024) and the regional climate model MAR (1940‒2024). Taking the southwestern percolation-zone site KAN_U as an example, multiple parameterizations are evaluated by comparing simulated firn density and temperature with in-situ observations, and the Brils2022 (BRI) scheme is selected for subsequent simulations. The results show that the mean annual refreezing at KAN_U during 2009‒2024 is 207 mm w.e. (45.3% refreezing ratio on average). The CFM-BRI simulations show 21%‒24% lower mean refreezing ratios relative to MAR. Long-term simulations for 1940‒2024 show relatively stable refreezing capacity at northern sites, but a 30% decline in refreezing ratio at the southwestern percolation zone (KAN_U) since the 1990s. Extreme melt events have become more frequent at KAN_U (five events during 2010‒2024), extending firn recovery from one cold season to 2‒5 years and increasing runoff, which peaked at 1160 mm w.e. in 2012. The frequency of extreme melt events is negatively correlated with the refreezing rates ( r = −0.72, p < 0.001), indicating a delayed refreezing response to extreme melting. These findings improve our understanding of the long-term variability of refreezing processes in the GrIS firn zone and its response to increasing extreme melt events.