Spatial response of Greenland’s firn layer to NAO variability

Changes in the Greenland ice sheet (GrIS) firn layer may impact its ability to retain meltwater. These changes also need to be accounted for when converting measured ice sheet volume changes to mass changes. With a firn model (IMAU-FDM v1.2G) forced by a regional climate model (RACMO2.3p2), we investigate how the GrIS firn layer depth and pore space have evolved since 1958 in response to variability in the large-scale atmospheric circulation. On interannual timescales, the firn layer’s depth and pore space shows a spatially heterogeneous response to variability in the North Atlantic Oscillation (NAO). Notably, a stronger NAO following the record warm summer of 2012 led the firn layer in the south and east of the ice sheet to regain thickness and pore space after a period of thinning and reduced pore space. The main driving forces behind these changes vary between GrIS sectors: in the southwest, a decrease in melt dominates, whereas in the east an increase in snow accumulation dominates. However, these trends are not uniform across the GrIS, and over the same period, the firn in the northwest continued to lose pore space. The NAO is also stronger in winter than in summer and we observe that this impacts the seasonal cycle of the firn. In the wet southeastern GrIS, most of the snow accumulates during the winter, when firn compaction is slow, amplifying the seasonal cycle in firn depth and pore space. The opposite occurs in other regions, where snowfall peaks in summer or autumn, at the same time as densification and melt, damping the seasonal oscillations in the firn thickness and pore space.