Spatial Response of Greenland's Firn Layer to NAO Variability

Firn on the Greenland Ice Sheet (GrIS) buffers meltwater, and has a variable thickness, complicating observations of volume change to mass change. In this study, we use a firn model (IMAU-FDM v1.2G) forced by a regional climate model (RACMO2.3p2) to investigate how the GrIS firn layer thickness and pore space have evolved since 1958 in response to variability in the large-scale atmospheric circulation. On interannual timescales, the firn layer thickness and pore space show 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. In the southwest, a decrease in melt dominated after 2012, whereas in the east, the main driver was an increase in snow accumulation. At the same time, the firn in the northwestern ice sheet continued to lose pore space. The NAO also varies on intra-annual timescales, being typically stronger in winter than in summer. This impacts the amplitude of the seasonal cycle in GrIS firn thickness and pore space. In the wet southeastern GrIS, most of the snow accumulates during the winter, when melting and densification are relatively weak, leading to a large seasonal cycle in thickness and pore space. The opposite occurs in other regions, where snowfall peaks in summer or autumn. This dampens the seasonal amplitude of firn thickness and pore space.