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. Plain Language Summary: Most of the Greenland ice sheet's surface is covered by a layer of firn, which is compressed snow that can retain meltwater within its pores, and in this way buffer runoff into the ocean. Loss of pore space thus amplifies future Greenland ice‐sheet mass loss and accelerates sea‐level rise. We use computer models of the firn layer to investigate how firn has evolved from 1958 to 2020. The firn layer has gotten substantially thinner, less porous, and warmer over this period, but the changes are not the same everywhere. Moreover, after 2012, the firn in some regions recovered some of its lost pore space. These changes are linked to changes in the jet stream, a band of strong westerly winds encircling the Northern Hemisphere. A weaker and more meandering jet stream enhances the loss of pore space and vice versa. The jet stream also impacts the seasonal distribution of snowfall, and in this way, it influences the seasonal cycle of the firn depth and pore space. Key Points: The thickness and pore space of Greenland's firn layer respond spatially heterogeneously to changes in the North Atlantic Oscillation (NAO)After 2012, a more positive NAO led to thickening and cooling of the firn layer in some regions, but not everywhereThe amplitude of the seasonal cycle in firn thickness is linked to the seasonality of the polar jet stream [ABSTRACT FROM AUTHOR]