Anthropogenic influence on Northern Hemisphere blocking during the winter 1960/1961–2012/2013

Atmospheric blocking (‘blocking’) in the Northern Hemisphere (NH) is a crucial driver of extreme cold spells in winter. Here we investigate the anthropogenic influence on the NH blocking and its impact on surface air temperature (SAT) during the winter 1960/1961–2012/2013 using two HadGEM3-GA6-N216 simulations with 15 ensemble members: (a) with anthropogenic and natural forcing (All-hist) and (b) with natural forcing only (Nat-hist). Compared to the Nat-hist run, the blocking frequency in the All-hist run decreases in the Euro-Atlantic, the Urals and the western Pacific, whereas it increases in the eastern Pacific and Greenland. These responses can be explained by the response of planetary waves and storm tracks. On the other hand, the decrease in SAT downstream of the blocking regions in the All-hist run is more pronounced than the Nat-hist run, especially in Europe and the Urals. Correspondingly, the proportion of cold days during all blocking days in these sectors is higher in the All-hist run than the Nat-hist run. These responses can be explained by the wind response associated with blocking. Overall, the spatiotemporal characteristics of blocking is crucial for evaluating the impact of blocking on extreme weather, and their response to anthropogenic forcing should be investigated by more models.