01/09/2021, 10:35 am EST
The arctic oscillation was -1.7 in December and (traditionally) representative of blocking high pressure in the polar latitudes while a split in the polar vortex southward causes widespread mid-latitude chill. But! December 2020 was mild across most of North America and Europe and only Central Eurasia was (very) cold (about as far away from a maritime influence as possible).