Thesis: That cold air is prevented from traveling in west-to-east Rossby style jetstream flow during winter by the warm oceans disrupting such and warming the air which turns north.
Observing wind and temperatures on earth.nullschool.net has shown below freezing air cannot exist long over the oceans and to reach low latitudes it can only move south over land, the continents and not over the sea.
Then, the warmer oceans have large-scale bound vortices, cyclones & anti-cyclones that work together in the North Pacific to move warm moist air northeast where it cools, dries then circulates south over land.
These prevent the Holocene jetstream from forming, no longer is there a procession of storms tied to a cold, dense jetstream that migrates eastward across North America in Rossby wave flow, that's disrupted.
Watching this winter in the Seattle area where cold northerlies dominated and temperatures stay below 50F/10C for weeks and this cold has persisted to create the expectation that it could continue and be the new normal for global atmospheric circulations.
So the thesis is that cold air cannot create Rossby wave flow due to the ocean heat disallowing transit at Holocene latitudes and cold only reaching low latitudes over land during winter and possibly all year; posting here for others to consider and comment.