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.
Background: From about the last decade the jetstream took to looking like a goat's head at times and couldn't penetrate the west coast sending most flow back north over the blockage that my thinking was from it being warmer thus less dense to manage getting over the mountains with a persistent high-pressure over the midwest and central USA.
Today: The cold air from East Siberia, a northwesterly hits the Bering Sea and turns right back north aided by a persistent cyclone over the eastern Bering Sea that's quasi-stationary, fading & reappearing along with a bound cyclone in the Gulf of Alaska and typically an anti-cyclone off the Oregon-California latitudes.
These have one side over warm ocean the other cold land, significantly they can be defined as bound votexes by the heat-transfer of warm-moist air north and cold south exclusively over land what's being observed.
This implies weather is now being driven by the ocean heat, not polar cold circulating in a jetstream driving cyclonic storms tied to a Rossby wave progression west-to-east.
Tentative Conclusion: Weather in the northern hemisphere is now being driven from the bottom of the atmosphere up, not a cold polar circulation forcing its way south using a high-density stream of air.