The high-altitude polar circulation that can weaken and split in winter is the polar vortex.
A polar vortex is a large region of cold, low-pressure air rotating around one of Earth’s poles. The strongest and most discussed version forms in the stratosphere during the polar winter, when the lack of sunlight allows the air over the pole to become extremely cold.
Sudden stratospheric warming events can rapidly disturb the northern polar vortex. Temperatures in the stratosphere rise, and the circulation may be displaced from the pole or split into separate lobes. Such events can influence the jet stream and increase the chance of cold outbreaks in some mid-latitude regions, but they do not guarantee severe cold everywhere.
The term is often confused with ordinary winter weather. A polar-vortex disturbance occurs high in the atmosphere, while local temperatures also depend on pressure patterns, snow cover, winds, and ocean conditions.