The cooling of the polar regions give rise to the polar vortex in the winter
The polar vortex is a vortex of descending air over a polar region in the winter. The warmer the lowest and the densest part of the atmosphere called troposphere is the higher its upper edge is. At the equator, the troposphere is about 15 kilometres high and over the poles its average height is about eight kilometres. That's because cold air is denser than warm air. In the winter, the difference becomes pronounced as a polar region does not get any solar irradiation in the middle of the winter. Air flowing towards a polar region in the upper troposphere sinks as it circulates it in a vortex. Where the slope is the steepest, there is a band of high velocity winds called the jet stream.
The jet stream determines the trajectories of the centers of low pressure at ground level to large degree. A well-formed polar vortex effectively keeps the centers of low pressure on a northerly course and the cold air mass in the polar region fenced in preventing it from making forays into lower latitudes. The strength of the polar vortex fluctuates during the course of the winter. If the polar vortex is weak it means that the jet stream will become slow and it begins to meander letting cold polar air into the lower latitudes and causing unusually cold winter weather in normally warm areas. This year's cold snap in Europe and North America in February and early March was caused by the polar vortex breaking up into two vortices. One ended up over Northern Europe and one over North America. The polar region was penetrated by an unusually warm mass of air disrupting usual weather conditions. Climate change will increase the likelihood of events like this. The polar regions are warming faster than the rest of the globe.