The planetary-scale cyclonic circulation, centered generally in the polar regions, extending from the middle troposphere to the stratosphere. The westerly airflow is largely a manifestation of the thermal wind above the polar frontal zone of middle and subpolar latitudes. The vortex is strongest in winter when the pole-to-equator temperature gradient is strongest. In the Northern Hemisphere, the vortex has two centers in the mean, one near Baffin Island and the other over northeast Siberia.