The stratospheric polar vortex in winter

In winter the polar stratospheric air within the earth's shadow forms the core of an intense ‘cold low’ which extends from about 10 km to at least 50 km and possibly to the base of the ionosphere. Compared with the tropospheric general circulation, this vortex seems to be remarkably stable, particularly in the Southern Hemisphere. Recent research work in Canada and the United States on the characteristics of the vortex is reviewed. The chief conclusions are that the vortex is more stable in the Southern than in the Northern Hemisphere, that ‘explosive warmings’ in the lower stratosphere of the Northern Hemisphere follow the breakdown of the vortex at high levels close to the pole, and that the breakdown extends from above downward over a period of several days. It is suggested that the high-level breakdown is correlated with solar activity.