A vigorous, westerly flow of air in the upper and middle troposphere over mid-latitudes. Frauenfeld and Davis (2003) J. Geophys. Res. 108 think the Northern Hemisphere circumpolar vortex could be a good indicator of climate change. ‘During the summer–fall season, the trend towards stronger circumpolar flow [in the Southern Hemisphere] has contributed substantially to the observed warming over the Antarctic Peninsula and Patagonia, and to the cooling over eastern Antarctica and the Antarctic plateau’ (Thompson and Solomon (2002) Science 296).