An update on dynamical changes in the Arctic and Antarctic stratospheric polar vortices

Dynamical changes in the Arctic and Antarctic lower stratosphere from autumn to spring were analysed using the NCEP/NCAR, ERA40 and FUB stratospheric analyses for three periods: 1979-1999, 1979-2005, and 1965-2005. We found a weakening of the Arctic vortex in winter and a strengthening in spring between 1979/1980 and 1998/1999, with corresponding changes in the zonal mean circulation. The vortex formed earlier in autumn and broke down later in spring. These changes however were statistically not significant due to the high interannual dynamical variability in northern hemisphere (NH) winter and spring and the relatively short time series. In the Antarctic, the vortex formed earlier in autumn, intensified in late spring, and broke down later. The changes of the Antarctic vortex were at all levels and for both autumn and spring transitions larger and more significant than the changes of the Arctic vortex. These changes of the 1980s and early to mid 1990s were however not representative of a long-term change. The dynamically more active winters in the Arctic and Antarctic since 1998/1999 led to an enhanced weakening of the polar vortex in winter, and to a reduction of the polar vortex intensification in spring. As two of the recent Arctic major warmings occurred rather early in winter the polar vortex could recover in late winter and the delay in spring breakdown further increased. In contrast, the increase in Antarctic vortex persistence did no longer appear when including the recent winters due to the dominant impact of the three recent dynamically active Antarctic winters in 2000, 2002, and 2004. The longterm changes of 1965/1966-2005 were smaller in amplitude and partly opposite to the trends since the 1980s. There is no significant long-term change in the Arctic vortex lifetime or spring persistence, while the Antarctic vortex shows a long-term deepening and shift towards later spring transitions. The changes in the stratospheric dynamical situation could be attributed in both hemispheres to changes in the dynamical forcing from the troposphere.