Interannual variability of the North Polar Vortex in the lower stratosphere during the UARS Mission

Northern winters since the 1991 launch of UARS are compared to earlier years (1978–1991) with respect to the potential for formation of Polar Stratospheric Clouds and for isolation of the north polar vortex. Daily NMC temperature minima at 465 K late in the 1993–94 winter and again in December 1994 were the lowest values experienced at those times of year (since 1978). Northern PV gradients were unusually strong in 1991–92 prior to late January and throughout the winter in both 1992–93 and 1994–95. Of all northern winters since 1978, 1994–95 with its early extended cold spell and persistently strong PV gradients most resembled the Antarctic winter lower stratosphere. Even so, temperatures were never as low, nor was the polar vortex as large, as during a typical southern winter. Judged by daily temperature minima and PV gradients at 465 K, meteorological conditions in the Arctic winter lower stratosphere during the UARS period were more conducive to vortex ozone loss by heterogeneous chemistry than in most previous winters since 1978–79.