ISENTROPIC MIXING IN THE ARCTIC STRATOSPHERE DURING THE 1992-1993 AND1993-1994 WINTERS

Dynamic isolation of the winter Arctic circumpolar vortex during 1992- 1993 and 1993-1994 (the second and third northern hemisphere winters o f the UARS mission) is studied using quasi-horizontal isentropic traje ctories. Ejection of vortex air and entrainment of mid-latitude air in to the vortex are quantified and compared with climatological values o btained from the analysis of 16 Arctic winters. A number of unusual fe atures of both winters are discussed. The most notable features are th e anomalous isolation experienced by the vortex during December 1992 a nd the unusual degree of isolation acid persistence of the vortex duri ng February and March of both years. The 1992-1993 winter season is th e most consistently isolated vortex on record. Only during January 199 3, when entrainment is large, is this pattern of extreme isolation bro ken.