Evolution of potential vorticity in the winter stratosphere of January‐February 1979

Abstract

Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) observations are used to study the evolution of potential vorticity in the stratosphere, January–February 1979. Daily analysis of this quantity at 850° and 1200°K provides circumstantial evidence of planetary wave “breaking” by which air parcels undergo rapid and irreversible separation from the circumpolar vortex during stratospheric warmings. Complementing this effect is the advection of subtropical, low‐vorticity air into the polar region. Temporal evolution of the size, shape, and orientation of the main circumpolar vortex is revealed very clearly by the potential vorticity field. All three factors are important, although some have been emphasized more strongly in previous literature. The size of the vortex determines the range of latitudes over which planetary Rossby waves are able to propagate vertically. Diminution of vortex area during the observed warmings is believed to precondition the flow, focusing subsequent Rossby wave activity into the polar cap, as in the major warming of late February 1979. The shape of the vortex undergoes both reversible and irreversible deformation. Examples of irreversible deformation are seen in the advective formation of extended high‐vorticity tongues over subtropical latitudes in connection with the warmings of late January, early February, and late February 1979. Two of these were recently discussed by McIntyre and Palmer. Reversible deformation is observed in the sudden cooling and concurrent wave 1, wave 2 vacillation, after the January warming. The orientation of the vortex can also be important, as in the period of rotation leading up to the major wave 2 warming of late February 1979. We suggest that the orientation of the vortex be included as part of the preconditioning process, in accord with numerical results of Butchart et al. We briefly consider the vertical structure of potential vorticity and ozone on two disturbed days in late January 1979. Meridional cross sections of potential vorticity in the middle stratosphere resemble ozone cross sections shown in the work of Leovy et al. and exhibit good vertical coherence extending into the lower mesosphere, where ozone (unlike potential vorticity) no longer serves as a tracer of the motion.