The Propagation and Mean-Flow Interaction of Waves in the Upper Troposphere and Lower Stratosphere.

By viewing the upper troposphere and lower stratosphere in terms of wave propagation and effects, new insight is gained into the general circulation and dynamical events of this region. Anomalous periods of wave propagation indicate that planetary-scale waves often propagate quite differently from the time-mean view. During many winter months, wavetrains are observed to propagate northeastward out of the tropical Pacific. Bursts of wave propagation into the stratosphere and convergence onto the polar cap occasionally result in stratospheric warmings. Although the mid-stratospheric height field is characterized mainly by zonal wave numbers one and two, the flow at this level is rich with small-scale structure. This structure must be considered for an understanding of the climatology of the stratosphere. The movement of air, as traced by potential vorticity (PV) or material tracers shows that the stratospheric flow is far from uniform, and that small-scale eddies, created as planetary-scale waves "break", act to shape and erode the vortex. By comparing stratospheric vorticity and PV maps for the same days, it is clear that outside the polar vortex, vorticity contains almost all of the information in PV, with PV being greater than absolute vorticity in the vortex due to reduced static stability. By examining nineteen years of data at the 850K isentropic surface, it is found that McIntyre and Palmer's wavebreaking/vortex erosion hypothesis has strong support. The process of vortex erosion ("preconditioning") occurs before every major warming. Moreover, the process of preconditioning together with the warming are seen as one continuous event when viewed in terms of the wavebreaking process. During most winters bursts of planetary-wave activity cause wavebreaking and marked decreases in vortex area. Such wavebreaking episodes initially involve low values of PV, but involve higher values as the vortex area is decreased. The culmination of many such events is a displacement and/or breakup of the polar vortex as low PV air moves over the pole. These events are identified as stratospheric warmings in zonal mean wind and temperature profiles. Thus, a consistent and dynamically-based definition of a sudden warming is a displacement of the polar vortex off the pole, with low PV air moving over the pole.