Interannual Variations of Planetary Waves in the Southern Hemisphere Stratosphere.

Abstract

A statistical analysis is made of the interannual variability of stratospheric planetary waves in the Southern Hemisphere (SH) in late winter using NCEP/NCAR reanalysis data gathered over a 20 year period from 1979 to 1998. The study investigates the dynamical coupling between the planetary waves and the tropospheric circulation, by focusing on the activity of transient baroclinic disturbances. With the aid of EOF analysis applied to the 10 hPa monthly mean height anomalies and tropospheric circulation patterns, it is found that the year-to-year variation of quasi-steady planetary waves in late winter (September and October) is characterized by a longitudinal phase shift of zonal wavenumber 1 as well as by a variation in wave amplitudes. An eastward (westward) shift of the stratospheric planetary waves corresponds to a double-jet (single-jet) structure of the upper tropospheric (300 hPa) zonal winds. On the other hand, planetary wave amplitude variation is closely related to that of the meridional heat flux in the upper troposphere. These results are reinforced by the Eliassen-Palm flux diagnosis composed of the stratospheric EOF scores, and hence the effect of tropospheric transient waves on the forcing to the stratospheric planetary waves is stressed.