Stratospheric Long Waves: Comparison of Thermal Structure in the Northern and Southern Hemispheres

Conway B. Leovy Department of Atmospheric Sciences, University of Washington, Seattle 98195

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Peter J. Webster Department of Atmospheric Sciences, University of Washington, Seattle 98195

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Abstract

With the aim of comparing planetary-wave behavior in the Northern and Southern Hemispheres, we have examined stratospheric temperature/thickness data for 1971–72 using Nimbus 4 Selective Chopper Radiometer (SCR) data, National Meteorological Center (NMC) gridded data, and Eole constant-level balloon data. Properties of planetary waves 1 and 2 derived from the SCR and NMC data are shown to agree well. Large-amplitude oscillations with energy in the 20–30 day period range and eastward propagating wave 2 in the Southern Hemisphere have been noted previously and are prominent in these data. Strong vertical coherence in wave phase at all levels between 200 mb and the upper stratosphere is observed. Additional features of interest in this data set include the following: 1) the upper stratosphere in mid-winter appears to satisfy the necessary condition for instability derived by Charney and Stern (1962), especially in the Southern Hemisphere; 2) wave amplification is closely associated with eastward phase propagation; 3) wavenumber 2 amplitude is highly asymmetric with respect to the solstice in both hemispheres and at least in the north the asymmetry is prominent in the upper troposphere as well as the stratosphere; and 4) the meridional extent of the wave amplitude is directly related to zonal wind speed. Some tentative interpretations of these observations are offered.

Abstract

With the aim of comparing planetary-wave behavior in the Northern and Southern Hemispheres, we have examined stratospheric temperature/thickness data for 1971–72 using Nimbus 4 Selective Chopper Radiometer (SCR) data, National Meteorological Center (NMC) gridded data, and Eole constant-level balloon data. Properties of planetary waves 1 and 2 derived from the SCR and NMC data are shown to agree well. Large-amplitude oscillations with energy in the 20–30 day period range and eastward propagating wave 2 in the Southern Hemisphere have been noted previously and are prominent in these data. Strong vertical coherence in wave phase at all levels between 200 mb and the upper stratosphere is observed. Additional features of interest in this data set include the following: 1) the upper stratosphere in mid-winter appears to satisfy the necessary condition for instability derived by Charney and Stern (1962), especially in the Southern Hemisphere; 2) wave amplification is closely associated with eastward phase propagation; 3) wavenumber 2 amplitude is highly asymmetric with respect to the solstice in both hemispheres and at least in the north the asymmetry is prominent in the upper troposphere as well as the stratosphere; and 4) the meridional extent of the wave amplitude is directly related to zonal wind speed. Some tentative interpretations of these observations are offered.

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