Transient Planetary Waves in the Winter Stratosphere

Power spectra and vertical structure of transient planetary waves are investigated using a space-time cross spectrum analysis. From the analysis of pressure-heights of three levels (30, 100 and 300mb) in the Northern Hemisphere for two winters, it is found that for the shorter period (around 15 days) waves in the stratosphere westward moving component prevails for the zonal wavenumber 1 while for the wavenumber 2 and 3 only eastward components have a significant power. It is also found that an eastward moving wave of wavenumber 1 with a period of about 30 days is present in the stratosphere which have<br>no corresponding wave in the troposphere. The vertical structure of eastward moving waves of the wavenumber 2 exhibits the characteristics of upward propagation whereas the westward moving waves of the wavenumber 1 exhibit somewhat complex features.<br>In the latter half, the characteristics of the vertical propagation of traveling planetary waves into the stratosphere are examined. The plausible vertical and meridional distribution of the mean zonal wind and the periodic amplification of waves at a lower boundary are prescribed. We select 6, 12, 24 and 48 days as the period of forcing. It is revealed that the planetary waves which have periods of either 12 or 24 days can penetrate into the upper stratosphere. As to wavenumber 2, the eastward moving modes with a period of 10-20 days become dominant at the 30km. On the other hand, as to wavenumber 1, the dominance of either eastward moving or westward moving depends upon the period of the wave. For periods longer than 20 days, eastward moving waves are more propagatable while for the period about 12-15 days, the westward moving wave becomes predominant at higher levels. All these waves have a westward tilt between the 15 km and 30 km levels. Thus, apart from the structure of the westward moving mode of wave 1, the agreement between the observa-<br>tion and computation is quite good. Especially, for the eastward moving mode of wavenumber 2, its transient features may be interpreted to be a manifestation of “selective propagation” due to the gross field of westerly shear flow and the phase velocity of wave itself.