Normal Mode Rossby Waves Observed in the Upper Stratosphere. Part I: First Symmetric Modes of Zonal Wavenumbers 1 and 2

Abstract

A global analysis is made of large-scale traveling planetary waves in the upper stratosphere by the use of stratospheric height and thickness data up to the 1 mb level derived from the Stratospheric Sounding Unit (SSU) on TIROS-N and NOAA-A satellites for the period December 1979 through November 1981. The results of space-time power and cross-spectral analyses show the global existence of a westward traveling wave of zonal wavenumber 1 with a period of about 5 days and a westward traveling wave of zonal wavenumber 2 with a period of about 4 days. The structure and behavior of these waves are investigated by using the components of these period bands separated by a numerical band-pass filter. The 5-day wave is very similar to the (1, 1) mode, which is the first symmetric wavenumber 1 Rossby normal mode in an isothermal atmosphere. Similarly, the 4-day wave appears to correspond to the (2, 1) mode. The vertical structure of both waves agrees well with the theoretical expectation of free external waves. With regard to the seasonal variation, both waves are irregularly predominant throughout the year except for the December?January?February season, and this evidence is significantly related to the temporal variation of the refractive index of the zonal mean field.