Seasonal variation of planetary waves in the stratosphere observed by the Nimbus 5 SCR

Abstract

The structure and behaviour of planetary-scale temperature waves in the stratosphere are described by the use of observations from the Selective Chopper Radiometer on the Nimbus 5 satellite during the two years from January 1973 to December 1974. From a latitude-height diagram of monthly root-mean-square wave amplitudes, it is found that the planetary wave of wavenumber one is predominant in the upper stratosphere at middle and high latitudes throughout the year for both hemispheres. Waves of higher wavenumbers are rather weak in the summer hemisphere. Power spectral analysis of monthly r.m.s. wave amplitude for 24 months shows that the 12-month period oscillation is prominent in middle and higher latitudes, while a 6-month periodicity is observed in tropical latitudes for wavenumber one. The phase of maximum of the 6-month cycle is about one month after the equinoxes, suggesting that planetary wave activity in lower latitudes is closely related to the semiannual oscillation of the equatorial zonal current in the upper stratosphere. Further analyses are made of seasonal characteristics of ‘transient’ waves to find the relationship between the wave motion and the mean zonal wind velocity. The predominant period (or phase velocity) is determined by a power spectral analysis using the Doppler effect due to the east-west motion of waves. With wavenumber one in the upper stratosphere, a 15- to 20-day period standing oscillation is observed in wintertime, while a 10-day period westward moving wave is found in the summer hemisphere. During the spring and autumn, a standing oscillation with a period of about 3 weeks can be seen together with an 8- to 10-day period westward motion. Higher wavenumbers show eastward motion in winter and no significant periodicity is found in other seasons as the wave activity is weak. These characteristic features of transient waves are explained by the theory of critical line absorption for vertical wave propagation through the mean zonal wind.