Equatorial Waves in the Upper Stratosphere and Mesosphere in Relation to the Semiannual Oscillation of the Zonal Wind

Abstract

An analysis is made of equatorial wave disturbances in the upper stratosphere and mesosphere by the use of meteorological rocket and satellite observations, to clarify their structure and behavior in relation to the semiannual oscillation of the mean zonal wind. From a power spectral analysis of wind and temperature over Ascension Island during the four years from 1969 to 1972, it is found that in the height region between 25 and 60 km there exists a wave disturbance with a characteristic vertical scale of 15–20 km; the wave is more active in the easterlies than in the westerlies, showing marked semiannual variation. Compared with the well-known characteristics of equatorial waves in the lower stratosphere, this wave is likely to he identified as a Kelvin wave with a period of about 10 days. It is suggested that this wave plays an essential role in producing the semiannual reversal of the mean zonal wind by supplying the westerly momentum to the equatorial mesospheric levels. Further discussions are made of the long-term behavior of mesospheric planetary Rossby waves as observed from the Nimbus 6 Pressure Modulator Radiometer, in connection with the semiannual cycle in the upper mesosphere.