Observational evidence of the semiannual oscillation in the tropical middle atmosphere—A review |
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| Authors: | Isamu Hirota |
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| Institution: | (1) Geophysical Institute, Kyoto University, 606 Kyoto, Japan |
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| Abstract: | Observational studies on the semiannual oscillation in the tropical stratosphere and mesosphere are reviewed. Results of many statistics based on rocket and satellite observations reveal that the long-term behavior of the mean zonal wind exhibits two semiannual cycles which have their maximum amplitudes centered at the stratopause level and the mesopause level, each one being associated with the semiannual temperature variations predominating at levels about 10 km lower.Observational evidence obtained from recent studies of the dynamical properties of upper stratospheric waves strongly supports the theoretical consideration that the stratospheric semiannual oscillation is the manifestation of the wave-zonal flow interaction with alternating accelerations of the westerly flow by Kelvin waves and the easterly flow by planetary Rossby waves.Regarding the semiannual variation in the upper mesosphere, however, very little is known about the possible momentum source. Therefore, emphasis is placed on the need for further observations of the structure and behavior of the tropical middle atmosphere. |
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| Keywords: | Semiannual cycles Kelvin waves Rossby waves Wave-zonal flow interaction |
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