On the day-to-day wind and semidiurnal tide variations at heights of the mid-latitude summer mesopause: Zonal wavenumber estimations and its consequences, case-study in 1998

The mean zonal wind undergoes several significant variations (“transitions”) during this time interval. These changes were accompanied by a frequency decreasing of the QTDW (quasi-2-day wave) in such a way that oscillations with a period of roughly about 36-44 h at the beginning of the measurements were replaced by strong 48-h waves, which in its turn after the end of the QTDW burst are replaced by a set of 3-6 day oscillations. Before the strong 48-h wave appearance we observe mean zonal wind variations with a temporal scale of 4 days. The set of 3-6 days includes 3-4 day waves with s∼3 and 5-6 day waves with s=1. The last wave is absent in the upper stratosphere during the time of observations and suggests to appear in the mesosphere. The both waves could represent a signature of the simultaneous non-linear excitation of a triad of waves. The zonal wind “transitions” also reflected in the semidiurnal tide by modulations of the tidal amplitudes with periods of 3-4 days and about 5 days. Some features observed at the end of summer 1998 could be explained by interaction between a zonally averaged wind variation and the semidiurnal tide. In a spectral representation, this variation looks like an oscillation, but is not a planetary wave. The strongest long-period modulations of the semidiurnal tide were observed during strong geopotential perturbations in the stratosphere of the Southern hemisphere.