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

This work presents an analysis of spectra and estimations of zonal wavenumbers for mean wind and semidiurnal tide amplitude oscillations observed during July-September 1998 by 3 meteor radars and one MF radar. A number of mean wind and tidal amplitude oscillations are found that are possibly related to the evolution of the summer mesospheric zonal wind jet. 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 similar to 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. (c) 2005 Elsevier Ltd. All rights reserved.