The 6.5-day wave observed by meteor radar over Cachoeira Paulista (22.7 S)

The 6.5-day wave in the mesosphere and lower thermosphere at low latitudes in the southern hemisphere is investigated through analysis of horizontal winds data, obtained by meteor radar over Cachoeira Paulista (22.7 S, 45 W), Brazil, from April 1999 to December 2003. The zonal wind spectral analysis shows that the presence of the 6.5-day wave is a common phenomenon during austral spring equinoxes in this region. Unlike the HRDI/UARS satellite observations for the equatorial region (Talaat et al., 2001), oscillations of 6.5-day were not observed during austral autumn equinoxes. Vertical amplitude and phase structures were determined by harmonic analysis and the results show maximum wave amplitudes of about 20 m/s above 90 km and vertical wavelengths taking values up to 110 km. Year-to-year variability was observed with the wave activities being more intense during QBO westward phase years. On some occasions, the quasi simultaneous presence of the 4-, 5-, 6.5-, 10-, and 16-day oscillations suggests the possibility that they might be inter-related.