Study of the ultra-fast Kelvin wave with meteor radar observations over a Brazilian extra-tropical station

Characteristics of the 3.5-day ultra-fast Kelvin (UKF) wave are investigated with 4 years of meteor radar wind observations over Cachoeira Paulista (22.7°S, 45°W). The UFK signature manifests itself as discrete bursts spanning over various times of the year with generally less amplitude during fall equinox and early winter. Daily as well as seasonal mean amplitude of the UFK component is found to be considerably smaller than that observed previously from equatorial stations. Estimated vertical wavelengths of the UFK wave come out to be larger than 40 km. Comparison of the present results with another Brazilian equatorial station implies latitudinal invariance of the wave as well as transient local dynamical influence to its variability. High zonal acceleration caused by momentum flux convergence due to the UFK wave just after equinoxes is believed to be responsible for driving the westerly phase of the mean zonal wind immediately after the equinoxes and hence the mesospheric semiannual oscillation to some extent. There is a clear signature of modulation of the UFK wave amplitude by various harmonics of the annual oscillation.