On the variability of the diurnal tide and coupling with planetary waves in the MLT over Cachoeira Paulista (22.7°S, 45°W)

• Temporal variability of the diurnal tide amplitude is studied in the MLT.
• Possible coupling between the diurnal tide and planetary waves is observed.
• Bispectral analysis showed triplets of interacting diurnal tide and planetary waves.

Using meteor radar observations of four years in the mesosphere and lower thermosphere (MLT) over a subtropical Brazilian station, Cachoeira Paulista (22.7°S, 45°W), the temporal variability characteristics of the diurnal tide have been studied. In addition to the semiannual, annual and interannual variations, the diurnal tide amplitude exhibits clear intermittent modulation at periods of planetary waves. The tidal amplitude exhibits clear seasonal pattern with largest amplitude in fall equinox. The dominant periods of modulation of the diurnal tide are found to be greater than 10 days in the MLT. The diurnal tide, as detected in the power spectra of the horizontal winds, shows a spread in period around the central period (24 h) which is an indication of nonlinear interactions between the diurnal tide and planetary waves. A bispectral analysis reveals prominent triplets (two primary waves and a secondary wave) confirming the interaction of the diurnal tide with planetary waves persistent over a broad spectral range. Also there is an indication of coupling of the diurnal tide with the intraseasonal oscillations at various times of the year.