Variability of the nonlinear interaction between ultrafast Kelvin waves and the Diurnal tide over the Brazilian equatorial region

Nonlinear interactions between atmospheric tides and planetary-scale waves play a key role in the redistribution of momentum and energy in the mesosphere and lower thermosphere (MLT). In this study, we investigate the occurrence and characteristics of nonlinear interactions between ultrafast Kelvin waves (UFKWs) and the diurnal tide over the Brazilian equatorial region, using one year of neutral wind measurements from an all-sky meteor radar at São João do Cariri (7.4°S, 36.5°W). Six UFKW events were identified through wavelet analysis. During these intervals, clear signatures of nonlinear interactions were detected, including modulation of the diurnal tide amplitude at UFKW periods and the presence of secondary waves at the sum and difference frequencies (1.25 and 0.75 cycles per day-cpd) with amplitudes of 5–15 m/s. Secondary waves also exhibited upward propagation and vertical wavelengths of 26–58 km, allowing them to reach the lower thermosphere. Comparisons between observed and theoretical vertical wavelengths revealed partial agreement with nonlinear interaction theory, indicating the importance of local atmospheric conditions. These results indicate that UFKWs play a significant role in short-term tidal variability and could contribute to vertical coupling processes in the equatorial atmosphere.