Quasi-16-Day Waves in the Low-Latitude MLT: Meteor Radar Observations of Seasonal Variability, Propagation Preference, and Cross-Equatorial Coupling

Planetary-scale quasi-16-day waves (Q16DWs) are important dynamical features of the mesosphere and lower thermosphere (MLT), yet their characteristics at low and equatorial latitudes remain incompletely understood. This study presents a multi-station analysis of Q16DW activity using meteor radar wind measurements from São João do Cariri (7.4°S), Cachoeira Paulista (22.7°S), Santa Maria (29.7°S), and Thumba (8.5°N). Seasonal amplitudes exhibit a clear latitudinal contrast: tropical stations display summer maxima, while subtropical sites show classical winter peaks. Zonal propagation preference transitions from westward dominance near the equator to eastward dominance at higher latitudes, consistent with background-wind filtering under the Charney-Drazin criterion. At Thumba, temperature leads the zonal wind by 5 ± 1 days, and a semi-annual oscillation signature is evident in temperature. Simultaneous observations support cross-equatorial ducting of winter-hemisphere wave energy into the summer MLT along the zero-wind line. Recent evidence further indicates that quasi-2-day waves can act as intermediary carriers transporting Q16DW signatures across the equator. These results demonstrate that the low-latitude MLT is an active conduit for interhemispheric coupling rather than a region of weak planetary-wave activity. The findings provide observational constraints for global models and highlight the need for improved representation of equatorial waveguides, zero-wind-line migration, and wave-wave interactions in simulations of middle-atmosphere dynamics.