On the variability of the terdiurnal tide over a Brazilian equatorial station using meteor radar observations

Features of the terdiurnal tide in the mesosphere and lower thermosphere (MLT) are investigated using four years (2005-2008) of meteor radar wind data from a southern hemisphere equatorial station, São João do Cariri (7.4°S, 36.5°W). Prominent existence of the terdiurnal tide is found to be a permanent feature of the MLT over the present site. It exhibits considerable inter-annual variability with smaller amplitude during 2007. Maximum monthly mean amplitudes of the terdiurnal tide are found to be ~11 and 14 m/s in the zonal and meridional winds, respectively. The meridional wind shows sharp amplitude enhancement during fall equinox and a broader peak during late winter and spring equinox. Phase profiles indicate large vertical wavelength and possible wave evanescence. The generation mechanism of the terdiurnal tide has been investigated and it is inferred that nonlinear interaction between the diurnal tide and semidiurnal tide/gravity waves can partially excite the terdiurnal tide during fall equinox and solar heating is believed to be the primary source at the present latitude throughout the year.