We studied the occurrence characteristics and variability of the terdiurnal tide (8 hour period) in the equatorial mesosphere and lower thermosphere (MLT), using a meteor radar at Koto Tabang (0.2°S, 100.3°E) and MF radars at Tirunelveli (8.7°N, 77.8°E) and Pameungpeuk (7.4°S, 107.4°E). These locations, one being located right over the equator and the other two at conjugate points around the equator within 10, form a unique experimental setup to study equatorial MLT dynamics. The terdiurnal tide exists as a distinct wave signature at all three locations. While the daily amplitudes can be as large as 15 m s-1, the monthly mean amplitudes lie between 1 and 10 m s -1. The amplitude of the terdiurnal tide at Pameungpeuk is generally smaller than that observed at Tirunelveli and Koto Tabang. The seasonal variation in amplitude shows both annual and semiannual oscillations of ∼1 m s-1 at all three locations. The present observations combined with previous reports indicate that the timing of the primary maximum of the terdiurnal tide amplitude shifts from autumn to late spring and early summer as one moves from high latitudes to the equator (all with respect to the Northern Hemisphere). The amplitudes and seasonal variation in the present observations show good comparison with that simulated by the General Circulation Model (GCM) developed by Kyushu University, Japan. This study supports the occurrence of nonlinear interaction between diurnal and semidiurnal tides and shows that gravity waves play an important role in the generation of the terdiurnal tide.
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