| Summary: | Ph. D. University of KwaZulu-Natal, Durban, 2013. In this work, observational results of atmospheric dynamics caused by upward propagating atmospheric waves (tides, planetary waves and their interactions) in mesosphere-lower thermosphere (MLT) region are presented. This study is imperative as it contributes toward an understanding of various physical and dynamical processes that take place in this region. The seasonal and inter-annual variations of tides are investigated using MLT winds recorded simultaneously by SuperDARN HF radars situated at Halley (75°S, 26°W), SANAE (72°S, 3°W) and Syowa (69°S, 36°E) from 1998 to 2007. The seasonal variation of tides was found to be characterized by maximum amplitudes in summer and minimum amplitudes in winter. The semidiurnal tides showed additional enhancement of amplitude in autumn. The seasonal behavior of the diurnal tide (semidiurnal tide) was found to be similar to that of tropospheric specific humidity (stratospheric ozone mixing ratio) which suggests a forcing mechanism as a possible source of tidal variation. Long-term variation of semidiurnal tide was found to be correlated to F10.7 solar flux, which suggests solar activity as a possible driver of the semidiurnal tide variation. The variability of tides prior and post 2002 sudden stratospheric warming (SSW) event was studied using MLT winds derived from SuperDARN HF radars at Halley, SANAE and Syowa. Forcing mechanism using the ozone mixing ratio was found to be a possible source of semidiurnal tide (SDT) variability before the SSW event (160-250). Nonlinear interaction between planetary waves and tides on the other hand, was found to be a possible source responsible for the SDT variation just before, during and after the SSW event (250-300). Nonlinear interaction between planetary waves and tides in the MLT region was studied using wind velocity data collected from meteor radar located at Rothera (68°S, 68°W) Antarctica during the year 2005. Wavelet analysis conducted on the wind data showed that the MLT ...
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