Internal gravity waves which propagate upwards from the lower atmosphere transport energy and momentum into the middle and upper atmosphere. As these waves dissipate, they alter the dynamics and composition of the background atmosphere. Changes in the background also affect the dissipation of these waves. Through this interplay, gravity waves couple the lower and upper atmosphere. Recent improvements in our understanding of gravity waves have allowed detailed physical representations of these waves to be included in global-scale atmospheric models. Three models have been developed for this study which are used to examine the role of gravity waves in the dynamics of the middle and upper atmosphere over a range of time-scales.\ud The effects of the solar sunspot cycle on gravity waves in the middle atmosphere are explored. Changes in the wave-induced wind variance of up to 10 % are simulated in the mesosphere. It is shown that a strong change in the tropospheric gravity-wave source could explain observed variability in gravity waves in this region, but changes in the mesospheric winds also play a crucial role in the model results.\ud Varying the amplitude of the lower atmosphere gravity-wave source affects gravity-wave drag and the residual circulation. These are shown to influence conditions in the stratosphere and can alter the strength of sudden warmings. This has important consequences for the whole atmosphere.\ud The interaction between gravity wave-drag and the diurnal tide is investigated using a coupled middle atmosphere and thermosphere model. This is shown to have a strong influence on dynamics and airglow emissions in the equatorial MLT region. Further constraint of the lower-atmosphere gravity waves and time dependent propagation effects are needed to improve our understanding of the role which gravity waves play in the dynamical coupling of the atmosphere
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