Gravity waves play a fundamental role in driving the large-scale circulation of the atmosphere. They are influenced both by the variation in their sources and the filtering effects of the winds they encounter as they ascend through the atmosphere. In this thesis we present new evidence that gravity waves play a key role in coupling the troposphere, stratosphere and mesosphere. In particular, we examine the connection of gravity waves to two important large-scale oscillations that occur in the atmosphere, namely the Madden-Julian Oscillation (MJO) in the troposphere and the Mesospheric Semi-Annual Oscillation (MSAO). We present the first ever demonstration that the MJO acts to modulate the global field of gravity waves ascending into the tropical stratosphere. We discover a significant correlation with the MJO zonal-wind anomalies and so suggest that the MJO modulates the stratospheric gravity-wave field through a critical-level wave-filtering mechanism. Strong evidence for this mechanism is provided by consideration of the winds encountered by ascending waves. The Ascension Island meteor radar is used for the first time to measure momentum fluxes over the Island. These measurements are then used to investigate the role of gravity-wave in driving a dramatic and anomalous wind event that was observed to occur during the first westward phase of the MSAO in 2002. Gravity waves are shown to play an important role in driving this event, but the observations presented here also suggest that the current theory of the mechanism describing these anomalous mesospheric wind events is not valid. Both of these studies highlight the critical importance of gravity waves to the dynamics of the atmosphere and highlight the need for further work to truly understand these waves, their processes and their variability.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:707571 |
| Date | January 2017 |
| Creators | Moss, Andrew |
| Contributors | Mitchell, Nicholas |
| Publisher | University of Bath |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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