| Summary: | <p>The polar vortex is a band of strong westerly winds that forms each winter and encircles the pole, high up in the atmosphere in a region known as the stratosphere. However this band of winds is not stable in the northern hemisphere, and often dramatically disintegrates in events known as sudden stratospheric warmings (SSWs). When SSWs happen they influence the weather occurring below, often leading to cold winters for northern Europe. SSWs are driven by the vertical propagation of waves from lower down in the atmosphere. Recent studies have suggested that the winds in the upper tropical and subtropical stratosphere may affect how these waves propagate, and whether they are able to influence the development of SSWs. </p> <p>In this thesis I investigate the various contributions different parts of the atmosphere make to the evolution of the SSW in the winter of 2008/9 by fixing different areas of the atmosphere to reality in a computer model, whilst letting the polar vortex evolve freely and comparing its evolution to reality. Fixing the lower atmosphere in this way prescribes the wave forcing entering the stratosphere, but this is not found to be enough on its own to generate an SSW close to that seen in 2008/9. Neither is fixing the upper stratosphere, above the polar vortex. It is only when the winds in the upper tropical stratosphere are fixed in addition to that of the lower atmosphere that a similar SSW occurs, consistent with the idea that this area of the atmosphere is important for determining the evolution of SSWs by affecting wave propagation.</p>
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