%0 Thesis
%9 Doctoral
%A Matthewman, N.J.
%B Department of Mathematics
%D 2009
%F discovery:18711
%I UCL (University College London)
%P 326
%T A vortex dynamics perspective on stratospheric sudden warmings
%U https://discovery.ucl.ac.uk/id/eprint/18711/
%X A vortex dynamics approach is used to study the underlying mechanisms leading  to polar vortex breakdown during stratospheric sudden warmings (SSWs). Observational  data are used in chapter 2 to construct climatologies of the Arctic polar vortex  structure during vortex-splitting and vortex-displacement SSWs occurring between  1958 and 2002. During vortex-splitting SSWs, polar vortex breakdown is shown to  be typically independent of height (barotropic), whereas breakdown during vortex-displacement  SSWs is shown to be strongly height dependent (baroclinic).  In the remainder of the thesis (chapters 3-7), a hierarchy of models approach is  used to investigate a possible resonant excitation mechanism which is responsible for  the vortex breakdown seen in our observational study. A single layer topographically  forced vortex model is shown to exhibit vortex-splitting behaviour similar to that  observed during SSWs. Two analytical reductions, the first a fully nonlinear analytical  model of an elliptical vortex in strain and rotation velocity fields, the second a weakly  nonlinear asymptotic theory applied to a topographically forced vortex, show that  vortex-splitting in the model occurs due to a self-tuning resonance of the vortex with  the underlying topography.  Resonant excitation of an idealized polar vortex by topographic forcing is then investigated in a three-dimensional quasi-geostrophic model, with emphasis on the  vertical structure of the vortex during breakdown. It is shown that vortex breakdown  similar to that observed during displacement SSWs occurs due to a linear resonance  of a baroclinic mode of the vortex, whereas breakdown similar to that observed during  splitting SSWs occurs due to a resonance of the barotropic mode. The role of  self-tuning in these resonant behaviours is then discussed in relation to the analytic  reductions of the single layer model.