A vortex dynamics perspective on stratospheric sudden warmings
Matthewman, N. J.
- Publisher: UCL (University College London)
arxiv: Physics::Atmospheric and Oceanic Physics | Condensed Matter::Superconductivity
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.
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