Abstract
The stratospheric winter polar vortex consists of strong westerly winds;
this winter flow regime can undergo a complete breakdown during sudden
stratospheric warming events. In the Northern Hemisphere these events are
often accompanied by the descent of anomalous flow regimes which can result
in extreme surface weather.
The focus of this thesis is to assess sudden stratospheric warmings and
their place in the coupled climate system. Portions of this work are dedicated
to algorithm development with the aim of quickly and accurately isolating
and subcategorising events. A method is successfully created that is computationally
cheap, easy to implement, based on dynamically relevant criteria
and has error rates clearly outlined.
Impacts on the surface and ocean are assessed with focus on specific subclasses
of sudden stratospheric warmings. It is found that there is, on average,
stronger surface and oceanic impacts following events that split the polar vortex.
The ocean system is impacted via modifications to the implied Ekman
heat transport and the net atmosphere-surface heat flux. Furthermore, there
is a relationship between the initial location of the disturbed polar vortex and
the strength of anomalous flow regime at the surface.
Analysis is conducted predominantly using general circulation model output,
with direct comparison between an atmosphere-only model and a coupled
atmosphere-ocean model. For the coupled model there is a reduction in the
number of simulated sudden stratospheric warmings, a result of altered atmospheric
wave dynamics. This is partially attributed to a cold bias over the
equatorial Pacific. The frequency of sudden stratospheric warmings is found
to be insensitive to North Atlantic sea surface temperature anomalies
