Dynamical Influences of Sudden Stratospheric Warmings on Surface Climate

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