Quasi-geostrophic influence of the polar stratosphere on the troposphere

This thesis considers the tropospheric response to extreme events in the stratosphere known as sudden stratospheric warmings (SSWs). These events have been shown to influence the troposphere through persistent, descending circulation anomalies. The thesis quantifies and tests the hypothesis that changes in stratospheric potential vorticity remotely influence mass flux into the polar troposphere, and so influence tropospheric pressure, in the aftermath of SSWs. Firstly, the thesis introduces a new metric of stratosphere-troposphere coupling, defining a pressure index on height surfaces over the polar region which is demonstrated to capture the same behaviour as the traditionally used Arctic Oscillation (AO) index. Secondly, quasi-geostrophic (QG) theory is used to derive expressions for horizontal and vertical mass flux into the polar region, which force the polar pressure index. The QG horizontal mass flux is found to be non-linear in the streamfunction associated with a QG potential vorticity (QGPV) distribution, with the interaction of mass flux associated with stratospheric QGPV and with tropospheric QGPV being substantial in the troposphere. Pressure index tendencies are approximated using these mass flux expressions, and found to generally compare with reanalysis moderately well in the stratosphere but poorly in the troposphere. Two algorithms used to define and categorise SSWs as splits and displacements are compared in this mass flux framework, with the approximation of the influence of vortex displacements by QG theory being more accurate than the approximation of the influence of vortex splits. There is little difference seen in the tropospheric pressure response to vortex split and vortex displacement SSWs. Lastly, the stratosphere is not demonstrated to substantially influence the polar troposphere via QG mass fluxes in the aftermath of SSWs given streamfunctions calculated in this thesis.