Extreme variability of the stratospheric polar vortex

• Main Author: Mitchell, Daniel M.
• Published: University of Reading 2010
• Subjects: 551.55
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.646015

Over the last decade a number of studies have detected a degree of tropospheric predictability from knowledge of the stratospheric polar vortex, for example whether or not it is disturbed by planetary waves that propagate vertically from the troposphere, often resulting in sudden stratospheric warmings (SSWs). This thesis has built a comprehensive set of diagnostics to characterise the state of the stratospheric polar vortex using a 2D moment technique. This has allowed the area, centroid and aspect ratio of the vortex to be diagnosed in both reanalysis datasets and stratosphere resolving chemistry-climate models (CCMs). The first part of this thesis extends the technique of moment analysis so that it can be applied to all conceivable states of the vortex. These techniques are applied to the ERA-40 dataset and composites are built that characterise the structure and evolution of the vortex throughout the Northern Hemisphere winter and specifically during SSW events. It is found that use of the moment based diagnostics alone can determine whether the vortex is in a stable, displaced or split phase. The second part of this thesis compares the vortex diagnostics calculated for CCM output with those calculated from ERA-40 using a mixture of Gaussian and extreme value distributions. It is found that the models are able to capture the essential characteristics of SSW extreme events, but in general under estimate the frequency and over estimate the magnitude of these events.