Stochastic and dynamical models of sudden stratospheric warmings

点击翻译按钮获取中文摘要

Research Area: Continuum MechanicsThe aim of the project is to investigate the fundamental fluid dynamics of sudden stratospheric warmings, by developing a range of simplified models capturing the key fluid dynamical processes involved.Sudden stratospheric warmings (SSWs) are atmospheric phenomena that typically occur every two to three years during the Northern hemisphere winter. They involve a rapid increase in polar stratospheric temperatures of 20-30K over a period of just 1-2 days. There has also been a single recorded event in the Southern hemisphere, from nearly 70 years of recorded observations, which took place in 2002. SSWs are important for climate because considered to be the main source of interannual variability in the extratropical stratosphere, and over the last two decades have been the subject of much research interest, because they have been discovered to significantly influence Northern Hemisphere surface climate.SSWs are understood to be primarily fluid dynamical phenomena. The rapid increase in polar temperatures occurs because cold polar air, which is trapped within a columnar structure known as the polar stratospheric vortex, is displaced from the pole by the action of planetary-scale waves (Rossby waves). The aim of the current project is to bring understanding from fluid dynamics, particularly concerning the propagation of Rossby waves on vortices, to bear on the problem of understanding the occurrence of SSWs. The key question we will be concerned with is how the frequency of SSWs might change in a changing climate.The project will proceed by developing a hierarchy of simplified models of SSWs, each of which captures more detail of the fluid dynamical behaviour. The simplest model considered will be that of an elliptical vortex in a random straining flow. The question of interest is how the probability that the vortex becomes elongated (`strained out') depends upon the statistics of the straining flow. The random straining flow here represents the influence of the dynamically active troposphere on the stratosphere, and its statistics will be subject to change in a changing climate. Interestingly, the same model can be used to analyse vortex lifetimes in two-dimensional turbulent flows. The next model in the hierarchy will be a dry primitive equation model on the sphere, which will be set up to capture the fundamental fluid dynamical interactions between troposphere and stratosphere in a more realistic setting. The statistics of long integrations will be analysed, to determine whether insights from the simple elliptical vortex model are useful for predicting changes in model SSW occurrence. Conclusions will be drawn from a comparison between the models and the observed behaviour, with a view to explaining (a) predicted changes in SSW frequency, and (b) variations in behaviour between different future scenarios.