Most of the time, the winter circulation of the stratosphere over the Northern Hemisphere polar cap, or the stratospheric polar vortex, is approximately symmetric about the pole, but occasionally it transitions to a dramatically wave-like state, in which stratospheric temperatures rise by as much as 40 Celsius degrees in less than a week. These transitions are termed sudden stratospheric warmings (SSWs), and they constitute the dominant form of variability in the polar stratosphere. This research project will examine the extent to which the stochastic variability of small-scale gravity waves (i.e. buoyancy oscillations) are an important factor in producing SSWs. The work is based on the idea of noise-activated transitions which are known to occur in simple dynamical systems in which two stable equilibrium sates are possible. The research will be conducted with a hierarchy of models of differing complexity, starting from a three-parameter system which is known to have two stable states and progressing to a global atmospheric model with a full three-dimensional representation of the flow in the stratospheric polar vortex.
The work will have broader impacts by contributing to the education of a graduate student. Results of the research will contribute to our understanding of SSWs, which have been shown to be consequential for surface weather, and may be of value for improving weather and climate models.
Sudden stratospheric warmings (SSWs) are abrupt transitions of the polar winter stratospheric circulation, from strong eastward to weak westward circumpolar flow. They are accompanied by explosive local warming (up to 50 K within a week) of the polar winter stratosphere, giving these phenomena their name. SSWs represent the dominant source of variability in the winter stratosphere, with implications for ozone chemistry, but also for surface weather and climate through stratosphere-troposphere coupling. Despite many improvements in our understanding of the phenomenology and dynamics of SSW events, we still lack a detailed understanding of the mechanisms at play forcing SSWs. The interaction between planetary waves and the background mean circulation in the stratosphere is known to be crucial. Planetary waves are generated by surface inhomogeneities such as land-sea contrast and large mountain ranges and can propagate up to stratospheric levels during winter. Strong enough planetary wave forcing from below is necessary, but does not appear to represent a sufficient condition in forcing SSWs. In this project we have studied the dynamics of SSWs using a hierarchy of idealized models, from simple toy models that can be partially solved analytically to idealized general circulation models. These model simulations were complemented by meteorological reanalysis data for the past ~35 years. Our work has elucidated several aspects of the dynamics of SSWs. A preferred planetary wave forcing time scale of SSWs was found (of the order of 10 days) in both, the models of varying complexity and reanalysis data. Our analysis revealed that wave forcing of shorter time-scale either falls short of accumulating enough total forcing or leads to wave reflection instead of absorption. On the other hand, wave forcing of long time-scales is increasingly radiatively damped preventing effective forcing of the background flow. We have furthermore elucidated the role of small scale gravity wave forcing. This small scale forcing, despite being insufficient by itself to lead to significant stratospheric background flow changes, was found to play an important role in the preconditioning of the stratospheric circulation in order to make it conducive to amplified wave growth and eventually the breakdown of the circulation (a SSW). Gravity wave effects need to be parametrized in current global scale weather forecast and climate models. Highlighting their role in important large-scale phenomena such as SSWs can help improve these models. It can also motivate pushing toward finer resolutions, in particular for climate models. Our research results support the notion that SSWs represent a type of resonant interaction between waves of planetary scale and the background circulation. Specifically, our results underline the importance of a positive feedback between the waves and the background flow, in which wave amplitudes grow as the background flow decelerates leading to further wave growth with further background flow deceleration and so on. That is, the background flow decelerations leading to SSWs are as much causing enhanced wave fluxes as they are caused by these enhanced wave fluxes. SSWs appear to be primarily forced by wave fluxes that are constructively redistributed or generated at or above the tropopause, and appear to be forced much less by anomalous wave fluxes from the troposphere below.
