The role of annular modes in the dynamics of climate variability will be investigated, focusing especially on (i) the dynamical mechanisms by which changes in the structure and circulation of the stratosphere can affect the tropospheric zonal flow, and thence surface climate, (ii) the role that tropospheric and stratospheric eddy feedback processes play in the dynamics of this interaction, and (iii) the applicability of the fluctuation-dissipation theorem to the understanding of the response of the zonal flow to external perturbations. While this investigation is part theoretical analysis, the bulk of the research will comprise suites of experiments in a simplified stratosphere-troposphere general circulation model. Future experiments will include surface topography, in order to ensure a realistic level of stratospheric Rossby wave activity, as well as a realistic degree of non-zonality in the model climate. These studies will further understanding of atmospheric annular modes, and identify the processes that may provide sufficient coupling to lead to significant stratospheric influences on the troposphere.
Broader impacts of this research are for the practice of climate modeling: if modeling the stratosphere correctly is an important prerequisite to modeling surface climate, there are important implications for climate models. Further, this work will have implications for the understanding of the dynamical impact of imposed perturbations, especially of increasing greenhouse gases and ozone depletion on climate.
The aim of this project was the deeper understanding of the influence of stratospheric events on surface weather and climate. There is compelling evidence that the spectacular, if intermittent, events of northern hemisphere winter known as stratospheric warmings influence surface behavior, especially in the North Atlantic region, and that stratospheric ozone depletion over Antarctica has led to the poleward shift of the southern hemisphere westerly wind belt. How the coupling between these different altitude regions occurs is not well described, however. In one part of this project, we found that the magnitude of the tropospheric response is dependent not only on the strength of the stratospheric anomalies, but also on the receptiveness of the tropospheric flow to such perturbation. Some states are much more sensitive than others. One important consequence of this finding is that some climate models may be too sensitive, and thereby exaggerate the surface signal. A second component of the research was to use sophisticated dynamical/statistical techniques to clarify the nature of troposphere-stratosphere variability. We found that the variability is described by two dynamically separate modes, one tropospheric, one stratospheric, which interact together. This conclusion is different from what purely statistical techniques describe, which combine all the variability into a single mode. Thirdly, diagnosis of the angular momentum budget in a model simulation led us to conclude that exchange of momentum with the Earth’s surface during a stratospheric warming event (when large-scale waves amplify suddenly) is an important but subtle pathway for communication between stratosphere and troposphere. Publications: Chan, C., and R. A. Plumb: The response of the troposphere to stratospheric perturbations: dependence on the state of the troposphere. J. Atmos. Sci., 66, 2107-2115 (2009). Chen, G., and R. A. Plumb: Quantifying the eddy feedback and the persistence of the zonal index in an idealized atmospheric model. J. Atmos. Sci. 66, 3707-3720 (2009). Chen, G., R. A. Plumb and J. Lu: Sensitivities of Zonal Mean Atmospheric Circulation to SST Warming in an Aqua-planet Model. Geophys. Res. Lett., 37, L12701 (2010). Plumb, R. A.: Planetary waves and the extratropical winter stratosphere. In: "The Stratosphere: Dynamics, Transport and Chemistry," Polvani, Sobel & Waugh, eds., AGU Publications (2010).
