The downward impact of the stratosphere in current and future climates

ended

One of the most striking long-lived influences on the weather in Europe comes from the stratosphere, a highly-stratified layer of the atmosphere starting at about 10 km above the surface. The stratosphere impacts surface weather through coupling between the stratospheric polar vortex and the tropospheric jet streams: Extreme events in the stratosphere, so-called sudden stratospheric warmings, tend to be followed by an equatorward shift of the North Atlantic jet stream and significant changes in weather patterns over Europe, including cold spells over Scandinavia and increased rainfall over the Mediterranean. It is however unclear how the coupling between the stratosphere and the troposphere will change in a future climate. Projections demonstrate great uncertainty in the stratospheric response to climate change, with a significant spread among the models, even in the sign of the response. Uncertainty also exists in the tropospheric response to climate change, with a considerable spread in climate model predictions in terms of the jet position, particularly in winter, when the stratospheric influence is strongest. It remains unclear how the combination of the changes in both the stratosphere and troposphere may alter the downward impact of the stratosphere. I propose to address these questions through observational data analysis and a hierarchy of numerical models. I will use an idealized modelling framework, designed to capture major underlying processes of stratosphere-troposphere coupling, to isolate the dominant factors that control the downward impact of the stratosphere, and their response to climate change. The role of transient eddies in maintaining the downward response will be investigated. A comparison to more complex climate models will provide further insights into dynamical coupling. Better understanding the connection between the stratosphere and surface climate is essential for reducing the uncertainty in climate models and extreme events prediction.