During this project, the researcher has worked on understanding the physical processes that affect the downward impact of the stratosphere, in present and future climate.
Present climate (WP1+WP2): the work performed in this part of the project has included analysis of the changes in weather and climate following extreme events in Earth’s stratosphere. For this purpose, the researcher has analyzed observational (reanalysis) data as well as model output from an intermediate-complexity model. We show that in observations, a “canonical” downward impact after Sudden Stratospheric Warming events (for example, a southward shift of the jet stream and the storm track over Europe and the North Atlantic) is linked to a strong signal that descends to the lower stratosphere. In the model, there is a stronger connection between the eastern North Pacific and the North Atlantic, leading to a more consistent response in these two basins, compared to the observations.
To better understand what physical drivers influence the downward impact of the stratosphere, we used a new dataset: sub-seasonal to seasonal forecasts from the ECMWF. These forecasts provide an extended-range (up to 46 days) forecast of the atmospheric conditions. By analyzing the stratospheric influence on midlatitude storm track in the Euro-Atlantic sector, we show that the midlatitude storm track tends to shift equatorward after Sudden Stratospheric Warming (SSW) events, with reduced cyclone frequency in northern Europe. After strong vortex events, cyclone frequency is increased in northern Europe, and reduced further south. While in observations only two thirds of SSW events have a downward impact, the forecasts tend to be over-confident and predict a storm track response more often than it occurs in reality.
Future climate (WP3): using state-of-the-art climate models from the CMIP6 project, we explored, together with colleagues, what are the changes that stratospheric polar vortex is expected to undergo in climate change conditions. In another project, we show that climate change is associated with increased storm damage over central Europe, whereis a decrease in storm damage is expected in northern and southern Europe. These results indicate that predicting the storm track response to extreme events in the stratosphere would require reducing the uncertainty in more than one layer of the atmosphere: both the stratosphere and the troposphere.
To summarize, episodes of stratosphere-troposphere coupling have a long-lived influence on surface weather, especially in winter. Using a combination of reanalysis data, idealized models and operational forecast models, we are able to study what processes determine if there is going to be an impact of the stratosphere on surface weather in Europe and the North Atlantic, or not, and how well it is represented in forecast models. By addressing these fundamental questions, the goal of this research project was to improve the predictability of climate extremes on sub-seasonal to seasonal timescales, which is critical for a better management of their associated impacts on infrastructure and human lives.
