One of the most striking long-lived influences on the weather in Europe comes from the stratosphere, a highly stratified, stable layer of the atmosphere at 10 to 50 km above the surface. The stratosphere influences surface weather through coupling between the stratospheric polar vortex and the tropospheric jet streams[1],[2],[3]. Particularly, extremely weak polar vortex events, known as sudden stratospheric warmings (SSW), can have a prolonged downward impact on the large-scale atmospheric circulation, including an equatorward shift of the tropospheric jet stream over the North Atlantic, cold spells over Scandinavia and increased rainfall over the Mediterranean. These anomalies in surface weather can persist for up to two months. Future climate projections demonstrate great uncertainty when it comes to the stratospheric response to climate change. In particular, the strength of the stratospheric circulation exhibits a wide spread among the models, even in the sign of the response[4],[5]. Such uncertainty in the stratospheric circulation response is suggested to significantly affect the downward influence on the troposphere. Uncertainty also exists in the tropospheric circulation response to climate change[6]. While most general circulation models (GCMs) predict a poleward shift of the midlatitude jet streams and the associated storm tracks in a warmer climate[7],[8],[9], the model spread is considerable, particularly in winter, when the stratospheric influence is strongest. It is therefore unclear how the combination of these changes in both the stratosphere and the troposphere may alter the tropospheric response to stratospheric forcing under climate change. Furthermore, storms are expected to become more intense as the storm tracks shift poleward, increasing rainfall and the risk of flooding in the extratropics[10],[11]. 

 

[1] Baldwin, M. P., & Dunkerton, T. J. (1999). Propagation of the Arctic Oscillation from the stratosphere to the troposphere. Journal of Geophysical Research: Atmospheres, 104(D24), 30937-30946.

[2] Charlton, A. J., O'Neill, A., Stephenson, D. B., Lahoz, W. A., & Baldwin, M. P. (2003). Can knowledge of the state of the stratosphere be used to improve statistical forecasts of the troposphere?. Quarterly Journal of the Royal Meteorological Society, 129(595), 3205-3224.

[3] Scaife, A. A., Knight, J. R., Vallis, G. K., & Folland, C. K. (2005). A stratospheric influence on the winter NAO and North Atlantic surface climate. Geophysical Research Letters, 32(18).

[4] Manzini, E., Karpechko, A. Y., Anstey, J., Baldwin, M. P., Black, R. X., Cagnazzo, C., ... & Gerber, E. (2014). Northern winter climate change: Assessment of uncertainty in CMIP5 projections related to stratosphere‐troposphere coupling. Journal of Geophysical Research: Atmospheres, 119(13), 7979-7998.

[5] Simpson, I. R., Hitchcock, P., Seager, R., Wu, Y., & Callaghan, P. (2018). The downward influence of uncertainty in the Northern Hemisphere stratospheric polar vortex response to climate change. Journal of Climate, 31(16), 6371-6391.

[6] Shepherd, T. G. (2014). Atmospheric circulation as a source of uncertainty in climate change projections. Nature Geoscience, 7(10), 703.

[7] Yin, J. H. (2005). A consistent poleward shift of the storm tracks in simulations of 21st century climate. Geophysical Research Letters, 32(18).

[8] Chang, E. K., Guo, Y., & Xia, X. (2012). CMIP5 multimodel ensemble projection of storm track change under global warming. Journal of Geophysical Research: Atmospheres, 117(D23).

[9] Barnes, E. A., & Polvani, L. (2013). Response of the midlatitude jets, and of their variability, to increased greenhouse gases in the CMIP5 models. Journal of Climate, 26(18), 7117-7135.

[10] Scaife, A. A., Spangehl, T., Fereday, D. R., Cubasch, U., Langematz, U., Akiyoshi, H., ... & Gettelman, A. (2012). Climate change projections and stratosphere–troposphere interaction. Climate Dynamics, 38(9-10), 2089-2097.

[11] Tamarin-Brodsky, T., & Kaspi, Y. (2017). Enhanced poleward propagation of storms under climate change. Nature geoscience, 10(12), 908.