Stratospheric Polar Vortex Variability in the Northern Hemisphere: The Effects of Climate Change on Polar Vortex Trends and Future Projections

Regions that have experienced recent successive cold winters such as the Atlantic Northeast and Siberia have endured critical social and economic impacts from anomalous low temperatures in recent years, despite warming global temperatures. It is well known that tropospheric circumpolar air flow is the primary influence on many mid-latitude weather patterns, extreme winter weather events or climate anomalies, however, Stratospheric Polar Vortex (SPV) variability can have an impact on the tropospheric circulation and thus winter weather in the Northern Hemisphere. Due to the dramatic contrast in land and sea topography in the Northern Hemisphere, strong upward propagating waves can affect the spatial extent and strength of the SPV edge resulting in a weakened polar vortex state, which can in turn bring persistent weather events to the mid-latitudes at the surface. Though there is model agreement that the SPV has been weakening since the 1980s and has more recently experienced frequent Sudden Stratospheric Warmings (SSW’s), it remains unclear if these trends in variability are expected to continue into the 21st century under continued global climate warming. Here, we present an index of SPV spatiotemporal variability using reanalysis of zonal wind and geopotential height to show changes in SPV behavior at a monthly and seasonal scale from 1950-2018. Using the CMIP5 multimodel ensemble, we show averaged simulations of the SPV edge and strength in 30-year runs from present to the end of this century, taking into account models with enhanced stratospheric representation. GIS-based multidimensional tools and interpolation methods are implemented to identify the SPV ‘edge’ through the reanalysis period as well as for the CMIP5 21st century SPV climatology projections. Preliminary results from the multimodel outputs are largely consistent with trends in the observational data, which suggest continued weaker SPV winters along with increased annual variability and size. In the case of more frequent wintertime perturbations in the stratosphere, implications could have significant outcomes in the Northern Hemisphere, ranging from deviations in ecosystem health, infrastructure damage, to human adaptation to climate change. As variability in the SPV can influence tropospheric circulation and in turn winter weather, early predictions of anomalous SPV states could improve forecasts of cold air outbreaks and severe or persistent winter weather.