What is already known:
What this study adds:
The state of the Northern Hemisphere (NH) stratospheric polar vortex is an indicator of NH wintertime weather. As such, its projected trends provide insight into the nature of future winters. Stratospheric vortex trends since 1980 have been studied in detail, but the trends of the entire twentieth century and of the twenty-first century have been studied very little. Previous work (Zhang et al. 2016) has explored connections between vortex trends and climate change impacts, implying that climate change has the potential to impact stratospheric polar vortex trends. We determine the polar vortex trends projected for the twenty-first century under conditions of increasing carbon dioxide emissions using the CMIP5 RCP8.5 emission scenario climate models. In addition, we use reanalysis data to explore the polar vortex trends of the twentieth century to better understand how it has behaved in the past. Also, we assess the CMIP5 Historical ensemble’s ability to reproduce the past polar vortex trends, thereby putting the ensemble’s future projections into context. Analysis of polar cap height (PCH) during the twentieth century reveals high internal variability, which must be taken into account when assessing the role of climate change on these trends. The CMIP5 Historical ensemble does not adequately reproduce the polar vortex trends of past; there is high spread among the ensemble members. However, certain individual models perform well. For the twenty-first century, the CMIP5 RCP8.5 ensemble projects a vortex shift toward Northeast Asia, with inconclusive results regarding future polar vortex strength.