May 22 - July 28
Stratospheric Polar Vortex Trends of the Twentieth and Twenty-First Centuries and an Assessment of the CMIP5 Historical Ensemble
Carly Narotsky and Jason Furtado
What is already known:
- The position and strength of the Northern Hemisphere stratospheric polar vortex can be used as indicators of winter temperatures at the middle and high-latitudes.
- Previous studies have found a directional shift in the position of the stratospheric polar vortex away from the pole and toward the Eurasian continent. One study has linked this trend to Arctic sea-ice loss and increased snow cover over the Eurasian continent associated with anthropogenic climate change (Zhang et al. 2016), while another has attributed the trend to internal variability (Seviour 2017).
- Sudden stratospheric warming events, which are associated with a breakdown of the stratospheric polar vortex, are not projected to increase in frequency during the twenty-first century. However, a climatological equator-ward shift of the vortex centroid is projected (Mitchell et al. 2012).
What this study adds:
- Analysis of polar cap height (PCH) during the twentieth century reveals high variability throughout the century, with multi-decade time periods sustaining high PCH, and multi-decade time periods sustaining low PCH.
- The CMIP5 Historical ensemble does not adequately reproduce the stratospheric vortex trends of the past. Only two models out of nine in the ensemble reproduce the directional shift of the stratospheric polar vortex toward the Eurasian continent, and there is high spread among the model results for all diagnostics.
- With low variability among the models, stratospheric geopotential heights are projected to rise in both the middle and high-latitudes during the twenty-first century, leaving projections of polar vortex strength unclear.
- With some variability among the models, the position of the stratospheric polar vortex is projected to shift toward Northeast Asia during the twenty-first century.
Abstract:
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.