Abstract
Abstract. The Northern Hemisphere stratospheric polar vortex (SPV) plays a key role in mid-latitude weather and climate. However, in what way the SPV will respond to global warming is not clear, with climate models disagreeing on the sign and magnitude of projected SPV strength change. Here we address the potential role of Barents and Kara (BK) sea ice loss in this. We provide evidence for a non-linear response of the SPV to global mean temperature change, which is coincident with the time the BK seas become ice-free. Using a causal network approach, we demonstrate that climate models show some partial support for the previously proposed link between low BK sea ice in autumn and a weakened winter SPV but that this effect is plausibly very small relative to internal variability. Yet, given the expected dramatic decrease in sea ice in the future, even a small causal effect can explain all of the projected ensemble-mean SPV weakening, approximately one-half of the ensemble spread in the middle of the 21st century, and one-third of the spread at the end of the century. Finally, we note that most models have unrealistic amounts of BK sea ice, meaning that their SPV response to ice loss is unrealistic. Bias adjusting for this effect leads to pronounced differences in SPV response of individual models at both ends of the spectrum but has no strong consequences for the overall ensemble mean and spread. Overall, our results indicate the importance of exploring all plausible implications of a changing Arctic for regional climate risk assessments.
Highlights
The stratospheric polar vortex (SPV), a band of fast-blowing westerlies forming during boreal winter, is a central component of the Northern Hemisphere atmospheric circulation (Waugh et al, 2016)
To test for evidence of a non-linear SPV response related to sea ice loss in the CMIP5 models, we plot the projected SPV change in January–March (JFM), the projected vertical wave activity flux (vT) change in December–February (DJF), and Barents and Kara (BK)-SIC change in October– December (OND) for different levels of global mean warming in the RCP8.5 high-emissions scenario (Fig. 1a–c)
When plotting SPV change and vT change as a function of Barents and Kara sea ice concentrations (BK-SIC) change, we find them to be approximately linear, with most models showing a weakening of the SPV and a strengthening of vT, while BK-SIC decreases (Fig. 1d and e)
Summary
The stratospheric polar vortex (SPV), a band of fast-blowing westerlies forming during boreal winter, is a central component of the Northern Hemisphere atmospheric circulation (Waugh et al, 2016). Variability in vortex strength is linked to stratospheric ozone concentrations, but, due to downward coupling to the troposphere, it strongly affects mid-latitude weather. For instance, Mediterranean precipitation is expected to strongly decrease, while days of extreme storminess in northern Europe are expected to increase (Simpson et al, 2018; Zappa and Shepherd, 2017). If the vortex weakens, the pace of Mediterranean drying is likely to be more moderate and changes in storminess less pronounced
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