A stretched polar vortex increased mid-latitude climate variability during the Last Glacial Maximum

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Abstract. The Arctic stratospheric polar vortex (SPV) is a key driver of winter weather and has been found to modify winter climate variability and its predictability in Eurasia and North America on inter-annual and decadal timescales. However, to what extent this relationship plays a role in driving climate variability on glacial–interglacial timescales is still unknown. Here, by systematically analysing SPV changes in four sets of PMIP4 (Paleoclimate Modelling Intercomparison Project – Phase 4) simulations for the Last Glacial Maximum (LGM) and the pre-industrial (PI) period, we explore how the SPV changed during the glacial climate and how it influenced climate variability. Our results show that under LGM conditions, the SPV stretched towards the Laurentide ice sheet; this was accompanied by anomalous upward wave propagation and enhanced SPV variability, which increased the likelihood of cold-air outbreaks into the mid-latitudes. During the LGM, this stretched SPV pushed cold Arctic air further equatorwards, increasing winter climate variability over the more southern mid-latitudes. In particular, in winter, this strengthened cooling over the mid-latitudes extended beyond the coverage of the Laurentide ice sheet (unlike in summer). SPV-induced temperature variability also explains the inter-model spread as removing SPV variation from the model results reduces the inter-model spread by up to 5°C over mid-latitude Eurasia. These results highlight the critical role of the SPV in connecting the polar region and mid-latitudes on glacial–interglacial timescales. These connections are reminiscent of intra-seasonal stratosphere–troposphere coupling.