The forecasting community has therefore suggested that the polar vortex provides useful information for improving subseasonal-to-seasonal forecasts of Northern Hemisphere regional weather 19, 20, 21, 22, 23. Persistent anomalies in the polar vortex strength are associated with a robust and persistent regional surface weather response in the northern extratropics 12, 13, 14, 15 and are known to affect the occurrence of CAOs 16, 17, 18. The Arctic stratospheric polar vortex (hereafter referred to as the polar vortex) is known to affect Northern Hemisphere winter weather on subseasonal-to-seasonal timescales 7, 8, 9, 10, 11. The error bars denote 95% confidence intervals estimated by 1000-time bootstrap samples with replacement. The filled markers denote that the likelihood is statistically significant and the circles with a black outline indicate that the differences between the risk of severe and moderate CAOs are statistically significant with a 95% confidence level based on 1000-time bootstrap samples with replacement. e As shown in ( d), but for the likelihood ratio in mid-latitudes. d The simultaneous likelihood ratio of severe CAOs (circle) and moderate CAOs (triangle) over high-latitude Europe (in orange), East Asia (in green), and North America (in blue) under weak polar vortex conditions for the reanalysis dataset (left column) and the multi-model mean (right column). The thick black solid contours indicate zero isotherms. Green (purple) boxes represent the high-latitude (mid-latitude) regions in this study. Stippled regions indicate that the anomalies are −1.2 times lower than the local standard deviation of SAT anomaly and show which regions could be defined as severe in this study. According to previous studies 61, 62, the occurrences of European CAO and East Asian CAO are influenced by the polar vortex weakening. Using a novel method to assess Granger causality, we show that the polar vortex provides predictive information regarding severe cold air outbreaks over multiple regions in the Northern Hemisphere, which may help with mitigating their impact.Ī– c Anomaly of surface air temperature on 27 February 2018, 22 January 2016, and 1 January 2018 when a severe CAO occurred in Europe, East Asia, and North America, respectively. By analysing the stream of polar cold air mass, we show that the polar vortex affects severe cold air outbreaks by modifying the inter-hemispheric transport of cold air mass. Such a disproportionate increase is also found in Europe, with an elevated risk persisting more than three weeks. Here, through reanalysis data for the period 1958–2019 and climate model simulations for preindustrial conditions, we show that weak stratospheric polar vortex conditions increase the risk of severe cold air outbreaks in mid-latitude East Asia by 100%, in contrast to only 40% for moderate cold air outbreaks. Anomalous behavior of the Arctic stratospheric polar vortex provides an important source of subseasonal-to-seasonal predictability of Northern Hemisphere cold air outbreaks. Severe cold air outbreaks have significant impacts on human health, energy use, agriculture, and transportation.
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