Northern hemisphere cold air outbreaks are more likely to be severe during weak polar vortex conditions

Severe cold air outbreaks have significant impacts on human health, energy use, agriculture, and transportation. Anomalous behavior of the Arctic stratospheric polar vortex provides an important source of subseasonal-to-seasonal predictability of Northern Hemisphere cold air outbreaks. 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. Such a disproportionate increase is also found in Europe, with an elevated risk persisting more than three weeks. 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. 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. A weak stratospheric polar vortex in the Arctic leads to a disproportionally increased risk of severe cold air outbreaks over northern extratropical regions, compared to moderate outbreaks, suggest analyses of reanalysis data and climate model simulations.

Automated summary

Research shows that a weakened Arctic stratospheric polar vortex significantly increases the risk of severe cold air outbreaks in mid-latitude East Asia and Europe; the vortex affects the outbreaks by modifying the transport of cold air masses; using Granger causality analysis can provide predictive information for severe cold air outbreaks in multiple Northern Hemisphere regions.