No abstract

Many recently updated climate models show larger future warming than previously.Separate lines of evidence suggest their warming rates may be unrealistically high, but the risk of such eventualities only emphasizes the need for rapid and deep reductions in emissions.

To explore the mechanisms linking Arctic sea ice loss to changes in midlatitude surface temperatures, we conduct idealized modeling experiments using an intermediate general circulation model and with sea ice loss confined to the Atlantic or Pacific sectors of the Arctic (Barents‐Kara or Chukchi‐Bering Seas). Extending previous findings, there are opposite effects on the winter stratospheric polar vortex for both large‐magnitude (late 21st century) and moderate‐magnitude sea ice loss. Accordingly, there are opposite tropospheric Arctic Oscillation (AO) responses for moderate‐magnitude sea ice loss. However, there are similar strength negative AO responses for large‐magnitude sea ice loss, suggesting that tropospheric mechanisms become relatively more important than stratospheric mechanisms as the sea ice loss magnitude increases. The midlatitude surface temperature response for each loss region and magnitude can be understood as the combination of an “indirect” part induced by the large‐scale circulation (AO) response, and a residual “direct” part that is local to the loss region.

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.