Trends in extreme cold air outbreaks in a changing climate and the role of blocking

Extreme cold air outbreaks (CAOs) over land and over the ocean are associated with a range of hazardous conditions, posing risks for human health and infrastructure. Recent trends in wintertime CAOs suggest a strong regional variability in the frequency of CAOs under climate change. Specifically, in the northeastern North Atlantic, CAOs have been predominantly associated with blocked weather regimes, particularly Greenland blocking. Other blocked regimes, such as European/Scandinavian blocking, have been linked to a suppression of CAOs in this domain. It remains however unclear how changes in the observed trend of CAOs are linked to trends in atmospheric circulation regimes.

Here, we investigate the physical drivers that are responsible for the trend in extreme cold air outbreaks in reanalysis data for the period 1940-2023. We show that there is a negative trend in the number of CAOs over land (defined as the number of days below the 10th percentile for a minimum of three consecutive days) over Greenland and Scandinavia. The relation between changes in CAOs in the North Atlantic and over Europe and potential dynamical drivers, including blocking and storm track variability, is explored. We further investigate the relation to changes in the stratospheric polar vortex. Understanding future trends in extreme weather, and cold air outbreaks in particular, has the potential to support more accurate and informed decision-making associated with these high-impact weather events.