Where Do Cold Air Outbreaks Occur, and How Have They Changed Over Time?

Cold air outbreaks (CAOs) occur in many regions of the globe and can be detrimental to the agricultural industry and human health, especially in less prepared regions. A systematic CAO classification was developed using two different climate reanalysis data sets from 1979–2018. Trends in CAOs were calculated for different regions across the globe and the results from each reanalysis data set compared with one another to identify discrepancies. CAOs occur more frequently in the Northern Hemisphere than the Southern Hemisphere with the highest number of CAOs in Europe, Central Eurasia, and North America. CAOs were found to have decreased in spatial extent, frequency, duration, and magnitude across much of the globe, particularly across Alaska, Canada, and the North Atlantic, while an increase in CAOs was observed in Eastern Europe, Central Eurasia, and the Southern Ocean. Early and late winter CAOs have also become much less frequent in most regions. Plain Language Summary On any given day, some region of the globe is likely to be affected by extreme cold. Depending on the severity, extreme cold periods may be deemed cold air outbreaks (CAOs). These CAOs can be detrimental to the agricultural industry, energy industry, and human health, especially in regions with a warmer climate that may be less well prepared to mitigate the impacts. This study uses a set of criteria to examine CAOs across the globe from 1979–2018 and to determine how CAOs have changed over the last 40 years. We found that CAOs occur most frequently in the Northern Hemisphere, with as many as eight CAO days per year in North America and Eurasia. CAOs were found to have decreased in size, intensity, frequency, and duration across much of the globe, with the largest decreases in Alaska, Canada, and the North Atlantic, while an increase in CAOs was observed in Eastern Europe, Central Eurasia, and the Southern Ocean. Early and late winter CAOs have also become much less frequent in most regions. Key Points Cold air outbreaks have decreased in frequency, duration, magnitude, and spatial extent across much of the globe The Northern Hemisphere has experienced a larger decrease in cold air outbreaks than the Southern Hemisphere There are regional differences between the NNR and ERA5 data sets in the trends and spatial distribution of cold air outbreaks