Preface to the Special Issue on Extreme Cold Events from East Asia to North America in Winter 2020/2

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Along with anthropogenic global warming, the Northern Hemisphere (NH) has experienced more frequent cold air out-breaks and heavy snowfalls in recent decades (Petoukhov and Semenov, 2010; Liu et al., 2012; Cohen et al., 2014; 2020;Overland et al., 2015). The most recent example of extreme winter weather was the extreme cold events that occurred from East Asia to North America during the winter of 2020/21 (Cohen et al., 2021; Zheng et al., 2022a). An early observational study argued that Arctic amplification contributes to more extreme weather in all seasons (e.g., Francis and Vavrus, 2012), manifested by a warmer surface and lower troposphere and a consequent decline in sea ice. Newson (1973) first noted that Arctic amplification could reduce the zonal momentum of midlatitude westerly winds and favor blocking flows, a theoretic-ally justifiable response to a reduced meridional temperature gradient. More recent studies have suggested that frequent Euras-ian cold extremes were associated with the loss of Arctic sea ice (Liu et al., 2012; Tang et al., 2013; Mori et al., 2014; Kug et al., 2015) through the promotion of more blocking events and the subsequent invasions of Arctic air into Eurasia. However, McCusker et al. (2016) and Sun et al. (2016) argued that declines in Arctic sea ice do not play a role in the midlatit-ude cold extremes, citing the cold-air outbreaks of the 1960s when Arctic sea ice was abundant.
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