Under current global warming, extreme weather and climate events, such as extreme heatwaves and severe precipitation, are occurring frequently worldwide (Chen D. L. et al., 2020; Herring et al., 2020, 2022; Zhou et al., 2020). Extreme heavy snowfall (EHS), as another example of extreme weather, has drawn attention because of its potentially serious socioeconomic impacts (Zhou et al., 2011; Wang et al., 2016). Moreover, under global warming, EHS is also becoming more and more frequent—a trend that is expected to continue in the future (Ding et al., 2008; Sun et al., 2010; Kawase et al., 2016; Chen H. P. et al., 2020; Quante et al., 2021; IPCC, 2022). In recent decades, EHS has shown an increasing trend in northern China, on both interannual and interdecadal timescales (Zhao et al, 2016; Zhou et al., 2018, 2021), and it easily causes severe damage. For example, extraordinarily frequent and long-lasting snowstorms in January 2008 caused above-normal precipitation, below-normal temperatures, and severe icing conditions over central-southern China (Ding et al., 2008; Wang et al., 2008; Wen et al., 2009). Naturally, therefore, more and more researchers are paying attention to the attribution of such extreme events in China (Zhao, 2020; Sun et al., 2021), and elucidating the mechanism driving EHS in China will be of benefit in this respect for this type of extreme weather.

Abundant atmospheric moisture is one of the necessary conditions for EHS (Guan et al., 2010). For instance, the increased evaporation enabled by Arctic sea-ice loss can provide moisture for EHS over Eurasia (Liu J. P. et al., 2012; Bailey et al., 2021). Thus, exploring the moisture sources will help to comprehensively reveal the mechanism of extreme precipitation. To identify the moisture sources, Lagrangian diagnosis has been successfully applied over several regions during different seasons (Drumond et al., 2011; Jiang et al, 2017; Huang et al., 2018). Sun and Wang (2015) indicated that the moisture source for wintertime precipitation over South China is in the western Pacific, while that for precipitation over the middle and lower reaches of Yangtze River valley and North China is primarily the land areas around East China. Further research on extreme precipitation events over East China using Lagrangian diagnosis has shown that the moisture sources for snowfall and rainfall originate from land areas and sea areas, respectively (Yang et al., 2019). However, the moisture sources for EHS over northern China have been less well studied.

Abnormal atmospheric circulation is also an important condition causing EHS events. For example, Pei et al. (2022) indicated an anomalous anticyclone over the northwestern Pacific and a weakened East Asian winter monsoon (EAWM) contributed to a record-breaking wintertime precipitation event over Beijing. More specifically, Zhou et al. (2021) found that anomalous atmospheric circulation similar to the northern mode of the EAWM may favor the interaction of cold air with moist airflows over northern China, which is conducive to interdecadal increases in localized heavy snowfall. Besides, the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) also play key roles in climate and weather changes over Eurasia (Cohen et al., 2010; Li and Wu, 2012). In terms of snowfall, some studies have found that the NAO and AO tend to be in a negative phase when more intense snowfall events occur in East Asia (Park et al., 2010; Wang and Zhou, 2018; Sun et al., 2021). Also, the negative phase of the NAO/AO is related to sea-ice loss and a weak polar vortex (Honda et al., 2009; Nakamura et al., 2015). Kim et al. (2014) proposed that Arctic sea-ice loss in early winter causes the upward propagation of planetary-scale waves to the stratosphere, especially over the Barents and Kara seas, which will lead to a weakening of the polar vortex in late winter and ultimately to a negative phase of the AO. The weakening of the polar vortex can explain 60% of extreme cold events in winter in the Eurasian midlatitudes in recent decades (Kretschmer et al., 2018). In addition, some studies have confirmed that EHS in China is closely related to Arctic sea-ice loss (Liu N. et al., 2012; Sun et al., 2019, 2021).

A record-breaking EHS event occurred in northern China during 6–8 November 2021. As shown in Fig. 1, the snowfall process not only resulted in abnormally high snowfall in northern China in early November 2021, but also abnormally high total snowfall in the whole of November, compared with the climatology in November 1981–2010. There were two maximum snowfall centers, in Tongliao and Tianjin, where the snowfall was abnormally more than eight times more than the climatology. Besides, the regions around the two centers had more than twice as much snowfall as normal. Comparing Figs. 1a and 1b, it can be seen that this EHS event resulted in a snowfall anomaly for the whole month of November 2021. As the center of the EHS, Tongliao City, Inner Mongolia, reported a total snowfall amount (hereafter, snowfall refers to snow water equivalent in this study) that reached 87.4 mm. Disasters were experienced in eight districts, inflicting direct economic losses of 34.5 million Yuan.

Figure 1.  Distributions of snowfall anomalies (%) in northern China in (a) early November 2021 and (b) November 2021 compared with the climatology in November 1981–2010.

Elucidating the mechanism of typical EHS events such as this one is useful in a broader context, to further improve the predictability of extreme snowfall in this region (northern China). Accordingly, in this study, the aim was to reveal the mechanism of this EHS event from the perspective of the two key conditions (cold air and water vapor). Following this introduction, the data and methods applied in our study are described in Section 2. In Section 3, the development of the snowfall, weather systems, horizontal temperature advection, and moisture supply related to the EHS event is described. Then, in Section 4, the water vapor sources and the dominant factors involved in the development of the weather systems are investigated. In Section 5, the precursory signals of the snowfall are discussed, and then a summary of the study’s key findings is provided in Section 6.