Attribution of April 2020 Exceptional Cold Spell over Northeast China

Northeast China (105°–135°E, 37°–55°N) is an important agricultural production area for wheat, soybean, and maize, among other crops (Lu et al. 2017; http://country.cnr.cn/gundong/20160303/ t20160303_521524231_1.shtml), making a substantial contribution to the food security of China. Late boreal spring (mid-April to mid-May) is a transition period when most crops are at their critical growing stage. Under climate change, a widespread shift toward earlier spring greening, triggered by warmer temperature, has been documented (Schwartz et al. 2006; Piao et al. 2015; Tan et al. 2018). Extreme cold events occurred at growth stage are more likely to cause serious damage to crop yields and large economic loss (Wheeler et al. 2000; X. Li et al. 2015). In the recent decades, regional cold extremes, superimposed on a warmer climate, recurred throughout the Northern Hemisphere (Cattiaux et al. 2015; Qian et al. 2018; Christidis and Stott 2020; Zhang et al. 2021). April 2020 was the second hottest April (following April 2016) in the records, being 1.06°C above the twentieth-century average (www.noaa.gov/news/april-2020was-earth-s-2nd-hottest-april-on-record). However, Northeast China witnessed an exceptional cold spell from 19th to 25th April 2020, mainly affecting the provinces of Heilongjiang, Jilin, Liaoning, Inner Mongolia, Hebei, Hongyong Yu, Xiaojing Yu, Ziwei Zhou, Yu Wang, Yingxin Li, Nergui Nanding, Nicolas Freychet, Buwen Dong, Dongqian Wang, Fraser C. Lott, Simon F. B.