Unusually clear sky in Beijing during winter 2017 and the underlying large-scale climatic anomalies: With implication for "haze-climate" study

Haze over Beijing has received widespread attention in recent years. However, Beijing notably witnessed unusually few haze days in the winter 2017. In 2018, Pei et al proposed an observation-based mechanism linking the number of persistent haze events (PHE) in Beijing with large-scale climatic anomalies in surrounding regions, suggesting an important role of the sea surface temperature anomalies (SSTA) in a key region in the northwestern Pacific (K-region). This letter accordingly compares the large-scale winter climatic anomalies in 2017 with those in 2016, when Beijing encountered with the most serious haze in the observational history. In the winter 2016, the K-region was warmer than usual, giving rise to more frequent anomalous southerlies in North China and more stable atmospheric vertical structure, and hence more haze in Beijing. In the winter 2017, the K-region was cooler than usual with opposite atmospheric anomalies around, and hence less haze in Beijing. The analysis demonstrates that haze over Beijing is influenced by large-scale climatic anomalies. Current status of "haze-climate" studies in China is discussed and future research priorities are highlighted.