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Abstract. Over the past decades, Beijing, the capital city of China, has encounteredincreasingly frequent persistent haze events (PHE). While the increasedpollutant emissions are considered as the most important reason, changes inregional atmospheric circulations associated with large-scale climate warmingalso play a role. In this study, we find a significant positive trend of PHEin Beijing for the winters from 1980 to 2016 based on updated dailyobservations. This trend is closely related to an increasing frequency ofextreme anomalous southerly episodes in North China, a weakened East Asiantrough in the mid-troposphere and a northward shift of the East Asian jetstream in the upper troposphere. These conditions together depict a weakenedEast Asian winter monsoon (EAWM) system, which is then found to be associated with an anomalouswarm, high-pressure system in the middle–lower troposphere over the northwesternPacific. A practical EAWM index is defined as the seasonal meridional windanomaly at 850 hPa in winter over North China. Over the period 1900–2016,this EAWM index is positively correlated with the sea surface temperatureanomalies over the northwestern Pacific, which indicates a wavy positivetrend, with an enhanced positive phase since the mid-1980s. Our resultssuggest an observation-based mechanism linking the increase in PHE in Beijingwith large-scale climatic warming through changes in the typical regionalatmospheric circulation.

atmospheric chemistry and physics

Increasing persistent haze in Beijing: potential impacts of weakening East Asian winter monsoons associated with northwestern Pacific sea surface temperature trends