Understanding severe winter haze events in the North China Plain in 2014: roles of climate anomalies

Abstract. Atmospheric pollution has become a serious environmental and social problem in China. Over the past 30 years, the number of winter (December–February) haze days over the North China Plain (WHD_NCP) was greatest in 2014. In addition to anthropogenic influence, climate anomalies also played a role. Thus, it is necessary to analyze the anomalous atmosphere circulations associated with haze pollution of this year in detail. Near the surface, the weaker East Asian winter monsoon pattern, causing southerly winds over the North China Plain, could aggravate the situation of haze. In the lower and middle troposphere, taking the anticyclone circulation over North China as an intermediate system, the positive phases of the eastern Atlantic/western Russia (EA/WR), the western Pacific (WP), and the Eurasia (EU) patterns led to a worse air pollution dispersion condition that contributed to a larger number of WHD_NCP. In 2014, these three patterns could be recognized from the wind anomalies in the lower troposphere. The preceding autumn (September–November) Arctic sea ice (ASI) anomalies over the eastern Hemisphere and the warmer winter surface over Eurasia might have induced or intensified the positive EA/WR pattern in 2014. These two external forcings, together with the pre-autumn sea surface temperature anomalies in the Pacific, might have also stimulated or enhanced the positive EU-like patterns. The anomalous surface temperature in autumn 2014 was efficient in intensifying anomalous circulations such as the positive phase of the WP pattern. The opposite case of minimum WHD_NCP in 2010 further supports the mechanism of how EA/WR and WP patterns and associated external factors altered the local climate conditions to impact the WHD_NCP.