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.

Surface ozone has been severe during summers in the eastern parts of China, damaging human health and flora and fauna. During 2015–2018, ground-level ozone pollution increased and intensified from south to north. In North China and the Huanghuai region, the O₃ concentrations were highest. Two dominant patterns of summer ozone pollution were determined, i.e., a south–north covariant pattern and a south–north differential pattern. The anomalous atmospheric circulations composited for the first pattern manifested as a zonally enhanced East Asian deep trough and as a western Pacific subtropical high, whose western ridge point shifted northward. The local hot, dry air and intense solar radiation enhanced the photochemical reactions to elevate the O₃ pollution levels in North China and the Huanghuai region; however, the removal of pollutants was decreased. For the second pattern, the broad positive geopotential height anomalies at high latitudes significantly weakened cold air advection from the north, and those extending to North China resulted in locally high temperatures near the surface. In a different manner, the western Pacific subtropical high transported sufficient water vapor to the Yangtze River Delta and resulted in a locally adverse environment for the formation of surface ozone. In addition, the most dominant pattern in 2017 and 2018 was different from that in previous years, which is investigated as a new feature.

In the east of China, recent haze pollution has been severe and damaging. In addition to anthropogenic emissions, atmospheric circulations and local meteorological conditions were conducive factors. The number of December haze days over North China and the Huanghuai area has increased sharply since 2010 and was greatest in 2016. During 2016, the most aggressive control measures for anthropogenic emissions were implemented from 16 to 21 December, but the most severe haze pollution still occurred, covering approximately 25 % of the land area of China and lasting for 6 days. The atmospheric circulations must play critical roles in the sub-seasonal haze events. Actually, the positive phase of the East Atlantic–West Russia pattern in the middle troposphere strengthened the anomalous anti-cyclone over the NH area that confined vertical motion below. The associated southerly anomalies made the cold air and surface wind speed weaker, but enhanced the humid flow. Thus, the horizontal and vertical dispersion of atmospheric particulates was suppressed and the pollutants gathered within a narrow space. In December 2016, these key indices were strongly beneficial for haze occurrence and combined to result in the severest haze pollution. The influences of the preceding autumn sea surface temperature near the Gulf of Alaska and the subtropical eastern Pacific, October–November snow cover in western Siberia, and associated physical processes on haze pollution are also discussed.

Haze pollution in the Beijing–Tianjin–Hebei (BTH) region has become increasingly more severe and persistent in recent years. To better understand the formation of severe haze and its relationship with anticyclonic anomalies over northeastern Asia (AANA), this research focused on severe haze over the BTH region occurring in December during 2014–2016 and examined the impacts of AANA. The results indicated that local meteorological conditions were conducive to severe haze (such as weaker surface winds, a stronger temperature inversion, a shallower boundary layer, and higher relative humidity) and were all closely related to AANA. During severe haze episodes, AANA remained strong in the middle upper troposphere, generating anomalous southeasterly winds near the surface. This effect not only promoted the accumulation of pollutants due to the unique topographical conditions in the BTH region but also caused warm advection in lower levels, which was the main cause of the formation and development of a temperature inversion layer. As a synoptic-scale circulation, AANA were accompanied by anomalous vertical motions in the surrounding areas, which weakened the meridional circulation over the BTH region. Intrusions of clean air from upper levels to the surface and downward transport of westerly momentum at mid-levels and upper levels were suppressed, resulting in weaker northerly winds near the surface and a shallower boundary layer. The thermally indirect zonal circulation between the BTH region and western Pacific triggered by AANA provided a persistent source of moisture for the BTH region, which strengthened the development of severe haze by promoting the growth of fine particulates. The advance and retreat of AANA often corresponded with the emergence and dissipation of severe haze, illustrating that AANA could be effective forecast indicators for air quality.

Haze is a serious issue in China today, presenting a substantial pollution problem to the public. To enhance understanding of haze mechanisms in recent years, this study examined the relationship between the subtropical western Pacific sea surface temperature (SST) and the haze pollution over North‐Central North China Plain. The empirical orthogonal function result showed an opposite trend of boreal winter (December–January–February) haze days (WHD) between the south and north of the Huaihe River. The negative sea surface temperature anomaly (SSTA) over the subtropical western Pacific could intensify the WHD over North‐Central North China Plain. As an effective external forcing, the preceding boreal autumn (Sepember–October–November) negative SSTA could weaken the East Asian winter monsoon (EAWM) circulation, leading to conditions such as a weaker Siberian High, reduced wind speed, higher humidity and an abnormal southerly wind. These conditions provide a favourable environment for haze with static stability and potential for hygroscopic growth. A series of SST sensitivity experiments were carried out using version 3.1 of the Community Atmosphere Model. When negative SSTA with different strengths were added into the model, all the key factors exhibited favourable features for haze, such as smaller EAWM index, stronger inversion potential, weaker vertical velocity, lower boundary layer height and higher relative humidity.