Abstract. Severe haze occurred in the North China Plain (NCP) from November to December 2015, with a wide spatial range and long duration. In this paper, the combined effect of the anomalous stationary Rossby waves within two westerly jet waveguides on this haze event in the NCP is investigated based on observational visibility data and NCEP/NCAR reanalysis data. The results show that circulation anomalies in Eurasia caused by the propagation of anomalous stationary Rossby wave energy along two waveguides within the westerly jet originating from the Mediterranean were responsible for haze formation in the NCP. The Rossby waves propagated eastward along the subtropical westerly jet and the polar front jet, causing an anomalous anticyclone over the Sea of Japan and anticyclonic wind shear at 850 hPa over the NCP, which enhanced the anomalous descent in the middle and lower troposphere and subsequently resulted in a stable lower atmosphere. Furthermore, the anomalous stationary Rossby waves propagating along the polar front jet weakened the East Asia trough and Ural ridge and strengthened the anomalous southerly wind at 850 hPa over the coastal areas of eastern China, decelerating the East Asia winter monsoon. The above meteorological conditions modulated haze accumulation in November and December 2015. Meanwhile, continuous rainfall related to ascending motion due to Rossby wave propagation along the waveguide provided by the subtropical westerly jet occurred in a large area of southern China. The associated latent heat release acted as a heat source, intensifying the ascending motion over southern China so that the descending motion over the NCP was strengthened, favoring the maintenance of severe haze. This study elucidates the formation and maintenance mechanism of large-scale haze in the NCP in late fall and boreal winter.

The Rossby wave train in boreal winter sometimes bifurcates into two branches near the Mediterranean, sometimes not. However, causes for bifurcating of the wave train are still being debated. Additionally, discussions about the differences in the deterioration ability of these wave trains on air pollution are also insufficient. This study uses the empirical orthogonal function (EOF) to obtain two dominant modes of the November‐January season wave trains over Eurasia and discusses their different influences on air pollution over the North China Plain (NCP). From the EOF1 mode, bifurcation of the wave train appears to stem from the Rossby wave reflection near the Mediterranean because the Rossby wave source is located outside of a strong absolute vorticity gradient. From the EOF2 mode, the wave train propagates only along the subtropical westerly jet because the Rossby wave source is located inside of a strong absolute vorticity gradient, so that the wave train tends to be refracted near the Mediterranean. In addition, the wave train associated with the EOF1 is suggested to be related to the eastern Pacific El Niño. The eastern Pacific El Niño tends to force a wave train propagating into East Asia under the charger effect from positive feedback of air‐sea coupling near the far eastern Atlantic and Mediterranean, while the wave train associated with the EOF2 is suggested to be mainly induced by natural internal variability. Furthermore, the results indicate that the bifurcation type wave train tends to cause higher PM2.5 concentrations over the NCP through a strengthened Northeast Asia anomalous anticyclone.

Air quality over the North China Plain (NCP) and rainfall over southern China exhibit robust and correlated interannual variations during winter from 1979–2018. A correlation coefficient of 0.7 between the PM2.5 concentration over the NCP and rainfall in southern China indicates that poor air quality and rainfall are linked (South Rainfall–North Pollution), which is likely due to the strong eastern Pacific (EP) El Niño. In the mature phase of the strong EP El Niño, the northeast Asian anomalous anticyclone (NAAA) is strengthened by two Rossby wave trains modulated by warmer sea surface temperatures. One wave train is directly related to the strong EP El Niño and originates in the tropical eastern Pacific, propagating eastward to East Asia along mid– and high latitudes, which weakens the Ural ridge and strengthens the NAAA. The other wave train is derived from the northern Indian Ocean and propagates into East Asia along big circle route, which strengthens an anomalous cyclone in southern China and the NAAA. This result in an anomalous horizontal trough of 850 hPa being captured in eastern China at ∼30°N. The southwesterly flow along the southern margin of the trough transports abundant moisture to southern China, which leads to heavy rainfall in southern China combined by an anomalous ascending motion related to the anomalous cyclone. The southeasterly flow to the north of the trough weakens the East Asia winter monsoon due to a weakened Ural ridge, which then leads to a high PM2.5 concentrations over the NCP with the help of an anomalous descending motion related to the NAAA.