Abstract
Abstract. El Niño events differ widely in their patterns and intensities. The regional climate anomalies caused by different types of El Niño events likely lead to various impacts on winter haze pollution in China. Based on long-term site observations of haze days in China from 1961 to 2013, this study explores the effects of eastern Pacific (EP) and central Pacific (CP) types of El Niño events on the number of winter haze days (WHDs) in China's Jing-Jin-Ji (JJJ) region and the physical mechanisms underlying WHD changes. The results show statistically significant positive and negative correlations, respectively, between WHDs in the JJJ region and EP and CP El Niño events. At most sites in the JJJ region, the average WHD increased in all EP El Niño years, with the maximum change exceeding 2.0 d. Meanwhile, the average WHD decreased at almost all stations over this region in all CP El Niño years, with the largest change being more than −2.0 d. The changes in large-scale circulations indicate obvious positive surface air temperature (SAT) anomalies and negative sea level pressure (SLP) anomalies over North China, as well as southerly wind anomalies at the middle to low troposphere over eastern China in the winters of EP El Niño years. These anomalies are conducive to increases in WHDs in the JJJ region. However, there are significant northerly and northwesterly wind anomalies at the middle to low troposphere over eastern China, as well as stronger and wider precipitation anomalies in the winters of CP El Niño years, which contribute to decreased WHDs over the JJJ region. Changes in local synoptic conditions indicate negative SLP anomalies, positive SAT anomalies, and weakened northerly winds over the JJJ region in the winters of EP El Niño years. The total occurrence frequency of circulation types conducive to the accumulation (diffusion) of aerosol pollutants is increased (decreased) by 0.4 % (0.2 %) in those winters. However, the corresponding frequency is decreased (increased) by 0.5 % (0.6 %) in the winters of CP El Niño years. Our study highlights the importance of distinguishing the impacts of these two types of El Niño events on winter haze pollution in China's JJJ region.
Highlights
North China, with the Jing-Jin-Ji (JJJ) region at the core, has encountered continuous severe haze pollution in recent winters
Whether for eastern Pacific (EP) or central Pacific (CP) El Niño events, the indices feature a uniformly negative correlation with winter haze days (WHDs) at most of the stations in southern China. This result is in agreement with previous studies (e.g., Li et al, 2017; Zhao et al, 2018; He et al, 2019), which reported that the increase in precipitation over southern China due to the anomalous anticyclone over the west Pacific during the mature phase of El Niño events significantly reduced WHDs in this region
For more than 60 % of sites in the JJJ region, WHDs are positively correlated with the Iep index (121 sites, accounting for 62.1 % of all sites) but negatively correlated with the Icp index (126 sites, 64.6 %)
Summary
North China, with the Jing-Jin-Ji (JJJ) region at the core, has encountered continuous severe haze pollution in recent winters. These atmospheric calamities have seriously harmed traffic, economic development, and resident health in this region Anomalous meteorological conditions have significant influences on the development and maintenance of haze events; in particular, the explosive increase in local air pollutants is always accompanied by anomalous atmospheric circulation conditions
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