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

Abstract

Abstract. Over the past decades, Beijing, the capital city of China, has encountered increasingly frequent persistent haze events (PHE). While the increased pollutant emissions are considered as the most important reason, changes in regional atmospheric circulations associated with large-scale climate warming also play a role. In this study, we find a significant positive trend of PHE in Beijing for the winters from 1980 to 2016 based on updated daily observations. This trend is closely related to an increasing frequency of extreme anomalous southerly episodes in North China, a weakened East Asian trough in the mid-troposphere and a northward shift of the East Asian jet stream in the upper troposphere. These conditions together depict a weakened East Asian winter monsoon (EAWM) system, which is then found to be associated with an anomalous warm, high-pressure system in the middle-lower troposphere over the northwestern Pacific. A practical EAWM index is defined as the seasonal meridional wind anomaly at 850 hPa in winter over North China. Over the period 1900-2016, this EAWM index is positively correlated with the sea surface temperature anomalies over the northwestern Pacific, which indicates a wavy positive trend, with an enhanced positive phase since the mid-1980s. Our results suggest an observation-based mechanism linking the increase in PHE in Beijing with large-scale climatic warming through changes in the typical regional atmospheric circulation.

Multi‐Index Attribution of Extreme Winter Air Quality in Beijing, China

“…They also found that CESM may underestimate the recent decreases in Arctic sea ice and increases in Eurasian snow, meaning our results may underestimate the influence of climate change on the PPI distribution and its extremes (Zou et al, ). While we cannot conclude that anthropogenic warming has affected historical change in the WMI, Pei et al () hypothesized that positive SST anomalies in the northwest Pacific create anomalously high‐pressure systems off the eastern coast of China, which produce anomalous southerly winds and weaken the East Asian winter monsoon.…”

“… Western Monsoon Index (WMI). We define an index of the East Asian winter monsoon following the approach of Pei et al (). The weakness of the monsoon is defined as the area‐averaged meridional wind anomaly at 850 hPa over 105–125 °E and 30–50 °N (Figure f).…”

“…The WMI can be denoted as: where V 850 refers to meridional wind (m/s) at 850 mb. Stronger southerly (positive) meridional winds are associated with increased southerly flow, indicating a weaker monsoon and increased transport of pollutants and moisture from the regions south of Beijing (Pei et al, ). We normalize the observed WMI by subtracting the observed mean and dividing by the observed standard deviation, and we normalize both the preindustrial and historical simulations by subtracting the preindustrial mean and dividing by the preindustrial standard deviation.…”

Links Between the Large‐Scale Circulation and Daily Air Quality Over Central Eastern China During Winter

“…Recent studies on this topic have identified several key meteorological factors associated with the development of severe haze events, including weak surface winds (which reduce horizontal dispersion of urban aerosols; Wang et al, ; Zhang et al, ), a shallower and more stable atmospheric boundary layer (which inhibits vertical ventilation; Jeong & Park, ; Wang et al, ; Zhao et al, ), and enhanced relative humidity (RH; which intensifies secondary aerosol formation and hygroscopic growth; Quan et al, ; Sun et al, ; Wang et al, ). At larger scales, a weakened East Asian trough and a weak Siberian high may not only suppress vertical mixing and reduce boundary layer height (Pei et al, ; Zhang et al, ) but also reduce cold surges from high latitudes (e.g., Huang et al, ). Associated anomalous southerlies in the lower troposphere have been found to increase humidity and air temperature in North China, contributing to the formation and maintenance of severe haze (Chen & Wang, ; Yin et al, ; Zhang et al, ).…”

“…Moreover, the severe haze that developed in the Beijing‐Tianjin‐Hebei area during January 2013 (monthly mean PM 2.5 concentrations of ∼ 145 μg/m 3 ) has been attributed in part to a period of anomalously weak EAWM and SH (Wang et al, ; Zhang et al, ). Meteorological conditions during winter have grown increasingly favorable for the development of haze in eastern China since the mid‐1980s (Jeong & Park, ; Pei et al, ), coincident with a prolonged weakening of the EAWM (Huang et al, ; Lee et al, ), and reduced autumn Arctic sea ice has been dynamically linked to reduced ventilation of low‐level air over eastern China (Wang et al, ; Zou et al, ). Given these relationships, the frequency of haze formation in China is projected to increase under climate change (Cai et al, ; Hori & Ueda, ).…”