Interdecadal relationship between the wintertime haze frequency over Beijing and mega‐ENSO

Abstract

Observational analyses suggest that natural or internal climate variability plays a crucial role in modulating wintertime haze days in Beijing (WHD BJ) on decadal timescales, which may overwhelm the effect of human emissions to some extent. This study links the variations in WHD BJ to the mega-El Niño-Southern Oscillation (ENSO), a newly defined ENSO-related pattern with a vaster range of variability, on decadal timescales. The mega-ENSO delineates an apparent out-of-phase relationship with WHD BJ , which could be used to explain past and recent decreases in WHD BJ in 1961-1971 and 1997-2012, as well as the increase in WHD BJ in 1972-1996. The positive phase of the mega-ENSO can induce a high dynamical scavenging efficiency of pollutants over the Beijing area through notable in situ low-level northerly wind anomalies that are associated with a quasi-barotropic anticyclonic anomaly centered around Lake Baikal, thus reducing the frequency of haze days on decadal timescales, and vice versa for the negative phase. The mega-ENSO can influence interdecadal predictions of the long-term occurrence of haze events over Beijing. Therefore, it will be focused upon in future routine operations.

Future changes in the meteorological potential for winter haze over Beijing during periods of peak carbon emissions and carbon neutrality in China projected by Coupled Model Intercomparison Project Phase 6 models

“…This implies that the projected Northeast Asia cyclonic anomaly (Wang et al ., 2020c) under the future fossil fuel development pathway could enhance the EAWM, and therefore, excite the decreased haze frequency in Beijing compared to that under the medium emission pathway in the near future P CP . Under such circumstances, enhanced atmospheric ventilation around Beijing may be induced by strengthened in situ surface winds, repressing haze formation in Beijing through improved dissipation of near‐surface pollutants (Wang et al ., 2020b). The resultant percentage change of the WHWI BJ in between is −93.6%, thus denoting the fairly suppressed haze potential (Table 2).…”

“…We refer to this period as the present‐day winter haze‐favourable period, which facilitates comparative analyses of haze‐associated meteorology under all future scenarios in Section 3. EAWM climatology features a mid‐tropospheric East Asian coastal trough (EAT) (e.g., Chang et al ., 2020; Wang et al ., 2020b). Therefore, we selected the 500 hPa EAT index (areal mean Z500 over 25°N–45°N and 110°E–145°E; green box in Figures 4, 6, and 10) as the EAWM index (EAWMI) to measure the EAWM intensity following previous studies (e.g., Wang and He, 2012; Hao et al ., 2019).…”

“…Haze pollution in the North China Plain (NCP), a typical response to local and nonlocal manmade aerosol emissions and overlying in situ stagnant meteorological conditions, is generally accompanied by poor quality of outdoor air and impaired atmospheric visibility (Ding and Liu, 2014; Wang and Chen, 2016; Yang et al ., 2016, 2018; Ding et al ., 2017; Wang et al ., 2018, 2019, 2020b, 2020c, 2020d, 2020e; An et al ., 2019; He et al ., 2019; Callahan and Mankin, 2020; Gao et al ., 2020; Wei et al ., 2020; Yin et al ., 2021). Haze episodes, especially those with high PM 2.5 [particulate matter (PM) with an aerodynamic diameter of 2.5 μm or less] concentrations, can negatively impact economic development and lessen human well‐being via the burden of triggered cardiovascular and respiratory diseases (e.g., Yin et al ., 2017).…”

“…Haze in the NCP primarily occurs in boreal winter (e.g., Chen and Wang, 2015; Mao et al ., 2019). Owing to its prominent status in China, researchers have focused on haze pollution in Beijing over multiple timescales (e.g., Pei et al ., 2020; Wang et al ., 2020b, 2020c). Notably, after the occurrence of the so‐called ‘Airpocalypse’ over Beijing and its adjoining areas in January 2013 (Zhang et al ., 2014; Callahan et al ., 2019), the Chinese government attached great importance to the mitigation and prevention of Beijing haze.…”

“…Thus, a weakened EAWM can establish a conducive environmental background for the hygroscopic growth and aqueous‐phase heterogeneous chemistry reactions for PM 2.5 , contributing to enhanced in situ haze pollution (Ding and Liu, 2014; An et al ., 2019; Ma and Zhang, 2020; Wang et al ., 2020d). Furthermore, although increased human activities [e.g., aerosol emissions and anthropogenic greenhouse gas (GHG) warming] have enhanced haze pollution over Beijing and its surroundings on interdecadal timescales (e.g., Xu et al ., 2015; Yang et al ., 2016, 2018; Li et al ., 2018; Chen et al ., 2019), research has also found that external forcing factors [e.g., basin‐scale sea surface temperature (SST) anomaly patterns of mega‐El Niño–Southern Oscillation (ENSO; Wang et al ., 2020b) and Atlantic Multidecadal Oscillation (AMO; Xiao et al ., 2015)] exert non‐negligible remote modulation roles for long‐term variations in Beijing haze.…”