Nonlinear effects of the stratospheric Quasi-Biennial Oscillation on ENSO modulating PM _2.5 over the North China Plain in early winter

Abstract. The North China Plain (NCP) experiences severe air pollution, with PM2.5 (fine particulate matter with an aerodynamic diameter ≤ 2.5 µm) as the primary pollutant, especially in early winter (November to December). The PM2.5 concentrations in this period are significantly modulated by the El Niño-Southern Oscillation (ENSO). In this study, we have found that the stratospheric Quasi-Biennial Oscillation (QBO) exerts a nonlinear impact on the relationship between ENSO and PM2.5 concentrations over the NCP in early winter. During the easterly QBO (EQBO) phase, ENSO's influence on PM2.5 concentration is stronger compared to the westerly QBO (WQBO) phase. In El Niño and EQBO years, PM2.5 concentrations rise due to meteorological factors like a shallower boundary layer, higher relative humidity, and intensified southerly wind anomalies. Conversely, during La Niña and EQBO years, PM2.5 levels decrease due to opposite meteorological conditions. The study attributes these changes to planetary wave dynamics. During El Niño and EQBO years, upward-propagating planetary waves in mid-latitudes enhance upper-level divergence over Eurasia, strengthening westerlies. These westerlies guide Rossby wave trains into Northeast Asia, forming a strong anomalous anticyclone that worsens air pollution over the NCP. In La Niña and EQBO years, downward-propagating planetary waves induce divergence in sub-polar regions, strengthening westerlies that facilitate La Niña-related wave trains. These wave trains trigger cyclonic circulation over Northeast Asia, improving air quality in the NCP. These findings underscore the complex interplay between ENSO, QBO, and atmospheric dynamics in shaping regional air pollution.