Two pathways of how remote SST anomalies drive the interannual variability of autumnal haze days in the Beijing-Tianjin-Hebei region, China

Analogous to the circumstances in wintertime, the increasing severity of autumnal haze pollution over the Beijing-Tianjin-Hebei (BTH) region may also lead to impairment of the socioeconomic development and human health in this region. Despite man-made aerosol emissions, the interannual variability of autumnal (September-October-November) haze days (AHDs) in the BTH region (AHD(BTH)) is apparently tied to the global and regional meteorological anomalies. The present study suggests that an above-normal AHD(BTH) is closely associated with the simultaneous sea surface temperature (SST) warming in two regions (over the North Atlantic subtropical sector, R1, and over the western North Pacific sector, R2). When the autumnal SST warming in both R1 and R2 is significant, the likelihood of a higher AHD(BTH) is greatly enhanced. Observational and simulation evidence demonstrated how remote SST anomalies over R1 and R2 influence variation of AHD(BTH) via two different pathways. Firstly, SST warming in R1 can induce a downstream midlatitudinal Rossby wave train, leading to a barotropic high-pressure and subsidence anomaly over the BTH region. Secondly, SST warming in R2 can also result in air subsidence over the BTH region through an anomalous local meridional cell. Through these two distinct pathways, localized meteorological circumstances conducive to a higher AHD(BTH) (i.e., repressed planetary boundary layer, weak southerly airflow, and warm and moist conditions) can be established.