Characteristics of anomalous circulation and formation mechanisms of autumn and winter fog days over eastern China

The daily observational data from meteorological gauge stations in eastern China and ERA5 reanalysis data for the period of 1960-2022 are utilized to investigate the key circulation of late autumn and early winter fog days in eastern China and its formation mechanisms by the statistical analysis and dynamical diagnosis. Results show that when fog days are more prevalent in eastern China during late autumn and early winter, a significant positive geopotential height anomaly occurs in the middle and upper troposphere along the East Asian coast, the East Asian trough weakened, which is not conducive to the eastward and southward movement of cold air. The anomalous high pressure also inhibits the vertical upward motion. There is an anticyclonic circulation anomaly in the lower troposphere to the east of 120E and a cyclonic circulation anomaly to the west, with significant southerly wind anomalies along the East Asian coast, corresponding to the weakened East Asian winter monsoon, reduced sea level pressure gradient, and weak near-surface wind speed. The warm advection caused by the anomalous southerly wind is the dominant factor in the stability of the atmospheric stratification, which is conducive to the formation and maintenance of temperature inversion layers. Moreover, the southerly wind anomalies also favor the northward transportation of water vapor and cause the convergence of moisture flux in eastern China, providing sufficient water vapor conditions for the formation of fog. The large-scale circulation anomalies when fog is frequent are synergistically affected by the El Ni?o-Southern Oscillation (ENSO) and mid- and high-latitude wave trains. ENSO enhances the heat and moisture transportation from the ocean through the anomalous low-level northwestern Pacific anticyclone. The mid- and high-latitude Rossby wave train excited by the negative wave source in central Europe causes high pressure anomalies in the middle and upper troposphere along the East Asian coast, weakening the East Asian trough and conducive to the occurrence of fog days. The results of this study deepen the understanding of the formation mechanisms of late autumn and early winter fog days in eastern China, and lay foundation for the fog seasonal prediction.