Characteristics of circulation anomalies and formation mechanisms for late autumn and early winter fog days over eastern China

Daily observations collected at meteorological gauge stations in eastern China and ERA5 reanalysis data for the period of 1960—2022 are utilized to investigate the key circulation in late autumn and early winter fog days in eastern China and its formation mechanisms based on statistical analysis and dynamical diagnosis. Results show that when fog days are 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 and the East Asian trough weakens, which is not conducive to the eastward and southward movement of cold air. There are an anticyclonic circulation anomaly in the lower troposphere to the east of 120°E and a cyclonic circulation anomaly to the west accompanied with significant southerly wind anomalies along the East Asian coast, corresponding to weakened East Asian winter monsoon, reduced sea level pressure gradient, and weak near-surface wind speed. The warm advection caused by the anomalous southerly winds is the dominant factor that affects the atmospheric stability, promoting the formation and maintenance of temperature inversion layers. Moreover, the southerly wind anomalies are also favorable for the northward transport of water vapor and cause moisture flux convergence in eastern China, providing a sufficient water vapor condition for the formation of fog. The large-scale circulation anomalies associated with fog days in eastern China are synergistically affected by El Niño-Southern Oscillation (ENSO) and mid- and high-latitude wave trains. ENSO is conducive to anomalous low-level anticyclone in the western North Pacific. The mid- and high-latitude Rossby wave trains triggered by the negative wave source in central Europe cause 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 enhance our understanding of the formation mechanisms for late autumn and early winter fog days in eastern China, and lay a foundation for seasonal fog prediction.