Interannual Variability Of Winter Fog Days Over Eastern China And Its Association With Sea Surface Temperature Anomaly

The observational and reanalysis datasets for the period of 1980-2017 are utilized to analyze the spatial-temporal characteristics of fog over China,and on this basis,the effects of El Ni(?)o-Southern Oscillation(ENSO)and North Atlantic sea surface temperature(SST)on winter fog days over eastern China and their physical mechanisms are explored.The main conclusions are as follows:(1)Fog is mainly concentrated over eastern China and the fog there has strong seasonal variations with less frequent fog in summer,and more in winter.The winter fog days over eastern China exhibit strong year-to-year variability.Favorable meteorological factors for fog formation on interannual time scales include reduced wind speed,stable atmosphere,and the decreased difference between air temperature and dew point temperature(T-T_d).The first two dominant modes of winter fog days over eastern China are the monopole mode and dipole mode,respectively.The monopole mode shows that winter fog days exhibit a monopole spatial pattern.The dipole mode shows that winter fog days exhibit a meridional dipole spatial pattern.(2)The monopole mode has a significantly positive relationship with ENSO.During El Ni(?)o winters,an anomalous anticyclone tends to appear over the western North Pacific(WNP),which can weaken the climatological winter northerly winds and enhance water vapor transport from oceans,thus increasing winter fog days over eastern China.Roughly opposite anomalies of fog frequency are displayed during La Ni(?)a winters.However,the linear relationship between ENSO and winter fog days is mainly contributed by La Ni(?)a and partial El Ni(?)o events.EP El Ni(?)o and the further eastward CP El Ni(?)o events correspond to the increased winter fog days.The further westward CP El Ni(?)o events tend to coincide with the less frequent foggy weather.(3)The dipole mode is closely linked to North Atlantic tripole SST.Theoretical analysis and numerical experiments show that the effect of North Atlantic tripole SST on winter fog days over eastern China is mainly be through a quasi-barotropic Rossby wave train that extends from the North Atlantic through Eurasia to East Asia.During positive North Atlantic tripole SST anomaly years,a positive phase of the North Atlantic Oscillation-like mode appears over North Atlantic.Over northern China,there are significant cyclonic circulation anomalies,and over southern China,there are significant anticyclonic circulation anomalies.These atmospheric circulation anomalies will increase meridional wind and T-T_d in the northern region,resulting in the decreased number of winter fog days,and increase static stability in the southern region,leading to the increase of winter fog days.Finally,winter fog days over eastern China exhibit a spatial pattern of positive anomaly in the south and negative anomaly in the north.