Effects of Atlantic multidecadal oscillation and Pacific decadal oscillation on interdecadal variability of fog frequency in autumn-winter season in Southwest China

Based on the Atlantic multidecadal oscillation (AMO) and the Pacific decadal oscillation (PDO) indexes, the observation of fog frequency in China and the reanalysis data from the National Centres for Environmental Prediction/National Center for Atmospheric Research during 1958-2007 are selected to study the effects of the AMO and PDO on the interdecadal variability of fog frequency in autumn-winter in Southwest China (SWC). The results suggest that the fog frequency in SWC is highest in autumn-winter season and shows a characteristic of strong interdecadal variation. When the north Atlantic and the north Pacific are abnormally cold (warm), which corresponds to the negative (positive) phase of the AMO and the positive (negative) phase of the PDO respectively, the fog in SWC occurs more (less) frequently. The atmosphere is unstable because of the eastward propagation of Rossby wave train and the anomaly of Siberian high-pressure, which are affected by the AMO and PDO, respectively. The AMO particularly affects the atmospheric stability in the middle and lower level, while the PDO mainly affects that of the whole layer. The water vapour is modulated by the southward movement of cold air in Northern China during the negative phase of the AMO and positive phase of the PDO. This results in the decreasing of temperature and specific humidity, and further leads to the supersaturation of water vapour in the lower atmosphere of SWC. The AMO particularly affects the temperature and water vapour supersaturation in the lower atmosphere, while the PDO mostly modulates the atmospheric stability of the whole layer and the southward movement of cold air.