资源环境科技发展态势分析平台

The influence of El Nino-Southern Oscillation (ENSO) on wintertime fog days in eastern China was investigated through the analysis of fog station data in China and the National Centers for Environmental Prediction reanalysis data during the period of 1954-2007. A composite analysis shows distinctive anomaly patterns of fog days in early and late winter during ENSO. More frequent fogs appear in Central and North China (around 30 degrees-40 degrees N) in November-December (ND), while more frequent fogs occur over Middle Reach of Yangtze River (27 degrees-32 degrees N) and less frequent fogs appear over 32 degrees-40 degrees N in January-February (JF) during El Nino. The cause of this distinctive sub-seasonal evolution feature is discussed as the following. Actually we analyzed all meteorological fields during El Nino and La Nina, and found that they are approximately in a mirror image. To reduce the number of figures, we presented the difference fields in most of cases. We found that anomalous southwesterlies associated with El Nino penetrate into North China (up to 40 degrees N) in ND. The northward moisture transport leads to near-surface air saturation over the region. Moreover, atmosphere becomes convectively unstable in South China (20 degrees-30 degrees N), which leads to a positive precipitation anomaly there. The local convergence leads to an anomalous divergence and subsidence north of 30 degrees N. Both the saturated air and stable stratification lead to more frequent fog days in 30 degrees-40 degrees N. Compared to ND, anomalous southwesterly flow in JF is weaker, and can only transport moisture up to Central China (near 30 degrees N). As maximum precipitation anomaly is confined near the southeastern coast of China, anomalous divergence and subsidence appear north of 27 degrees N. This stable stratification together with the anomalous moisture transport causes the increase of fog over Central China but decrease of fog in North China (33 degrees-38 degrees N) due to the subsidence induced dryness.