Observational and Modeling Studies of Impacts of the South China Sea Monsoon on the Monsoon Rainfall in the Middle-Lower Reaches of the Yangtze River During Summer

Based on the ERA-40 and NCEP/NCAR reanalysis data,the NOAA Climate Prediction Center＇s merged analysis of precipitation（CMAP）,and the fifth-generation PSU/NCAR Mesoscale Model version 3（MM5v3）,we defined a monsoon intensity index over the East Asian tropical region and analyzed the impacts of summer（June-July） South China Sea（SCS） monsoon anomaly on monsoon precipitation over the middle-lower reaches of the Yangtze River（MLRYR） using both observational data analysis and numerical simulation methods.The results from the data analysis show that the interannual variations of the tropical monsoon over the SCS are negatively correlated with the southwesterly winds and precipitation over the MLRYR during June-July.Corresponding to stronger（weaker） tropical monsoon and precipitation,the southwesterly winds are weaker（stronger） over the MLRYR,with less（more） local precipitation.The simulation results further exhibit that when changing the SCS monsoon intensity,there are significant variations of monsoon and precipitation over the MLRYR.The simulated anomalies generally consist with the observations,which verifies the impact of the tropical monsoon on the monsoon precipitation over the MLRYR.This impact might be supported by certain physical processes.Moreover,when the tropical summer monsoon is stronger,the tropical anomalous westerly winds and positive precipitation anomalies usually maintain in the tropics and do not move northward into the MLRYR,hence the transport of water vapor toward southern China is weakened and the southwest flow and precipitation over southern China are also attenuated.On the other hand,the strengthened tropical monsoon may result in the weakening and southward shift of the western Pacific subtropical high through self-adjustment of the atmospheric circulation,leading to the weakening of the monsoon flows and precipitation over the MLRYR.

Relationship Between Winter Snow Cover Days in Northeast China and Rainfall near the Yangtze River Basin in the Following Summer

Based on observed snow and precipitation data and NCEP/NCAR reanalysis data, the relationship between the number of winter snow cover days in Northeast China and the following summer＇s rainfall in the northern part of southern China is analyzed and the possible underlying mechanisms are discussed. The results indicate that a negative relationship is significant throughout the study period, especially more obvious after the 1980s. The pre-winter circulation patterns in years with more snow cover days and less summer rainfall in the south bank of the Yangtze River are almost the same. In years with more snow cover days, lower temperatures at the lower level over Northeast China are found in winter and spring. The winter monsoon is weaker and retreats later in these years than in those with fewer snow cover days. In spring of years with more snow cover days, anomalous cyclonic circulation is observed over Northeast China, and anomalous northerly wind is found in eastern China. In summer of these years, anomalous northeasterly wind at the lower level is found from the area south of the Yangtze River to the East China Sea and Yellow Sea; and with less southwesterly water vapor transport, the rainfall in the area south of the Yangtze River is less than normal, and the opposite patterns are true in years with fewer snow cover days. In recent years, the stable relationship between winter snow cover in Northeast China and summer rainfall in the Yangtze River basin can be used for summer rainfall prediction. The results are of great importance to short-term climate prediction for summer rainfall.