南海夏季风垂直结构的变化特征及其对中国东部降水的影响

采用NCEP/NCAR再分析月平均资料和NOAA全球逐日降水资料，首先利用EOF方法分析了南海夏季风的垂直结构时空变化特征，然后初步探讨了南海夏季风垂直结构对中国夏季降水的影响和机制。(1) 南海夏季风的垂直结构有明显的年际和年代际变化特征。EOF第一模态型主要表现为南海夏季风垂直结构的年际变化特征，为对流层低层西南风和对流层高层东北风同时增强(同时减弱)(简称“低层-高层同时增强”和“低层-高层同时减弱”)两种典型结构变化；EOF第二模态主要表现为南海夏季风垂直结构的年代际变化，为对流低层(高层)西南风(东北风)由下向上的增强到减弱变化和相反的对流层低层(高层)西南风(东北风)减弱到增强的变化(简称“低层强弱-高层强弱”和“低层弱强-高层弱强”)的两个不同年代(时段)的垂直结构变化。(2) 南海夏季风垂直结构变化通过改变对流层低层、高层的环流异常变化来影响中国东部夏季降水的异常变化。南海夏季风呈“低层-高层同时增强”垂直结构时，南海低纬热带季风环流异常加强，长江流域低层辐散、高层辐合及异常下沉运动，其南侧的华南地区和北侧的东北地区是低层辐合、高层辐散和异常上升运动，导致华南降水异常偏多、长江流域降水偏少、北方降水偏多；在“低层-高层同时减弱”年，则相反。南海夏季风呈“低层弱强-高层弱强”垂直结构时，我国东部地区自华南到东北，分别是低层辐合(辐散)、高层辐散(辐合)的有利于上升(下沉)运动的环流条件，华南、江淮地区降水增多，江南、东北地区降水减少；对“低层强弱-高层强弱”时段，则相反。 

VARIATION CHARACTERISTICS OF THE VERTICAL STRUCTURE OF THE SOUTH CHINA SEA SUMMER MONSOON AND ITS INFLUENCE ON PRECIPITATION IN EASTERN CHINA

Based on the NCEP/ NCAR reanalysis monthly mean data and NOAA global daily precipitation data, the present study analyzes the temporal and spatial characteristics of the vertical structure variation of the South China Sea summer monsoon (SCS-SM) by using the empirical orthogonal function (EOF) method, and then discusses the influence and mechanism of the vertical structure of the SCS-SM on summer precipitation in China. The main conclusions are as follows: (1) The vertical structures of the SCS-SM feature obvious inter-annual and interdecadal variation. The first mode of EOF mainly shows the inter-annual variation of the vertical structure of the SCS-SM, which consists of two typical structural changes, i.e., the simultaneous increase (decrease) of the southwest wind in the lower troposphere and the northeast wind in the upper troposphere (referred to as "lower-upper troposphere simultaneous enhancement" and "lower-higher troposphere simultaneous weakening"); the second mode of EOF mainly features the interdecadal change of the vertical structure of the SCS-SM, which can be divided into two vertical structure changes of two different ages (periods): the change of the southwest wind (northeast wind) at the lower troposphere (upper troposphere) from enhancement to weakening from bottom to top with height, and the change of the southwest wind (northeast wind) at the lower troposphere (upper troposphere) from weakening to enhancement (referred to as "low troposphere from stronger to weaker-upper troposphere from stronger to weaker" and "low troposphere from weaker to stronger-upper troposphere from weaker to stronger"). (2) The changes of the SCS-SM vertical structure significantly affect the anomalous changes of summer precipitation in eastern China by changing the circulation anomalies in the lower and upper troposphere. When the SCS-SM presents a vertical structure of "lower-upper troposphere simultaneous enhancement", the low-latitude tropical monsoon circulation in the SCS is anomalously strengthened, and there are the low-level divergence, upper-level convergence and abnormal subsidence movement in the Yangtze River Basin. At the same time, southern China and northeastern China are characterized by low-level convergence, upper-level divergence, and abnormal ascending movements. Such a low-upper-level circulation configuration leads to abnormally high precipitation in southern China, less precipitation in the Yangtze River Basin, and more precipitation in the north. It is the opposite in the "lower-higher troposphere simultaneous weakening" years. When the SCS-SM vertical structure is in the period of "low troposphere from weaker to stronger-upper troposphere from weaker to stronger", from southern China to northeastern China, the circulation conditions of low-level convergence (divergence) and high-level convergence (convergence) are favorable to the rising (sinking) movement, which leads to the increase of precipitation in southern China and Jianghuai areas, and the decrease in the precipitation in Jiangnan and northeastern China. The opposite is true for the period of "low troposphere from stronger to weaker-upper troposphere from stronger to weaker".