A Circulation Index of the Spring Persistent Rainfall in the South of the Yangtze and Its Synoptic Characteristics

Using daily precipitation dataset of 2466 stations over China, daily and monthly reanalysis dataset from 1961 to 2016 by National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), a circulation index of spring persistent rainfall (I_SPR) is defined based on latitudinal differences of zonal winds in lower troposphere over the region from East Asia to the western Pacific. Relationships of I_SPR with spring persistent rainfall and general circulation is investigated. Results show that the westerly wind from the South of the Yangtze to South China and the easterly wind located in the region from Huanghuai to Jianghuai area in spring is beneficial to the spring persistent rainfall in the South of the Yangtze. Using this characteristic, the spring rainfall circulation index in the South of the Yangtze is defined. In high-index cases, rainfall increases in the South of the Yangtze; and in low-index cases, rainfall decreases. Meanwhile, the index defined not only reflect the annual variation of the spring persistent rainfall, but also can reflect the daily variation of the spring persistent rainfall. The index has good synoptic significance, and is positively correlated with the daily precipitation in the South of the Yangtze. A verification using data from 1961 to 2016 in the South of the Yangtze indicates that this definition of index can reflect the precipitation in most years in the South of the Yangtze in spring. Taking the year of 2016 as an example, results show that the index defined has a clear physical meaning. In high-index cases, the plateau trough and the southern branch of westerly trough are more active. The western Pacific high moves northward anomalously. Confluence of the southwesterly wind from the low trough, the western Pacific high and the cold air from the higher latitude occur from the South of the Yangtze to South China. Low level convergence and high level divergence provide dynamic uplifting conditions for spring persistent rainfall in the South of the Yangtze. In low-index cases, the western Pacific high moves southward anomalously, and anticyclone anomalies cover the mainland of China. The existence of weak divergence in the lower troposphere in the South of the Yangtze to South China goes against persistent rainfall.