Variation Characteristics of the Vertical Structure of the Summer Somali Cross-Equatorial Flow and Its Relationship with East Asia Summer Monsoon Activity

The spatiotemporal evolution of Somali cross-equatorial flow (SMJ) in summer at five levels (1000 hPa, 925 hPa, 850 hPa, 700 hPa, 600 hPa) are studied on the basis of National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and ERA40 reanalysis monthly mean data. The results show that vertical structures of SMJ in the middle and lower troposphere differ, with interannual and interdecadal change characteristics. Two main vertical structure modes are clarified. The first is a consistent mode in the entire region ("A" mode), and the second is the vertically opposite mode ("B" mode). Thus, four vertical anomalous types of SMJ are observed to include consistently strong pattern ("A+" pattern), consistently week pattern ("A-" pattern), higher weak and lower strong pattern ("B+" pattern), and higher strong and lower weak pattern ("B-" pattern). It is also indicated that the vertical structures of SMJ are closely related to East Asia summer monsoon activity. Specifically, the "A+" pattern (“A-" pattern) correlates strongly to the strong (weak) East Asia summer monsoon with more (less) rainfall in eastern Inner Mongolia and North China. The "B+" pattern ("B-" pattern) correlates strongly to the weak (strong) East Asia summer monsoon with less (more) rainfall in the southern area of the Yangtze River and Japan.