Changes of the Boundary Between the South Asian and East Asian Tropical Summer Monsoon Subsystems

NCEP/NCAR 65-year daily reanalysis wind data are used to divide the boundary of the two tropical Asian summer monsoon subsystems (the South Asian summer monsoon and the East Asian Tropical summer monsoon). According to the boundary derived from the wind data, it's concluded that both of the Asian summer monsoons have their own control domain. A deviation index is defined which can describe changes of two Asian tropical summer monsoon subsystems' boundary, positive value means that the boundary of two tropical Asian summer monsoon subsystems is further east, while the negative value means their boundary is further west. Then comparative study is carried out to check out which area of the boundary changes larger. The area with intense changes is further investigated to study the change regulation of two Asian tropical summer monsoon subsystems. At last, a monsoon strength index is designed which can describe the strength change of Asian tropical summer monsoon.According to the latitude of vorticity minimum value, and the boundary deviation index between the decadal changes, results show the boundary change is either eastern, central or western. When the pattern of the central changes to the pattern of western, there is a mutation passing the test of 0.01 level. The low, medium and high layers of the three boundary categories are also quite different. And then, 10°-17.5°N area can be chosen as the main study area according to the oscillation amplitude. By using the boundary deviation to define the two Asian Tropical Summer monsoon indexes, it's found that consistent change happens more than reversed phase change for the strength of two summer monsoons. And the meridional wind anomaly field of four kinds of strength type from the Bay of Bengal to the South China Sea reflect the four anomaly situations:'+ -', '- +', '- -', and '+ +'.