By partitioning the monthly mean horizontal winds at various tropospheric levels into the vertical mean and vertical shear components, the kinetic energy of the shear flow (Ks) averaged between June and September within the box (40¿--100¿E, 0--20¿N) is found to be strongly related to the South Asia summer monsoon (SASM), and is therefore defined as the SASM index (SASMI) to represent the intensity of the SASM. When the value of the SASMI is large, positive (negative) temperature anomalies are found over the Asian landmass (Indian Ocean). This gives rise to an above-normal north-south land-sea temperature gradient. As a result, low-level southwesterlies extend all the way from the Indian sub-continent to the Philippines, which result in a monsoon trough over south China. The southerlies to the north of this trough converge with the northerlies in the midlatitudes over northern China to form a monsoon frontal trough.