Relationship between the South Asian Summer Monsoon and Rainfall Anomalies Associated with the East Asia summer monsoon

This paper attempts to relate the intensity of the South Asia summer monsoon (SASM) to the rainfall anomalies associated with the East Asia summer monsoon (EASM). An index is first defined to represent the intensity of the SASM by partitioning the monthly mean horizontal winds at various tropospheric levels into the vertical mean and vertical shear components. The data are from the NCEP reanalyses for the period 1958-1997. The kinetic energy of the shear flow (Ks) over south Asia is found to be the largest between June and September, which is concomitant with the evolution of the SASM. The value of Ks averaged over these four months within the box (40º - 100ºE, 0 - 20ºN) is therefore defined as the south Asian summer monsoon index (SASMI) to represent the intensity of the SASM. A good correlation is found between the SASMI and the other intensity indices of the SASM. Based on the value of the SASMI, the SASM of individual years can be classified as either strong, normal or weak. The planetary circulations associated with the strong and weak SASM years are almost opposite. As expected, a strong SASM brings an abundant rainfall and cool air to south Asia while a weak monsoon leads to a drought and a hot summer. Besides the conditions in south Asia, the value of SASMI is also closely related to the summer rainfall anomalies over the eastern part of China. The monsoon trough and the associated rainband of the EASM shifts northward as the magnitude of the SASMI increases. Such a relationship be explained in terms of the planetary-scale flow. When the SASMI is large, strong low-level southwesterly anomalies extend into northern China and thus transport an abundant amount of moisture to the region, and hence heavy rainfall. When the SASM is weak, the situation is almost opposite.