THE ANALYSIS OF MECHANISM OF IMPACT OF AEROSOLS ON EAST ASIAN SUMMER MONSOON INDEX AND ONSET

RegCM4.3, a high-resolution regional climate model, which includes five kinds of aerosols (dust, sea salt, sulfate, black carbon and organic carbon), is employed to simulate the East Asian summer monsoon (EASM) from 1995 to 2010 and the simulation data are used to study the possible impact of natural and anthropogenic aerosols on EASM. The results show that the regional climate model can well simulate the EASM and the spatial and temporal distribution of aerosols. The EASM index is reduced by about 5% by the natural and anthropogenic aerosols and the monsoon onset time is also delayed by about a pentad except for Southeast China. The aerosols heat the middle atmosphere through absorbing solar radiation and the air column expands in Southeast China and its offshore areas. As a result, the geopotential height decreases and a cyclonic circulation anomaly is generated in the lower atmosphere. Northerly wind located in the west of cyclonic circulation weakens the low-level southerly wind in the EASM region. Negative surface radiative forcing due to aerosols causes downward motion and an indirect meridional circulation is formed with the low-level northerly wind and high-level southerly wind anomaly in the north of 25 degrees N in the monsoon area, which weakens the vertical circulation of EASM. The summer precipitation of the monsoon region is significantly reduced, especially in North and Southwest China where the value of moisture flux divergence increases.