Direct Effect of Sulfate Aerosols on the Process of the East Asian Subtropical Monsoon

The direct effect of sulfate aerosols on the process of the East Asian subtropical monsoon was investigated using the interactive global climate-chemistry model version 5 of the Community Atmosphere Model combined with the NCEP/NCAR reanalysis dataset. The simulation results suggest that the surface temperature in most parts of the East Asian land has decreased, and the zonal thermal difference between the East Asian mid-latitude land and the western Pacific has weakened. The reversal time of the land-sea zonal thermal difference is later/earlier in early spring/late summer, and the onset/retreat times of the subtropical monsoon have been shifted to later/earlier by approximately 3 pentads because of the direct radiation effect of sulfate aerosols. The decreasing land-sea zonal thermal difference has led to the weakening of the south wind over the East Asian land and the East Asian monsoon, and a regional adjustment in monsoon rainfall. The results also imply that the reversal time of the land-sea zonal thermal difference between the East Asian mid-latitude land and the western Pacific after 1980 is later/earlier in early spring/late summer than before 1980 from the analysis of the NCEP/NCAR reanalysis data, leading to a/an later/earlier onset/retreat of the subtropical monsoon, weakening of the south wind over the East Asian land, and weakening of the subtropical monsoon after 1980. The sulfate aerosol direct radiation effect is likely to be one of the causes of this result. Both previous observational and model studies have shown that the sulfate aerosol loading increased markedly in the East Asian land after 1980 as a result of human activity, and the East Asian monsoon weakened. This study further confirms the relationship between them.