Intraseasonal responses of the East Asia summer rainfall to anthropogenic aerosol climate forcing

The WRF Model is used to investigate intraseasonal responses of the summer rainfall to aerosol direct and cloud-adjustment effects over East Asia, where the anthropogenic aerosol loading has been increasing in the past few decades. The responses are evaluated by comparing two cases for each year during 2002–2008: a control case imposing the observed aerosol optical depth of the corresponding year and a sensitivity case having anthropogenic components of the control case reduced by 75%. Analyses of multiple-year simulations reveal that aerosol-induced changes of rainfall and circulation exhibit strong intraseasonal variability, and that the spatial pattern of changes in the monthly rainfall is related to the intensification and westward extension of the western North-Pacific subtropical high (WNPSH) by increased aerosols. This perturbation of the WNPSH induces surface air divergence over the southeast China and convergence over regions to the north and west of the WNPSH, causing, respectively, decreased and increased rainfall. As the WNPSH migration path varies year by year, however, the variability of rainfall changes over subregions of the eastern China (e.g., North China) is large within the decade. Meanwhile, the pattern of summer-gross rainfall changes also shows large interannual variation, but the general pattern of wetter in the west and dryer in the east persists. Results also suggest that the aerosol increase tends to reduce the number of Tibet Plateau vortices, which indirectly influence summer rainfall over the eastern China.