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Dynamics of Asian Summer Monsoon Response to Anthropogenic Aerosol Forcing

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  • 1 Physical Oceanography Laboratory, Collaborative Innovation Center of Marine Science and Technology, Ocean University of China, and Qingdao National Laboratory for Marine Science and Technology, and College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao, China
  • | 2 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, and Physical Oceanography Laboratory, Collaborative Innovation Center of Marine Science and Technology, Ocean University of China, and Qingdao National Laboratory for Marine Science and Technology, Qingdao, China
  • | 3 Research Center for Advanced Science and Technology, University of Tokyo, Komaba, Meguro-ku, Tokyo, Japan
  • | 4 Physical Oceanography Laboratory/CIMST, Ocean University of China and Qingdao National Laboratory for Marine Science and Technology, and College of Oceanic and Atmospheric Sciences, Ocean University of China, Qingdao, China
  • | 5 State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China
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Abstract

Anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in Asian summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST)-mediated change, the physical mechanisms for anthropogenic-aerosol-induced changes in the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM) are diagnosed. Using coupled and atmospheric general circulation models, this study shows that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20° and 40°N and weakens the EASM north of 20°N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to this direct atmospheric response to radiative forcing, although the aerosol-induced North Pacific SST cooling also contributes. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the north–south interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, with large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing, resulting in a reorganization of the regional meridional atmospheric overturning circulation. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the intermodel spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean–atmosphere coupling.

© 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Hai Wang, wanghai@ouc.edu.cn

Abstract

Anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in Asian summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST)-mediated change, the physical mechanisms for anthropogenic-aerosol-induced changes in the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM) are diagnosed. Using coupled and atmospheric general circulation models, this study shows that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20° and 40°N and weakens the EASM north of 20°N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to this direct atmospheric response to radiative forcing, although the aerosol-induced North Pacific SST cooling also contributes. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the north–south interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, with large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing, resulting in a reorganization of the regional meridional atmospheric overturning circulation. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the intermodel spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean–atmosphere coupling.

© 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Hai Wang, wanghai@ouc.edu.cn
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