Investigating Anthropogenic Aerosol Impacts on East Asia Summer Monsoon Using the NCAR CAM5

Aerosol can affect climate directly by scattering and absorption of radiation (both solar and terrestrial), and indirectly by modifying microphysical and radiative properties of clouds and precipitation processes. East Asia has experienced the fastest economic growth in the world during the last several decades. Along with severe aerosol pollution, significant changes in precipitation pattern, frequency and intensity have been observed in East Asia, e.g., the general trend of "South Flooding and North Drought", and the persistent reduction of frequency of light rain. In this study we analyze a series of simulations using the latest released NCAR Community Atmospheric Model version 5 (CAM5) to investigate the impacts of anthropogenic aerosols (sulfate and black carbon (BC)) on precipitation and summer monsoon system in East Asia. CAM5 includes treatment of all the important effects of aerosol on climate. We find that anthropogenic aerosols (sulfate and BC) reduce the solar flux reaching the surface and generate a surface cooling over the land, while the absorption of solar flux is enhanced by BC in the low troposphere. This reduces the surface temperature by 1-2 K and increases the low-level atmospheric stability over the land, which reduces land-sea thermal contrast and weakens the East Asia summer monsoon. The precipitation shifts southwardly in east China with the weakening of monsoon. Indirect effect from anthropogenic sulfate reduces cloud droplet effective radius, increases low-level cloud amount and liquid water path, while semi-direct effect of BC reduces cloud amount and liquid water path due to its "heating" effect. Precipitation pattern changes resulting from the semi-direct effect of BC (through heating clouds) and from the indirect effect of sulfate (through increasing cloud lifetime) are similar, although cloud responses to BC and sulfate are significantly different.