Indirect vs. Direct Effects of Anthropogenic Sulfate on the Climate of East Asia as Simulated with a Regional Coupled Climate-Chemistry/Aerosol Model

Abstract

We intercompare a series of multi-year simulations with a coupled regionalchemistry-climate model for east Asia to assess the relative importance ofdirect and indirect (Type I) effects of anthropogenic sulfate on the climateof the region. Both direct and indirect aerosol effects induce a negativeradiative forcing that results in a cooling of the surface and in a decrease of precipitation. Under present day sulfur emissions,the direct aerosol effects prevail during the cold season, while the indirecteffects dominate in the warm season (when cloudiness is maximum over the region). When both the direct and indirect effects are included, the surface cooling varies in the range of –0.1 to over –1 K throughout the region and extended areas ofstatistically significant cooling are found in all seasons except winter.The indirect effects largely dominate in inhibiting precipitation, especiallyduring the summer. When doubling the sulfur emissions, the direct effects aresubstantially strengthened, while the indirect effects are only marginally affected. This indicates that the indirect effects over the region might be asymptotically approaching their maximum efficiency. Overall, the indirect effects appear necessary to explain theobserved temperature record over some regions of China, at least in the warm season.A number of uncertainties need to be addressed, such as due to Type IIindirect effects, modeling of the relationship between aerosol concentration and cloud optical properties, and contribution of aerosols other than anthropogenic sulfate.