The response of East Asian Summer Monsoon (EASM) precipitation to long termchanges in regional anthropogenic aerosols (sulphate and black carbon) isexplored in an atmospheric general circulation model, the atmosphericcomponent of the UK High-Resolution Global Environment Model v1.2 (HiGAM).Separately, sulphur dioxide (SO2) and black carbon (BC) emissions in 1950and 2000 over East Asia are used to drive model simulations, while emissionsare kept constant at year 2000 level outside this region. The response of theEASM is examined by comparing simulations driven by aerosol emissionsrepresentative of 1950 and 2000. The aerosol radiative effects are alsodetermined using an off-line radiative transfer model. During June, July andAugust, the EASM was not significantly changed as either SO2 or BCemissions increased from 1950 to 2000 levels. However, in September,precipitation is significantly decreased by 26.4% for sulphate aerosol and14.6% for black carbon when emissions are at the 2000 level. Over 80% ofthe decrease is attributed to changes in convective precipitation. The coolerland surface temperature over China in September (0.8 °C for sulphateand 0.5 °C for black carbon) due to increased aerosols reduces thesurface thermal contrast that supports the EASM circulation. However,mechanisms causing the surface temperature decrease in September aredifferent between sulphate and BC experiments. In the sulphate experiment,the sulphate direct and the 1st indirect radiative effectscontribute to the surface cooling. In the BC experiment, the BC direct effectis the main driver of the surface cooling, however, a decrease in low cloudcover due to the increased heating by BC absorption partially counteracts thedirect effect. This results in a weaker land surface temperature response toBC changes than to sulphate changes. The resulting precipitation response isalso weaker, and the responses of the monsoon circulation are different forsulphate and black carbon experiments. This study demonstrates a mechanismthat links regional aerosol emission changes to the precipitation changes ofthe EASM, and it could be applied to help understand the future changes inEASM precipitation in CMIP5 simulations.
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