Abstract
Abstract. The response of East Asian Summer Monsoon (EASM) precipitation to long term changes in regional anthropogenic aerosols (sulphate and black carbon) is explored in an atmospheric general circulation model, the atmospheric component of the UK High-Resolution Global Environment Model v1.2 (HiGAM). Separately, sulphur dioxide (SO2) and black carbon (BC) emissions in 1950 and 2000 over East Asia are used to drive model simulations, while emissions are kept constant at year 2000 level outside this region. The response of the EASM is examined by comparing simulations driven by aerosol emissions representative of 1950 and 2000. The aerosol radiative effects are also determined using an off-line radiative transfer model. During June, July and August, the EASM was not significantly changed as either SO2 or BC emissions increased from 1950 to 2000 levels. However, in September, precipitation is significantly decreased by 26.4% for sulphate aerosol and 14.6% for black carbon when emissions are at the 2000 level. Over 80% of the decrease is attributed to changes in convective precipitation. The cooler land surface temperature over China in September (0.8 °C for sulphate and 0.5 °C for black carbon) due to increased aerosols reduces the surface thermal contrast that supports the EASM circulation. However, mechanisms causing the surface temperature decrease in September are different between sulphate and BC experiments. In the sulphate experiment, the sulphate direct and the 1st indirect radiative effects contribute to the surface cooling. In the BC experiment, the BC direct effect is the main driver of the surface cooling, however, a decrease in low cloud cover due to the increased heating by BC absorption partially counteracts the direct effect. This results in a weaker land surface temperature response to BC changes than to sulphate changes. The resulting precipitation response is also weaker, and the responses of the monsoon circulation are different for sulphate and black carbon experiments. This study demonstrates a mechanism that links regional aerosol emission changes to the precipitation changes of the EASM, and it could be applied to help understand the future changes in EASM precipitation in CMIP5 simulations.
Highlights
Performance of the coupled version of High-resolution Global Environment Model (HiGAM) has been ter to rain water in liquid clouds is linked to the cloud conevaluated by Shaffrey et al (2009), in which they noted that densation nuclei (CCN)
HiGAM captures the global pattern of precipitation, and compared to models with lower horizontal resolution HiGAM enhances the precipitation over the coast the maritime continent and the southern slope of the Himalayas
Over East Asia, the Meiyu/Baiyu/Changma front is weaker in HiGAM than in the observations, with an deficit over India
Summary
During June, July and August, the EASM was not significantly changed as either SO2 or BC emissions increased from 1950 to 2000 lev-. In September, precipitation is significantly de- strongest monsoon system due to the combination of thermal creased by 26.4 % for sulphate aerosol and 14.6 % for black contrast between the largest continent and the Pacific and Incarbon when emissions are at the 2000 level. The cooler land surface temperature over China in September (0.8 ◦C for sulphate and 0.5 ◦C for black carbon) athnedwDoirnlgd,’s2p0o0p6u).laIttioanffiencEtSsoatuhrtehtlhaivnedSsEyoafssmttAeosrmeiat.han a third of Over the past several decadesS, racpiiednincdeusstrialisation over due to increased aerosols reduces the surface thermal contrast Asia has resulted in a dramatic increase in emissions of that supports the EASM circulation. The sulphate direct and the 1st indirect rapatretchiepistuartfioacneb(yHmayowdouolOadticnangedraaBdnoiautSciohcnerii,en2n0th0ce0e)atamnodsbpyhecrheaanngddiative effects contribute to the surface cooling. In the BC ing cloud microphysics (Twomey, 1977; Albrecht, 1989)
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