Modeling Study Of Impacts Of Anthropogenic Aerosol Forcings On The East Asian Summer Monsoon

The impact of the total effects due to anthropogenic aerosols from global, East Asian, and non-East Asian sources on East Asian summer monsoon (EASM) system and the equilibrium response of the EASM system to increases in various anthropogenic aerosol species, including sulfate, black carbon (BC), and organic carbon (OC), from 1850 to 2000 are studied using an aerosol-climate online model BCC_AGCM2.0.1_CUACE/Aero.(1) The results show that the summer mean net all-sky shortwave fluxes averaged over East Asian monsoon region (EAMR) at the top of the atmosphere(TOA) and surface reduce by 4.8 and 5.0 W m-2, respectively, due to the increases of global aerosol emissions in 2000 relative to 1850. Changes in radiations and their resulting changes in heat and water transport and cloud fraction contribute together to the surface cooling over EAMR in summer. The increases in global anthropogenic aerosols lead to a decrease of 2.1 K in summer mean surface temperature and an increase of 0.4 hPa in summer mean surface pressure averaged over EAMR,respectively. It is shown that the changes in surface temperature and pressure are significantly larger over land than ocean, thus decreasing the contrast of land-sea surface temperature and pressure. This results in the marked anomalies of north and northeast winds over eastern and southern China and the surrounding oceans in summer, thereby weakening the EASM. The summer mean precipitation averaged over the EAMR reduces by 12%. The changes in non-East Asian aerosol emissions play a more important role in inducing the changes of local temperature and pressure,and thus significantly exacerbate the weakness of the EASM circulation due to local aerosol changes. The weakening of circulation due to both is comparable,and even the effect of non-local aerosols is larger in individual regions. The changes of local and non-local aerosols contribute comparably to the reductions in precipitation over oceans, whereas cause opposite changes over eastern China. Our results highlight the importance of aerosol changes outside East Asia in the impact of the changes of anthropogenic aerosols on EASM.(2) The results show that various anthropogenic aerosol species produce substantially different impacts on EASM by changing the local land-sea surface thermal contrast and atmospheric circulation. The increase in sulfate leads to decrease in the contrast of land-sea surface temperature and pressure, the southward shift of the East Asian subtropical jet (EASJ),and significant northerly wind anomalies at 850 hPa over eastern China and the surrounding oceans, thus weakening the EASM markedly. However, pronounced surface cooling appears and an anticyclonic anomaly is formed over the oceans north of 30°N due to the increase of OC. This causes a slight increase in land-sea surface thermal contrast and significant southerly flow anomalies to the west of the anticyclonic center, thereby strengthening the northern East Asian summer monsoon (NEASM). However, OC decreases the surface land-sea thermal contrast over southern China and weakens the southern East Asian summer monsoon (SEASM). The responses of 850 hPa winds and precipitation over the EAMR in summer to increase in BC are generally consistent with those to OC. The increase in BC leads to the strengthening of the NEASM north of 35°N and slight weakening of the SEASM south of 35°N. The summer mean precipitation averaged over EAMR reduces by 0.72 mm day-1 and 0.04 mm day-1, respectively, due to the increases of these two types of scattering aerosols and increases by 0.07 mm day-1 due to the increase in BC. Also, we find that the simulated responses of the EASM to increase in BC are not changed when BC emissions are significantly increased, but the intensities of responses are enhanced.Overall, the increase in sulfate primarily weakens the EASM, while the increases in BC and OC mitigate the weakening of the NEASM to some extent.