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Observations show a significant interdecadal decline of July and August (JA) mean upper-tropospheric temperature (UTT) over East Asia during the second half of the twentieth century, which is likely responsible for the weakening of the East Asian summer monsoon and the "southern flood and northern drought" phenomenon observed in eastern China. This study investigates the role of anthropogenic aerosol forcing in the interdecadal cooling for the period 1951-2001 and the physical mechanisms involved using a coupled Earth system model, combined with reanalysis data. Our results show that the aerosol forcing leads to a drop of 0.14 degrees C per decade in JA mean UTT averaged over East Asia (30-45 degrees N, 90-130 degrees E), which significantly contributes to the observed cooling of 0.2 degrees C per decade. The interdecadal cooling may be attributed to the weakening of interhemispheric differences in JA mean sea surface temperature in response to aerosols. The asymmetric hemispheric change in sea surface temperature alters the local eddy flux, thereby weakening upper-tropospheric westerlies and the transport of warm air from over the Tibetan plateau into downstream regions between 30 degrees N and 40 degrees N. We further find that changes in the geographic distribution of SO 2 emissions are the dominant cause of differences in JA mean UTT trends over East Asia between decades. The projected decrease in anthropogenic aerosols will lead to a rise of 0.05 degrees C per decade in JA mean UTT over East Asia during 2010-2050 under the Representative Concentration Pathways 8.5 scenario. This will likely have large impacts on the distribution of summer precipitation in East Asia in the future.

Plain Language Summary In the context of global warming, there are different levels of interdecadal cooling in the upper troposphere in most regions of Eurasia north of 30 degrees N in July and August (JA) during the second half of the twentieth century. The cooling trends are significant over midlatitude East Asia from 90 degrees E to 130 degrees E and 30 degrees N to 45 degrees N, which is likely responsible for the weakening of the East Asian summer monsoon and the "southern flood and northern drought" phenomenon observed in eastern China in the past few decades. However, the specific reasons for the decline in upper-tropospheric temperature remain subject to debate. Our study reveals that anthropogenic aerosol forcing significantly contributes to the observed interdecadal JA mean upper-tropospheric cooling over East Asia. The reversal of the interdecadal JA mean upper-tropospheric temperature trend over East Asia will occur in the future due to the expected decrease in anthropogenic aerosol burdens. This will likely have significant effects on the spatial distribution of summer precipitation over East Asia. This study has implications for better understanding the mechanism of climate change.