In this paper, we have compared and contrasted competing influences of greenhouse gases (GHG) warming and aerosol forcing on Asian summer monsoon circulation and rainfall based on CMIP5 historical simulations. Under GHG-only forcing, the land warms much faster than the ocean, magnifying the pre-industrial climatological land-ocean thermal contrast and hemispheric asymmetry, i.e., warmer northern than southern hemisphere. A steady increasing warm-ocean-warmer-land (WOWL) trend has been in effect since the 1950’s substantially increasing moisture transport from adjacent oceans, and enhancing rainfall over the Asian monsoon regions. However, under GHG warming, increased atmospheric stability due to strong reduction in mid-tropospheric and near surface relative humidity coupled to an expanding subsidence areas, associated with the Deep Tropical Squeeze (DTS, Lau and Kim, 2015b) strongly suppress monsoon convection and rainfall over subtropical and extratropical land, leading to a weakening of the Asian monsoon meridional circulation. Increased anthropogenic aerosol emission strongly masks WOWL, by over 60% over the northern hemisphere, negating to a large extent the rainfall increase due to GHG warming, and leading to a further weakening of the monsoon circulation, through increasing atmospheric stability, most likely associated with aerosol solar dimming and semi-direct effects. Overall, we find that GHG exerts stronger positive rainfall sensitivity, but less negative circulation sensitivity in SASM compared to EASM. In contrast, aerosols exert stronger negative impacts on rainfall, but less negative impacts on circulation in EASM compared to SASM.

中文翻译：

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温室变暖和气溶胶对亚洲夏季季风环流和降雨的竞争影响。

在本文中，我们基于 CMIP5 历史模拟，比较和对比了温室气体（GHG）变暖和气溶胶强迫对亚洲夏季季风环流和降雨的相互影响。在仅温室气体强迫下，陆地变暖速度比海洋快得多，放大了前工业化气候的陆地-海洋热对比和半球不对称性，即北半球比南半球温暖。自 1950 年代以来，海洋温暖陆地 (WOWL) 趋势不断增加，邻近海洋的水分输送大幅增加，亚洲季风地区的降雨量也随之增加。然而，在温室气体变暖的情况下，由于对流层中部和近地表相对湿度的大幅下降，再加上与深热带挤压有关的沉降面积不断扩大，大气稳定性增加（DTS，Lau和Kim，2015b），强烈抑制季风对流和降雨亚热带和温带陆地上空，导致亚洲季风经向环流减弱。人为气溶胶排放量的增加强烈掩盖了WOWL，在北半球超过60%，在很大程度上抵消了温室气体变暖导致的降雨量增加，并通过增加大气稳定性导致季风环流进一步减弱，很可能与气溶胶太阳能调光和半直接效应。总体而言，我们发现与 EASM 相比，SASM 中的温室气体对降雨具有更强的正降雨敏感性，但对环流的负敏感性较小。相比之下，与SASM相比，气溶胶对降雨的负面影响更强，但对东亚夏季风环流的负面影响较小。