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Abstract. Anthropogenic aerosols (AAs) induce global and regionaltropospheric circulation adjustments due to the radiative energyperturbations. The overall cooling effects of AA, which mask a portion ofglobal warming, have been the subject of many studies but still have largeuncertainty. The interhemispheric contrast in AA forcing has also beendemonstrated to induce a major shift in atmospheric circulation. However,the zonal redistribution of AA emissions since start of the 20th century, with anotable decline in the Western Hemisphere (North America and Europe) and acontinuous increase in the Eastern Hemisphere (South Asia and East Asia),has received less attention. Here we utilize four sets of single-model initial-condition large-ensemblesimulations with various combinations of external forcings to quantify theradiative and circulation responses due to the spatial redistribution of AAforcing during 1980–2020. In particular, we focus on the distinct climateresponses due to fossil-fuel-related (FF) aerosols emitted from the Western Hemisphere (WH) versus the Eastern Hemisphere (EH). The zonal (west to east) redistribution of FF aerosol emission since the1980s leads to a weakening negative radiative forcing over the WHmid-to-high latitudes and an enhancing negative radiative forcing over theEH at lower latitudes. Overall, the FF aerosol leads to a northward shift of the Hadley cell and an equatorward shift of the Northern Hemisphere (NH) jet stream. Here, two sets of regional FF simulations (Fix_EastFF1920and Fix_WestFF1920) are performed to separate the roles ofzonally asymmetric aerosol forcings. We find that the WH aerosol forcing,located in the extratropics, dominates the northward shift of the Hadley cell by inducing an interhemispheric imbalance in radiative forcing. On the other hand, the EH aerosol forcing, located closer to the tropics, dominates the equatorward shift of the NH jet stream. The consistent relationship between the jet stream shift and the top-of-atmosphere net solar flux (FSNTOA) gradient suggests that the latter serves as a rule-of-thumb guidance for the expected shift of the NH jet stream. The surface effect of EH aerosol forcing (mainly from low- to midlatitudes)is confined more locally and only induces weak warming over the northeastern Pacific and North Atlantic. In contrast, the WH aerosol reduction leads to a large-scale warming over NH mid-to-high latitudes that largely offsets the cooling over the northeastern Pacific due to EH aerosols. The simulated competing roles of regional aerosol forcings in drivingatmospheric circulation and surface temperature responses during the recentdecades highlight the importance of considering zonally asymmetric forcings(west to east) and also their meridional locations within the NH (tropicalvs. extratropical). 

Abstract The response of precipitation extremes (PEs) to global warming is found to be nonlinear in Community Earth System Model version 1 (CESM1) and other global climate models (Pendergrass et al., 2019), which led to the concern that it is not accurate to approximate the response of PE to a single forcing as the difference between simulations with all forcing agents and those that exclude one specific forcing. This calls into question previous model‐based results that the sensitivity of PE with warming due to aerosol forcing is significantly larger than that due to greenhouse gases (GHGs). We reevaluate the PE sensitivity to GHGs and aerosols using available CESM1 ensemble simulations. We show that although the PE response to warming is nonlinear in CESM1, especially for the high warming projected in the twenty‐first‐century, PE sensitivity to aerosols is still significantly stronger than that due to GHGs when the comparison is made within similar warming regimes to avoid the bias induced by the nonlinear behavior. But the difference is smaller than previously estimated. We also conclude that the additivity assumption is largely valid to isolate the PE response due to aerosol forcing from the paired simulations including the “all forcing” experiment when the warming regime is small (e.g., 1°C–2°C in the next few decades when aerosol forcing is projected to decline and becomes a major source of uncertainty for model projection).