Anthropogenic Aerosol effects on Tropospheric Circulation and Sea Surface Temperature (19802020): Separating the role of Zonally Asymmetric Forcings
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Abstract. Anthropogenic Aerosols (AA) induce global and regional tropospheric circulation adjustments due to the radiative energy perturbations. The overall cooling effects of AA since the pre-industrial (PI) era, to mask a portion of global warming, have been the subject of many studies with large uncertainty remaining. The interhemispheric contrast in AA forcing has also been demonstrated to induce a major shift in atmospheric circulation. The zonally heterogeneous changes in AA emissions since the late 20th century, with a notable decline in the Western Hemisphere and continuous increase in the Eastern Hemisphere, received less attention. Here we utilize four sets of single-model initial-condition large-ensemble simulations with various combinations of external forcings to quantify the different radiative and circulation responses due to aerosol emissions changes during 1980-2020. In particular, we focus on the distinct climate responses to Fossil-Fuel (FF) related aerosol from Western Hemisphere (WH) versus Eastern Hemisphere (EH). The zonal and meridional redistribution of FF aerosols from WH to EH results in negative radiative forcing over Asia and positive radiative forcing over North America and Europe. This leads to a counterclockwise anomaly of zonal mean stream function over the tropics (i.e. a northward shift of Hadley cell) and stronger equatorward shift of the Northern Hemisphere (NH) jet stream, consistent with the thermal wind argument with the gradient of surface air temperature (SAT) as a predictive metric. Two sets of regional FF simulations (Fix_EastFF1920 and Fix_WestFF1920) are performed and reveals the dominating role of WH forcing due to aerosol reduction in the NH. The Aerosol reduction over WH mid-to-high latitudes dominates the warming over NH mid-to-high latitudes. The increased aerosol over the EH low-to-mid latitudes is confined more locally but also induces slight warming over the northeastern Pacific and North Atlantic. The competing role of FF forcing originating from EH and WH in shaping tropospheric circulation and surface climate response indicates the importance of both zonal and meridional distribution of aerosol forcing within the NH, and previous idealized models that only consider the zonal difference of aerosol emission may oversimplify the real aerosol forcing.
