The tropical belt has widened during the last several decades, and both internal variability and anthropogenic forcings have contributed. Although greenhouse gases and stratospheric ozone depletion have been implicated as primary anthropogenic drivers of tropical expansion, the possible role of other drivers remains uncertain. Here, we analyze the tropical belt width response to idealized perturbations in multiple models. Our results show that absorbing black carbon (BC) aerosol drives tropical expansion, and scattering sulfate aerosol drives contraction. BC, especially from Asia, is more ecient per unit radiative forcing than greenhouse gases in driving tropical expansion, particularly in the Northern Hemisphere. Tropical belt expansion (contraction) is associated with an increase (decrease) in extratropical static stability induced by absorbing (scattering) aerosol. Although a formal attribution is dicult, scaling the normalized expansion rates to the historical time period suggests that BC is the largest driver of the Northern Hemisphere tropical widening but with relatively large uncertainty.

Several studies show anthropogenic aerosols (AAs) can perturb large-scale atmospheric circulation, as well as precipitation associated with the tropical rain belt and monsoons of the Northern Hemisphere, including the South and East Asian monsoons, as well as the West African monsoon. In the Northern Hemisphere mid-latitudes, however, the impact of AAs on regional climate and precipitation remains uncertain. This work investigates the influence of AAs on wintertime precipitation along the west coast of North America using the newest climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6). Over much of the 20th century, particularly from 1930-1975 when U.S. and European AA emissions rapidly increased, models show a robust wintertime precipitation dipole pattern along the Pacific Coast, with wetting southward of 40°N and drying to the north. This dipole pattern is consistent with a deepening of the Aleutian Low and a southeastward shift of the east Pacific jet stream, which in turn is associated with a southeastward displacement of the region of maximum baroclinicity. These changes in baroclinicity are related to strong aerosol-induced tropospheric cooling that maximizes at 40°N over the east Pacific. This strong cooling is consistent with aerosol-induced decreases in top-of-the-atmosphere radiation and changes in clouds, including decreases in cloud top effective radius and increases in cloud cover. Consistently, in response to 21st-century AA reductions an opposite hydro-dynamic dipole pattern occurs, including drying southward of 40°N and wetting to the north. Although uncertainties remain, including possible overestimation of aerosol forcing, our results suggest aerosols can perturb precipitation along the west coast of North America. However, the dipole pattern under AA forcing is partially offset by GHGs, resulting in a muted dipole response in all-forcing simulations.