Anthropogenic aerosols (AAs) have been found to drive significant historical precipitation changes, particularly in the tropics, such as a southward displacement of the Intertropical Convergence Zone (ITCZ), the mid-1980s Sahel and Amazonia droughts, as well as the weakening of the South and East Asian monsoons.  Similarly, the projected future reduction in AAs has been linked to opposite responses. Much less is known about AA impacts on mid-latitude precipitation, including over North America. Previous work shows that historical AAs increase in precipitation over central to eastern U.S. and northern Mexico. Idealized experiments have shown the potential of aerosols to perturb precipitation over the Pacific Coast of North America.

Historical AAs impact on wintertime precipitation along the Pacific Coast of

North America is investigated using CMIP6 multimodel archives. Over much of the 20th century, particularly from 1930-1975 when the U.S. and European AA and precursor gas emissions rapidly increased, a robust wintertime precipitation dipole pattern exists along the Pacific Coast, with wetting southward of 40N and drying to the north. A corresponding robust dynamical dipole pattern is found, including increases in the East Pacific jet and baroclinicity increases southward of ~40N and decreases to the north. These dynamical changes are related to AA-induced tropospheric cooling, consistent with decreases in top-of-the-atmosphere radiation and changes in clouds. In response to future AA reductions, an opposite hydro-dynamic dipole pattern occurs. The AA-induced dipole pattern is partially offset by greenhouse gases, resulting in a muted dipole response in both historical and future all-forcing simulations. 

Regression analysis suggests that the Pacific Coast precipitation dipole pattern is most sensitive to AAs from East Asia, relative to those from the U.S. and Europe. Although uncertainties remain, including possible overestimation of CMIP6 aerosol forcing, our results suggest aerosols can perturb precipitation along the Pacific Coast. Future reductions in AA emissions may promote an increase in precipitation for the northern Pacific Coast, but a decrease for the southern, including the southwest U.S. (Allen, R.J., Zhao X., 2022, Environmental Research: Climate)