Strong large-scale climate response to North American sulphate aerosols in CESM

The effects of increased North American sulphate aerosol emissions on the climate of Mexico and the United States (U.S.) during 1950-1975 are investigated by using two sets of transient coupled experiments with the Community Earth System Model, one with historically evolving emissions, and a second one where North American SO2 emissions are kept at their pre-industrial levels. The 1950-1975 increase in North American sulphate aerosols is found to have regional and remote impact. Over central U.S. and northern Mexico, the strengthening and westward expansion of the North Atlantic Subtropical High and subsequent intensification of the low-level easterlies, along with local aerosol interactions with radiation and clouds, cause a cooling trend and enhance precipitation. The interaction between the enhanced moisture transport across the Gulf of Mexico and the elevated topography of central Mexico favours positive rainfall on the Atlantic side while suppressing it on the Pacific side. These continental anomalies are embedded in a hemispheric-wide upper-tropospheric teleconnection pattern over the mid-latitudes, extending from the Pacific to the Atlantic basin. Details of the underlying mechanisms-in particular the prominent role of dynamical adjustments-are provided. With SO2 emissions considerably reduced in the U.S., and the expectation of a continued global decline throughout the 21st century, this study sheds light upon possible ongoing and future regional climate responses to changes in anthropogenic forcing.