Two Distinct Modes of Climate Responses to Anthropogenic Aerosol Forcing Changes

Unlike greenhouse gases (GHG), anthropogenic aerosol concentrations have increased and then decreased over the past century, with the timing of the peak concentration greatly varying in different parts of the world. To date, it has been challenging to separate the climate impact of these anthropogenic aerosols from that due to GHG and background natural variability. We use a pattern recognition method to isolate and extract the forced patterns for the surface ocean and associated atmospheric variables from the Community Earth System Model large ensemble with all-but-one external forcing. We find that the aerosol forced responses in all the variables we examined are dominated by two leading modes with one associated with the evolution of global mean aerosol concentrations while the other due to the transition of primary source regions of the anthropogenic aerosols. In particular, the transition effect exhibits a remarkable zonal asymmetry in the sea surface temperature and salinity responses. While the combination of the two modes explains most of the total response to the aerosol effects, the transition effect is particularly important in the long-term trend in 1965-2010.