Contrasting the Effects of the Historical Increase in Sulfate Aerosols from North America and Europe on the Atlantic

Variations in North Atlantic sea surface temperatures (SSTs), both on multi-decadal and even longer time scales, are known to be linked to a range of climate features globally, but the underlying causes are strongly debated, including the separate contributions of external forcing and internal variability as well as the role of atmosphere-ocean feedbacks. The extent to which observed fluctuations in Atlantic SSTs since pre-industrial times are driven by changes in anthropogenic aerosols from surrounding sources (North America (NA) and Europe (EU)) is particularly controversial. Here, we investigate the Atlantic response to historical (1850-1975) sulfur dioxide emissions from NA and EU separately in the coupled Community Earth System Model (CESM1-CAM5).

Using purposefully designed 8-member ensemble transient simulations to isolate the forced response from internal variability, we find the simulated multi-decadal variability of North Atlantic SSTs (Atlantic Multidecadal Variability (AMV)), similar to global SSTs, to be externally forced in agreement with previous studies, but not by NA or EU sulfate aerosols, which cause instead a long-term cooling. This cooling is due to a combination of atmospheric aerosol effects and a strengthening of the simulated Atlantic Meridional Overturning circulation. The SST response to NA emissions is larger, with stronger indirect aerosol effects due to a wider aerosol spread over the Atlantic and collocation with cloud cover. A southward shift of the Inter-Tropical Convergence Zone, affecting tropical precipitation globally and causing a small positive feedback to the North Atlantic cooling, is also found. The results further suggest the observed 'warming hole' found in other studies not to be driven by aerosol cooling, but by ocean feedbacks.

The analysis provides novel insights into the possible mechanisms of aerosol impact on the Atlantic, which is also relevant with respect to the ongoing reductions in emissions from NA and EU.