Reconciling roles of sulphate aerosol forcing and internal variability in Atlantic multidecadal climate changes

The interdecadal basin-wide warming and cooling cycle of the North Atlantic Ocean, known as the Atlantic multidecadal variability (AMV), influences not only the Euro-Atlantic climatic conditions, but also tropical storms and monsoons remotely. However, controversy still remains on the relative importance of external forcing and internal processes for the past AMV. Here, we use three attribution experiments, consisting of five-member ensemble for each, for 1931–2014 by a climate model and show that the temporal variation of the observed AMV can be reproduced in the model forced by historical changes in radiative forcing. Specifically, anthropogenic sulphate emissions are found to explain 46–63% of the forced SST variations at decadal time scales. The spatial pattern of the sea surface temperature (SST) associated with the AMV is also well captured by the model, in which externally forced components dominate and significantly increase precipitation over Europe and the Sahel during positive AMV. An exception is the subpolar region where internally generated SST variability coupled with the North Atlantic Oscillation plays a major role. Under declining scenarios of anthropogenic aerosol emissions, a multi-model ensemble of climate simulations shows that the North Atlantic decadal SST variability will be generated increasingly by internal processes, suggesting a decreasing impact on regional precipitation.