Quantifying Contributions of Internal Variability and External Forcing to Atlantic Multidecadal Variability Since 1870

Identifying the mechanisms behind the Atlantic Multidecadal Variability (AMV) is crucial for understanding and predicting decadal climate change. However, what is behind the AMV is still debated. A key issue is the relative role of internal variability (IV) and external forcing in causing the AMV. By analyzing observations and a large number of climate model simulations, here we show that IV and volcanic and anthropogenic aerosols all influenced the AMV over the last ~150 years. Although the AMV since 1870 resulted mainly from IV, decadal variations in aerosol forcing happen to be in phase with the IV-induced AMV and thus enlarged its amplitudes, especially since the late 1920s. Our results support the notion that the AMV resulted from both internal climate variability and decadal changes in aerosols but are inconsistent with the conclusion that the recent AMV is mainly a direct response to external forcing.