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 similar to 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. Plain Language Summary Decadal to multidecadal changes in North Atlantic sea surface temperatures (NASST), commonly known as Atlantic Multidecadal Variability (AMV), influence climate worldwide. The AMV is thought to result from Atlantic ocean circulation and its interaction with the atmosphere, but an ongoing debate is whether external forcing such as decadal variations in aerosols has played a major role in NASST's recent multidecadal variations. Here we show that while internal variability (IV) played a dominant role for the AMV since 1870, decadal variations in volcanic and anthropogenic aerosols acted to enlarge the IV- induced AMV cycles. Our new findings not only reconcile ongoing controversial debate on the role of IV and external forcing in causing the AMV but also provide new convincing evidence on the notion that the recent AMV resulted from both internal climate variability and decadal changes in aerosols.