Atlantic Multidecadal Variability since 1850 is largely externally forced

Abstract. Whether observed Atlantic Multidecadal variability (AMV) is truly an intrinsic internal mode of climate variability or an externally forced response remains contentious, with conflicting literature that North Atlantic SST variability arises from internal dynamics or external forcing. The availability of several single model initial-condition large ensembles (SMILEs) and new insights into potential biases in sea surface temperature (SST) variations offer a fresh opportunity to reassess this question. We show that SMILE ensembles provide strong evidence that AMV-like variability is largely externally forced. New insights into potential SST biases also raise questions about apparent early 20th Century oscillatory behaviour. SMILE models with stronger multidecadal variability show weaker agreement with observed AMV phasing, even in the best performing individual ensemble members, suggesting that large internal model variability may obscure the forced signal. We conclude that future variations in North Atlantic SST will very likely be driven primarily by future anthropogenic activities.