Abstract Whether multidecadal sea surface temperature (SST) variability in the Atlantic and Pacific basins originates internally or is externally forced has long been debated. Here, we apply a novel Rotated Low‐Frequency Component Analysis to large‐ensemble climate model simulations and observational SST data sets to disentangle the forced and unforced components of Atlantic Multidecadal Variability (AMV) and the Pacific Decadal Oscillation (PDO). By rotating low‐frequency modes from individual ensemble members toward externally forced reference modes derived from their corresponding ensemble means, we separate external and internal SST anomalies. Across the CESM1, CESM2, GFDL‐CM3, and E3SMv2 ensembles, we find that AMV is predominantly forced, while the PDO is largely internal. Application to observations agrees with these findings: the observed AMV is almost entirely explained by external forcing, whereas the PDO retains a substantial internal component. These results have important implications for the attribution, prediction, and understanding of historical and future multidecadal SST variability.

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