Pacific decadal variability (PDV), a low-frequency internal fluctuation in sea surface temperature (SST) in the Pacific, exerts a significant influence on climate worldwide. Understanding its response to external forcing is crucial, yet this response remains poorly understood. A major uncertainty to date has been the relative importance of forced changes in the mean state to internal changes. Here we use single-forcing large ensembles and preindustrial simulations across multiple climate models to separate intrinsic PDV modes from external influences. We show that historical SST responses in the North Pacific are in fact a reflection of the changing mean state rather than internal variability. Our analysis reveals that a Pacific Decadal Oscillation (PDO)-like leading mode is dominated by internal variability across multiple large ensembles. Conversely, a second mode, resembling the North Pacific Gyre Oscillation (NPGO), demonstrates apparent associations with changes in anthropogenic aerosols and greenhouse gases in CESM2 single forcing simulations. However, these associations are primarily a result of externally-driven mean-state changes which project onto the NPGO pattern; when the time-varying background trend is removed, both the PDO and NPGO-like modes exhibit no change in mean or variance, underscoring the presence of mean state changes that resemble internal modes in the context of external forcing impacts.