We perform decadal hindcast experiments with initialization every year and assess sensitivity of the hindcasted errors to the errors in the initial climate states. The hindcasted sea surface temperature (SST) over the extratropical North Pacific shows significant impacts of initialization, yet the hindcasted indices of the Pacific decadal variability usually suffer from limited predictability. Our sensitivity analyses reveal that, in the decadal SST hindcasts over the extratropical North Pacific, the annual-todecadal errors of the Aleutian Low fluctuation before the time of starting hindcast experiments work as a major source of uncertainty through delayed responses of the ocean. As we directly assimilate only the ocean states to the atmosphere-ocean coupled model in initialization, the Aleutian Low fluctuation in the assimilation exhibits large errors even though the SST is well correlated with the observation. These assimilation errors in the Aleutian Low fluctuation are primarily due to the distorted responses of the extratropical atmosphere to the tropical SST changes in the model. A close examination indicates that the observed and assimilated Aleutian Lows are sensitive to the eastern and central equatorial SSTs, respectively. Toward further reducing uncertainty in the Pacific decadal hindcasts, therefore, it should be an effective way to raise quality of initial conditions for the extratropical atmosphere and the tropical atmosphere-ocean coupling.
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