The role of ocean modes in Pacific Decadal Variability

It is likely that the ocean is a key player in low-frequency climate variability. However, the precise nature of the involvement of the ocean is still an open problem. In this study we address the hypothesis that decadal climate variability in the North Pacific reflects the excitation of ocean modes. A modal, deterministic origin of low frequency variability would have significant implications for decadal climate predictability and ocean initialization.

We will show results of our analysis of a 500 year control integration of the coupled climate model CCSM3. The dominant EOF of sea surface temperature (SST) resembles the observation-based Pacific Decadal Oscillation. Its time series displays a significant spectral peak of about 20 years, suggesting the presence of an oscillatory mode in the system. A multivariate Principal Oscillation Pattern (POP) analysis is performed on SST and a new metric of the subsurface density field. Regression analysis does not show unequivocal evidence for the involvement of an oceanic basin mode. Instead, the PDO in the model appears to result from a coupled mode of air-sea interaction.