Is the PDO a Coupled Atmosphere-Ocean Process?

This talk diagnoses how much of Pacific decadal variability is due to intrinsic air-sea coupling and how much is forced by internal atmospheric variability. The approach is to use the Interactive Ensemble (IE) coupling strategy for this diagnosis. The coupling approach is based on the Community Climate System Model v.4, and includes ten atmospheric model instances, coupled correspondingly to ten realizations of land-surface and sea-ice models. An ensemble average estimate from the multiple realizations subsequently forces a single ocean model. At the initial time atmospheric temperature perturbations introduced into each of the atmospheric models to ensure that there is ensemble spread in the atmosphere. Coupling to the single ocean model via the ensemble mean fluxes from the 10-atmospheric models filters out the ocean response to the stochastic weather variability, yet preserving the coupled response. The ensemble approach allows to differentiate the effects of weather variability vs. intrinsic coupling modes on the ocean dynamics around various parts of the global ocean. Furthermore, an empirical orthogonal mode analysis of the IE simulations allows to verify the robustness of the atmospheric teleconnections, and in particular, ENSO and PDO, as well as to compare the modes to those from the single-component control run. In addition to the coherence of the spatial patterns, time series of the modal amplitudes are indicative of their natural variability, under weather-independent assumptions.