Ocean-atmosphere coupled modes of variability appear in many climate system models. A decadal mode of variability identified in control simulations of CCSM3 (Community Climate System Model version 3) is centered in the subtropical North Pacific over the Kuroshio Extension. Spectral analysis of sea surface temperature in the Kuroshio Extension in the North Pacific in the model shows a large well defined peak at 16-20 years that is statistically significant at 95% confidence. In contrast, while there is a marginally significant peak in the observations, its energy is at least 3 times smaller.

The model coupled mode owes its origins to lateral geostrophic heat flux divergence, caused by a meridional shift of the Kuroshio Extension forced by basin-scale wind stress curl anomalies 3–5 yr earlier. Local surface heat flux and Ekman heat flux divergence act as a damping and positive feedback, respectively. The wind stress curl response to decadal SST anomalies in the Kuroshio Extension is similar in structure but opposite in sign and somewhat weaker than the wind stress curl forcing pattern. These results suggest that the simulated North Pacific decadal variability in the model owes its existence to two-way ocean–atmosphere coupling. However, as is typical of state of the art coupled models, the ocean component does not have eddy resolving resolution and as a result has insufficient non-linearity in the western boundary current. This results in the latitude of the current being too strongly coupled to the location of the gyre boundary predicted from wind-stress curl. As a consequence, the latitudinal excursions of the current are too large which results in excessive sea surface temperature and heat flux anomaly. In addition, an internal mode of variability in non-eddy resolving ocean models can be generated at a similar period that can enhance coupled modes of variability as described above. This internal mode of variability results from very long wave baroclinic instability at length scales resolved by the model, but much larger than the deformation radius. The combined effects of ocean model biases result in an enhanced coupled mode of variability that is centered in the western boundary current extension in the North Pacific.