The role of extra-tropical to tropical Pacific Ocean exchanges in the variability of ENSO in CCSM3

In this manuscript we have attempted to investigate mechanisms that force the interdecadal variability of ENSO magnitude and frequency in an effort to help explain the apparent shift in ENSO behaviour seen during the late 1970's. We have chosen to focus particularly on the extra-tropical Rossby wave mechanism. Results from oceanic shallow water models show that the Rossby wave theory is a valid mechanism for altering the equatorial thermocline depth and for modulating the amplitude of modelled ENSO events. However, questions remain over whether the mechanism operates in more complicated CGCMs (especially z- coordinate types) and if so, whether it generates a large enough equatorial or ENSO modulating response to be seen above the noise level of the equatorial Pacific Ocean. We firstly set out to confirm that the Rossby- Kelvin wave mechanism operates in more complicated (z-coordinate) CGCM models. We then used a model surgery approach called Partial Coupling (PC) to help isolate the role of wind stress forced oceanic exchanges between the extra-tropics and the tropics on modelled ENSO variability. We find that changes in the background state of the equatorial Pacific thermocline depth induced by a fixed off-equatorial wind stress anomaly forcing have a significant effect on the amplitude of the modelled ENSO events. Therefore, confirming the results from simpler models and further validating theories that rely on oceanic wave dynamics to generate Pacific Ocean decadal variability. As such, we believe that a comprehensive understanding of seasonal-to-interannual prediction of ENSO variability can be gained by a better understanding of extra- tropical to tropical Pacific Ocean transfers and western boundary processes. Further, we believe that these results may provide a physical basis to enhance multiyear probabilistic predictions of ENSO indices.