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A fundamental study of the nonlinear dynamics of the Tropical Pacific climate system was performed within this project. Using tools of dynamic systems theory and an intermediate coupled ocean-atmosphere model, the physical mechanisms determining the time-mean state of the Tropical Pacific and its variability on interannual time scales (El-Nino/Southern Oscillation, ENSO) are described. Coupled processes between ocean and atmosphere are not only responsible for the ENSO variability, but also determine the zonal spatial structure of the mean state (warm pool/cold tongue structure) and its equatorial asymmetry. Within an equatorial symmetric coupled model, which allows the interaction between the mean state and its variability, the ENSO mode is shown to be a robust eigenmode of the coupled system, which is destabilised as coupling strength is increased. The spatial structure of the mean state is central to the propagation mechanism and spatial pattern of the ENSO mode, which was shown to correspond well to the recently proposed recharge oscillator image, In reality, the mean state also displays substantial equatorial asymmetry. This asymmetry is important for the structure of the seasonal cycle in the Tropical Pacific; its physical state has been investigated in a conceptual model. A further and final study explored the impact of intra-seasonal oscillations, atmospheric noise and the seasonal cycle on the interannual variability of the coupled system.