The nonlinear interactions between the seasonal cycle and El Niño-Southern Oscillation (ENSO) in the coupled ocean-atmosphere system are examined using a newly developed intermediate coupled ocean-atmosphere model. The model permits coupling between total sea surface temperature (SST) and total surface winds and thus is able to produce its own seasonal cycle. This coupling approach allows for the examination of full dynamic interactions between the seasonal cycle and interannual oscillations. Numerical simulations with realistic surface heat fluxes indicate that this model is capable of capturing the essential variability of the coupled ocean-atmosphere system on seasonal-to-interannual timescale in the tropical Pacific. Model sensitivity experiments were carried out by independently varying the external forcing strength and coupling strength. These experiments reveal a very different behavior of the coupled system with and without the seasonal cycle. In the presence of the seasonal cycle, the co... Abstract The nonlinear interactions between the seasonal cycle and El Niño-Southern Oscillation (ENSO) in the coupled ocean-atmosphere system are examined using a newly developed intermediate coupled ocean-atmosphere model. The model permits coupling between total sea surface temperature (SST) and total surface winds and thus is able to produce its own seasonal cycle. This coupling approach allows for the examination of full dynamic interactions between the seasonal cycle and interannual oscillations. Numerical simulations with realistic surface heat fluxes indicate that this model is capable of capturing the essential variability of the coupled ocean-atmosphere system on seasonal-to-interannual timescale in the tropical Pacific. Model sensitivity experiments were carried out by independently varying the external forcing strength and coupling strength. These experiments reveal a very different behavior of the coupled system with and without the seasonal cycle. In the presence of the seasonal cycle, the co...