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 coupled model, in response to changes in the model parameters, undergoes several transitions between periodic (frequency-locking) and chaotic states. Chaotic response is found as the forcing amplitude approaches the observed value. In contrast, in the absence of the seasonal cycle, varying model coupling strength produces neither frequency-locking nor chaos. The coupled system simply undergoes a Hopf bifurcation from a nonoscillatory state to a periodic state as the coupling strength increases. This result suggests that nonlinear interactions between the forced seasonal mode and the intrinsic ENSO mode of oscillation are crucial for the irregular behavior of the model ENSO cycle. The experiments also show that a biennial oscillation can be excited by seasonal forcing even when air-sea coupling is so weak that a self-sustaining oscillation does not exist in the coupled system. This implies that the biennial oscillation observed as a fundamental element of ENSO variability in the low-latitude eastern Indian and western Pacific sector could be a subharmonic resonant to the seasonal forcing rather than a self-sustaining oscillation of the coupled system. Analysis of SST time series further demonstrates that major ENSO “episodes” in the coupled model exhibit a preferred phasing with the seasonal cycle. This phase-locking with the seasonal cycle occurs not only when the model ENSO cycle is periodic but also when it is chaotic. However, phase locking in the model appears to be tighter than that in nature. This study uncovers dual roles of the seasonal cycle in ENSO variabilities: it introduces a degree of regularity into the ENSO cycle by producing annual phase-locking and it generates chaos in the coupled system through inherent nonlinear interactions.