The quasi-periodicity of the annual-cycle forced ENSO recharge oscillator model

The annual-cycle impact on a class of nonlinear ENSO recharge oscillator model is studied by transforming the model equations into the van der Pol-Duffing oscillator with external forcing. The Hopf bifurcation and stable limit cycle are obtained by the multi-scale method. Qualitative analysis and numerical results show that the ENSO system turns into quasi-periodic oscillations as the annual-cycle forcing is increased.