Munnich et al. [1991] have examined a streamlined, nonlinear atmosphere/ocean
model and found that irregular interannual variability can result from the
coupling of the atmosphere and ocean. Recently, Mantua [1994] and Mantua
and Battisti [1994a] analyzed the Zebiak/Cane numerical model and
demonstrated the irregularity in the model ENSO cycle was due to the
interaction between two coupled unstable atmosphere/ocean modes. They
concluded that, though an individual model ENSO cycle was well described by
the delayed oscillator theory, the irregularity in the model ENSO
events is due to deterministic, nonlinear processes in the model (cf.,
section 3.2.1) associated with the interactions between the unstable modes.