IRREGULARITY AND LOCKING TO THE SEASONAL CYCLE IN AN ENSO PREDICTION MODEL AS EXPLAINED BY THE QUASI-PERIODICITY ROUTE TO CHAOS

The behavior of the Cane-Zebiak ENSO prediction model is analyzed as a function of model parameters measuring the strength of coupling betwe en the model ocean and atmosphere and the amplitude of the background seasonal cycle specified in the model. As either of these two paramete rs is increased, the model undergoes a transition from periodic to cha otic behavior according to the universal quasi-periodicity route to ch aos. Thus, the irregularity of model ENSO events and their partial loc king to the seasonal cycle can both be explained as low-order chaotic behavior driven by the seasonal cycle. The chaos is due to irregular j umping of the Pacific natural ocean-atmosphere oscillator between diff erent nonlinear resonances with the seasonal forcing. The periodic sea sonal forcing seems to be the main factor determining the chaotic beha vior of the model. However, the full irregularity of model ENSO events is only explained by considering additional factors, possibly includi ng the nonlinear interaction of different delay oscillator modes relat ed to the different model ocean Rossby modes.