Mechanisms of seasonal - ENSO interaction

The mechanisms by which the seasonal cycle in the equatorial Pacific affects ENSO are investigated using the Zebiak and Cane ENSO prediction model. The most dominant seasonal effect is found to be due to the wind divergence field, as determined by the seasonal motion of the ITCZ, through its effect on the atmospheric heating. The next-order seasonal effects are due to the seasonality of the background SST and ocean upwelling velocity, and the corresponding mechanisms are analyzed. It is suggested that the seasonal forcing has a first-order effect on ENSO's dynamics and that important aspects of the seasonal forcing may be included in idealized delayed oscillator ENSO models by making the model background shift seasonally from stable to unstable states.