  The mechanisms of interaction between the seasonal cycle and 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. Important aspects of the seasonal forcing may be
included in idealized delayed oscillator ENSO models by making the model
background seasonally shift from stable to unstable states.
