A hybrid coupled model (HCM) for the tropical Pacific ocean-atmosphere syst
em is used to test the effects of physical parametrizations on ENSO simulat
ion. The HCM consists of the Geophysical Fluid Dynamics Laboratory ocean ge
neral circulation model coupled to an empirical atmospheric model based on
the covariance matrix of observed SST and wind stress anomaly fields. In th
is two-part work, part I describes the effects of ocean vertical mixing sch
emes and atmospheric spin-uptime on ENSO period. Part II addresses ENSO pre
diction using the HCM and examines the impact of initialization schemes. Th
e standard version of the HCM exhibits spatial and temporal evolution that
compare well to observations, with irregular cycles that tend to exhibit 3-
and 4-year frequency-locking behavior. Effects in the vertical mixing para
metrization that produce stronger mixing in the surface layer give a longer
inherent ENSO period, suggesting model treatment of vertical mixing is cru
cial to the ENSO problem. Although the atmospheric spin-up time scale is sh
ort compared to ENSO time scales, it also has a significant effect in lengt
hening the ENSO period. This suggests that atmospheric time scales may not
be truly negligible in quantitative ENSO theory. Overall, the form and evol
ution mechanism of the ENSO cycle is robust, even though the period is affe
cted by these physical parametrizations.