ENSO in a hybrid coupled model. Part I: sensitivity to physical parametrizations

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.