Two intermediate ocean-atmosphere coupled models of the tropical Pacific are used to investigate the sensitivity of the forecasts of the 1997--1998 El Ni¿o to the configuration of the oceanic vertical structure. The models consist in a three baroclinic mode tropical Pacific Ocean and differ from their atmospheric part which is either a Gill (1980)'s tropical atmosphere or a statistical atmosphere. Forced with observed winds, the ocean model simulates dynamical fields in better agreement with observations compared to a single baroclinic mode model. In forecasting sea surface temperature anomalies in the eastern Pacific, the models are comparable to prediction systems of similar complexity. Results of sensitivity tests to the oceanic vertical structure indicate that the second and third baroclinic mode contributions are necessary to better capture the rise in SST anomalies from April 1997 along with the amplitude of the event at its mature phase.