An Ocean General Circulation Model is used to investigate the oceanic response to the March 1997 Westerly Wind Event that is suggested to have played an important role in the onset of the 1997--1998 El Ni¿o. Our results point out three distinct impacts. First a strong wind-forced downwelling Kelvin wave propagates eastward generating sea surface temperature anomalies up to 1¿C and large subsurface temperature and zonal current anomalies, mainly located in the core of the thermocline. Second the northward and westward extension of this wind event is responsible for a surface advection of cold waters from 130¿E--5¿N to the equator. Third it generates large zonal surface currents at the eastern edge of the warm and fresh pool by a nonlinear interaction between the wind-forced surface jet and the local thermohaline front. Salinity through both its contribution to the local zonal pressure gradient at the front and the barrier layer effect is crucial in the occurrence of this nonlinear mechanism. The fast displacement of the front (2000 km in a month) together with the cooling in the western Pacific is likely to be responsible for the eastward displacement of atmospheric deep convection and eastward winds observed in April--June 1997 and thus to have played a major role in initiating the El Ni¿o of the century.