The importance of the barrier layer during the onset of El Ni¿o is investigated using a coupled ocean-atmosphere general circulation model. Sensitivity experiments are done by removing or keeping the salinity stratification in the upper layer of the western equatorial Pacific warm pool. The barrier layer favors the maintenance and displacement of the warm pool into the central Pacific by isolating the mixed layer from the entrainment cooling at depth and by confining the response of westerly wind events (WWEs) to a shallow mixed layer. The increased zonal fetch of WWEs through the coupling with sea surface temperature (SST) enhances downwelling equatorial Kelvin waves and thus leads to El Ni¿o. In the absence of salinity stratification, slightly cooler SST and a reduced eastward displacement of the warm pool result in a reduced El Ni¿o or a return to the mean seasonal cycle of the model. The possibility that the barrier layer affects the onset of El Ni¿o pleads for a careful consideration of the salinity stratification in climate forecasts.