In the equatorial Pacific Ocean, the rapid transition from the 1997 El Ni¿o to the 1998 La Ni¿a was associated with an outcrop of cold water appearing at the surface around (0¿, 125¿W) end of May 1998. In a validated ocean general circulation model, the cold water of the outcrop appears to come from two pathways (one in each hemisphere) that connect the subtropics to the equatorial upwelling region through the western boundary currents and the Equatorial Undercurrent (EUC). The EUC and subtropics may have an influence on the rapid turn into La Ni¿a in 1998. Large variations in transport (up to 60 ¿ 106 m3 s-1) and in transport-weighted temperature (up to 3¿C) of the EUC are observed in TAO moored data and in the model. The rapid variability of EUC transport and transport-weighted temperature may interact with the long-term variability of the thermocline circulation linking the subtropics to the equator.