At El Ni¿o/Southern Oscillation (ENSO) frequencies, the ocean is mainly driven by zonal equatorial winds. Using zonal equatorial Pacific wind stress for the last 8 El Ni¿os, we demonstrate that anomalous equatorial westerly winds typically begin to blow in the far western Pacific in about November prior to the El Ni¿o year and then slowly move eastward, amplifying and reaching a maximum in the central Pacific in about the following October. The key phase-locking in the far western Pacific winds that leads to calendar-year phase-locking in El Ni¿o appears to be associated with quasi-biennial (QB) variability in the zonal wind. Recent theory suggests that evaporation and the seasonal wind cycle in the far-western equatorial Pacific are crucial to the generation of the QB variability. The biennial wind-stress phase-locking explains why there is a spring persistence barrier in often used ENSO indices. ¿ 2000 American Geophysical Union