Based on the NCEP ocean re-analysis data, the three-dimensional co-evolution of the tropical Pacific climate system is examined to explain the onset of the 1991--92 El Ni¿o event. A logical sequence is discovered that links subsurface and surface temperature anomalies off and on the equator in the western and central Pacific. Along the North Equatorial Countercurrent (NECC) path, subsurface temperature anomalies propagated coherently eastward from the western boundary in mid-1989 to the date line in mid-1990, at a speed of about 10--20 cm-1. As the thermocline shoals eastward along the NECC, the subsurface anomalies outcropped in the regions near the date line, initiating warm sea surface temperature (SST) anomalies which further amplified while advectively extending into the equatorial wave guide. These subsurface-induced SST anomalies could then trigger local coupled air-sea interactions producing atmospheric-oceanic anomalies that developed and evolved in 1991, thus setting onset conditions for the 1991--92 El Ni¿o. These results differ markedly from the delayed oscillator physics in that a major role can be played by the eastward advection of off-equatorial subsurface thermal anomalies and their outcropping along the NECC subsurface pathways, not necessarily involving wave reflections along the western boundary for triggering El Ni¿o. This mechanism explains well the observed, otherwise unaccounted for, surface warming near the date line during late 1989 and early 1990. ¿ 1999 American Geophysical Union