The onset of the 1991-92 El Nino event in the tropical Pacific Ocean: The NECC subsurface pathway

Based on the NCEP ocean re-analysis data, the three-dimensional co-evolutio n of the tropical Pacific climate system is examined to explain the onset o f the 1991-92 El Nino event. A logical sequence is discovered that links su bsurface and surface temperature anomalies off and on the equator in the we stern 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, t he subsurface anomalies outcropped in the regions near the date line, initi ating warm sea surface temperature (SST) anomalies which further amplified while advectively extending into the equatorial wave guide. These subsurfac e-induced SST anomalies could then trigger local coupled air-sea interactio ns producing atmospheric-oceanic anomalies that developed and evolved in 19 91, thus setting onset conditions for the 1991-92 El Nino. These results di ffer markedly from the delayed oscillator physics in that a major role can be played by the eastward advection of off-equatorial subsurface thermal an omalies and their outcropping along the NECC subsurface pathways, not neces sarily involving wave reflections along the western boundary for triggering El Nino. This mechanism explains well the observed, otherwise unaccounted for, surface warming near the date lint: during late 1989 and early 1990.