A possible link between off-equatorial warm anomalies propagating along the NECC path and the onset of the 1997-98 El Niño

By examining sea surface height (SSH) from the TOPEX/Poseidon (T/P) altimeter and upper-ocean temperature/current data from the NCEP reanalysis product, a well-defined, shallow pathway has been identified of subsurface temperature anomalies along the Pacific North Equatorial Countercurrent (NECC). A physical picture is developed to demonstrate how oceanic-atmospheric anomalies leading to the 1997-98 El Niño may be generated in the tropical Pacific climate system. Prior to its onset, a preconditioned oceanic state is set in the west during 1995-96, characterized by elevated sea level, depressed thermocline and warm temperature anomalies in the western boundary along 6-10°N. With the seasonal intensification of the NECC in the fall, the warm anomalies propagated coherently eastward along the relatively shallow NECC subsurface pathways (centered about 100 m), from the western boundary in mid-1996 to near the date line in late 1996 and early 1997. As the thermocline shoals eastward and upward along the NECC, the subsurface anomalies were exposed to the surface near the date line, likely initiating warm sea surface temperature (SST) anomalies north of the equator in late 1996. Due to the convergence of mean subsurface circulation onto the equator, the equatorward flows helped move these initially subsurface-produced SST anomalies southward onto the equator, which would induce westerly wind anomalies.