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

By examining sea surface height (SSH) from the TOPEX/Poseidon (T/P) altimet er and upper-ocean temperature/current data from the NCEP reanalysis produc t, a well-defined, shallow pathway has been identified of subsurface temper ature anomalies along the Pacific North Equatorial Countercurrent (NECC). A physical picture is developed to demonstrate how oceanic-atmospheric anoma lies leading to the 1997-98 El Nino may be generated in the tropical Pacifi c 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 degrees N. With the seasonal intensification of the NECC in the fall, t he warm anomalies propagated coherently eastward along the relatively shall ow NECC subsurface pathways (centered about 100 m), from the western bounda ry in mid-1996 to near the date line in late 1996 and early 1997. As the th ermocline shoals eastward and upward along the NECC, the subsurface anomali es were exposed to the surface near the date line, likely initiating warm s ea surface temperature (SST) anomalies north of the equator in late 1996, D ue to the convergence of mean subsurface circulation onto the equator, the equatorward flows helped move these initially subsurface-produced SST anoma lies southward onto the equator, which would induce westerly wind anomalies .