Role of off-equatorial subsurface anomalies in initiating the 1991-1992 ElNino as revealed by the National Centers for Environmental Prediction ocean reanalysis data

The National Centers for Environmental Prediction ocean reanalysis data for 1980-1995 have been analyzed along mean constant density surfaces (mean is opycnals) in order to describe better and understand the three-dimensional space-time evolution of the 1991-1992 El Nino event in the tropical Pacific . The major finding is of a well-defined, shallow pathway of off-equatorial temperature anomalies along the Pacific North Equatorial Countercurrent (N ECC) associated with the onset phase of the event. This pathway originates from the western boundary off the equator in the Northern Hemisphere along 6 degrees-10 degrees N and then basically follows the mean circulation east ward and upward along the NECC path toward the central basin. Along this pa thway, temperature anomalies show coherent phase relationships off and on t he equator and in surface and subsurface layers, respectively. A sequence o f events is described that lead to El Nino conditions in the tropical Pacif ic Ocean. Beginning in early 1989, a positive temperature anomaly progresse d coherently eastward along the NECC path off the equator toward the centra l basin, finally making its way equatorward with the expected mean circulat ion in late 1990. As the thermocline shoals eastward and upward along the N ECC, in due course the progressing subsurface anomaly outcropped near the d ate line off the equator and initiated and sustained a midbasin warm sea su rface temperature (SST) anomaly during the period from late 1990 through ea rly 1991. This SST anomaly induced westerly wind anomalies over the western tropical Pacific, favoring Ekman convergence onto the equator. These anoma lous surface currents advected these initially subsurface-produced SST anom alies equatorward, Subsequently, these SST and wind anomalies were coupled, which resulted in a large-scale relaxation of the trade winds over the wes tern and central tropical basin, generating eastward currents that transpor ted water mass eastward along the equator. These results are markedly diffe rent from the delayed oscillator physics in that the major role can be play ed by advection and outcropping of off-equatorial subsurface thermal anomal ies along the shallow NECC pathway, not necessarily involving the western b oundary reflection of equatorial Rossby waves.