Role of off‐equatorial subsurface anomalies in initiating the 1991–1992 El Niño as revealed by the National Centers for Environmental Prediction ocean reanalysis data

Abstract

The National Centers for Environmental Prediction ocean reanalysis data for 1980–1995 have been analyzed along mean constant density surfaces (mean isopycnals) in order to describe better and understand the three‐dimensional space‐time evolution of the 1991–1992 El Niño 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 (NECC) associated with the onset phase of the event. This pathway originates from the western boundary off the equator in the Northern Hemisphere along 6°–10°N and then basically follows the mean circulation eastward and upward along the NECC path toward the central basin. Along this pathway, temperature anomalies show coherent phase relationships off and on the equator and in surface and subsurface layers, respectively. A sequence of events is described that lead to E1 Niño conditions in the tropical Pacific Ocean. Beginning in early 1989, a positive temperature anomaly progressed coherently eastward along the NECC path off the equator toward the central basin, finally making its way equatorward with the expected mean circulation in late 1990. As the thermocline shoals eastward and upward along the NECC, in due course the progressing subsurface anomaly outcropped near the date line off the equator and initiated and sustained a midbasin warm sea surface temperature (SST) anomaly during the period from late 1990 through early 1991. This SST anomaly induced westerly wind anomalies over the western tropical Pacific, favoring Ekman convergence onto the equator. These anomalous surface currents advected these initially subsurface‐produced SST anomalies equatorward. Subsequently, these SST and wind anomalies were coupled, which resulted in a large‐scale relaxation of the trade winds over the western and central tropical basin, generating eastward currents that transported water mass eastward along the equator. These results are markedly different from the delayed oscillator physics in that the major role can be played by advection and outcropping of off‐equatorial subsurface thermal anomalies along the shallow NECC pathway, not necessarily involving the western boundary reflection of equatorial Rossby waves.