Unveiling the Mysteries of SST Evolutions in the Equatorial Pacific at the Onset of El Niño Events.

It is generally accepted that the El Niño–Southern Oscillation (ENSO) dominates interannual climate variability. Yet, its genesis and maintenance mechanisms are still under intense debate, with no scientific consensus. Some authors argued that the westerly winds originating over the equatorial Indian Ocean are significantly enhanced and extend eastward in the western and central equatorial Pacific during El Niño events, thus advecting the warm pool eastward along the equator and causing SST anomalies. However, this assertion is unlikely to be quantitatively supported by observational data. Here we present detailed observational data and modeling evidence to demonstrate that the westerly winds had little change in intensity in the western equatorial Pacific, with a wider zonal extent only during most El Niño events, and with a slight increase even in the most pronounced 1997 El Niño. Instead, an eastward equatorial current near the equator has been observed and is considered to play a significant role in shifting the eastern edge of the warm pool eastward, elevating SSTs in the central and eastern equatorial Pacific and giving rise to El Niño, with the interactions between the eastward warm pool and the upwelling in the eastern cold tongue ascertaining the amplitudes of SST anomalies. [ABSTRACT FROM AUTHOR]