Precursors of the El Niño/La Niña onset and their interrelationship

In this study, the equatorial sea level, western Pacific zonal wind, and high‐frequency (2∼90 day) wind variability are investigated as precursors of the onset of El Niño/La Niña. To a large extent, it is shown that each variable is a good indicator of El Niño–Southern Oscillation (ENSO) onset by exhibiting significant correlation to the ENSO index with 9 month lag. However, it is demonstrated here that the three precursors are remarkably correlated with each other. Based on the statistical analysis, we suggest here that the three precursors are governed by strong coupling processes: (1) scale interaction within the atmosphere between the low‐frequency (LF, i.e., interannual) and high‐frequency (HF, i.e., intraseasonal) winds and (2) positive air‐sea coupled feedback among the western Pacific westerly, SST, and zonal mean thermocline. Our observational analysis and statistical prediction experiments suggest that the three precursors are not independent and they reflect the different aspects of the same coupling process during the ENSO onset phase.