The advance of El Niño phase locking from period 1982–2000 to 2001–2022

The mature phases of El Niño-Southern Oscillation (ENSO) events exhibit a distinct tendency to peak towards the end of the calendar year, a phenomenon commonly referred to as ENSO phase locking. This phase locking is a fundamental property of the ENSO. The observed characteristics of phase locking are intricately tied to the seasonality of ENSO-related Sea Surface Temperature (SST) growth rate. In this study, notable observational evidence is presented: the strength of phase locking of El Niño weakened, and the phase locking advanced to peak at an earlier time during the period 2001 to 2022 compared to 1982 to 2000. The advancement of El Niño Phase Locking is explored by analyzing the contributions of different oceanic feedbacks to the El Niño phenomenon. Specifically, our findings highlight the significant role of nonlinear advective dynamic heating (NDH), which is influenced by the decrease in equatorial pacific surface zonal current anomaly and the equatorial pacific surface zonal gradient of sea SST anomaly. This investigation enhances our understanding of the evolving dynamics of El Niño phase locking, shedding light on the intricate interplay of oceanic feedbacks in influencing this fundamental aspect of ENSO behavior.