How the central-western equatorial Pacific easterly wind in early 2022 affects the third-year La Nina occurrence

After consecutive two years with the La Nina phenomenon in 2020-2021, cold sea surface temperature (SST) anomalies in the central-eastern equatorial Pacific persisted in 2022, known as a "triple-dip" La Nina event. These conditions have had a profound impact on global weather and climate. Understanding the underlying processes and mechanisms is crucial for improving ENSO prediction and has significant socioeconomic implications. In this study, we investigate the processes responsible for the evolution of the third-year La Nina event in 2022 based on related observations and an intermediate coupled model (ICM). The results show that two factors are essential for the development of the third-year La Nina event: surface easterly wind anomalies over the central-western equatorial Pacific in early 2022 and the induced cold SST anomalies by the former. In the first half of 2022, mainly due to the off-equatorial downwelling Rossby wave reflection at the western boundary, the equatorial downwelling Kelvin waves continuously induce warming effect in the upper ocean, acting to weaken the cold SST anomalies in the central-eastern Pacific. However, the easterly wind anomalies over the central-western equatorial Pacific during February-March 2022 induce upwelling Kelvin waves, whose cooling effect reverses the warming SST tendency. Afterward, despite the pronounced warming effect induced by the reflected downwelling Kelvin waves, the wind-induced cold SST anomalies persist on the equator during the equatorial cold season, which are further intensified by the Bjerknes feedback. Sensitivity experiments based on the ICM further confirm these outcomes. When the cooling effect induced by the easterly wind anomalies is included in ocean initial conditions, the ICM can accurately capture the reappearance of the La Nina phenomenon. These results highlight the importance of anomalous easterlies in the central-western equatorial Pacific during the decaying phase of the La Nina event.