Diverse Timing of El Niño Onset Linked to Preconditioned Recharge State and Occurrence of Westerly Wind Bursts

Abstract El Niño is generally phase‐locked to the boreal winter but displays significant variability in its onset timing, contributing to its diverse climate impacts. The physical mechanisms driving this variability remain inadequately understood. This study demonstrates that onset of El Niño events can occur over a broad range of months from March to September, with its onset timing closely linked to the precondition of oceanic recharged state and the occurrence of westerly wind bursts (WWBs) in the preceding spring. A stronger recharged state and increased frequency of WWBs promote earlier onset by efficiently transporting warm water to the equatorial eastern Pacific. Supporting evidence from MIROC6 simulations and a conceptual model underscores the crucial roles of both the recharged state and WWBs in determining the timing of El Niño onset. These results enhance our understanding of El Niño dynamics and hold important implications for seasonal climate prediction.