Boreal spring is typically considered the onset phase of El Niño–Southern Oscillation (ENSO), and extracting reliable precursors during this season is therefore essential for improving ENSO prediction. In this study, the spring precursors of Central Pacific (CP) and Eastern Pacific (EP) El Niño are investigated using a multivariable linear inverse model that incorporates sea surface temperature (SST), zonal wind stress (Taux), and thermocline depth. The results reveal distinct precursors for the two event types: EP El Niño events are preceded by westerly wind anomalies centered east of the dateline, along with widespread thermocline deepening across the CP and EP, indicating the dominance of the subsurface processes. These westerly wind anomalies excite downwelling equatorial Kelvin waves, which propagate eastward and deepen the thermocline in the EP, subsequently inducing positive SST anomalies (SSTAs). In contrast, the precursors of CP El Niño are characterized by westward-displaced Taux anomalies, together with SSTAs extending westward along the equator and southwestward from the Baja California toward the equator, reflecting the leading role of surface ocean-atmosphere feedback in the early development stage. Among the three variables examined, Taux consistently emerges as the influential precursor for both event types, with the zonal displacement of spring westerly anomalies critically modulating the location and intensity of subsequent SST amplification and ultimately determining the type of El Niño that develops. This study systematically compares the spring precursors of CP and EP El Niño events and highlights the diagnostic importance of Taux anomalies, providing new insights into the physical mechanisms underlying ENSO diversity.

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