Diversity of Developing El Niño Events and Its Relationship with the Zonal Contrast SSTA Mode in Summer

Abstract El Niño exhibits spatial diversity during its mature stage and is commonly categorized into eastern Pacific (EP) and central Pacific (CP) types. However, contrasting with matured warm sea surface temperature anomaly (SSTA) centers distributed around the Niño-3.4 region, intense warming or cooling often occurs in the far eastern equatorial Pacific in developing summer, like the 2023 extreme coastal event. Interevent differences of developing El Niño have been unintentionally overlooked. In this study, two tropical Pacific modes—the conventional El Niño–Southern Oscillation (ENSO) mode and zonal contrast SSTA mode—are found to play crucial roles in the diversity of developing El Niño, as they are comparable factors influencing zonal gradients of equatorial SSTAs. The former features a prominent SSTA center along the Peru–Ecuador coast. The latter features opposite SSTA centers located around 90°W and 180°, respectively. According to the spatial structure in summer, developing El Niños are classified into two types: the coast-centered (CC) type governed by the ENSO mode and the ocean-centered (OC) type regulated by the zonal contrast SSTA mode. These two types own different dynamic processes, where the CC type is driven by the thermocline feedback (TH), but the OC type is initially dominated by TH, with an increasing contribution of zonal advective feedback over time. The CC and OC developing types can be precursors to the EP and CP mature types, respectively, with approximately 70% of them developing into the corresponding mature types. These results enhance our understanding of ENSO diversity and provide new references for ENSO categorized monitoring.