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Abstract Multi-year El Niño-Southern Oscillation (ENSO) events, where the warming (El Niño) or cooling (La Niña) extends beyond a single year, have become increasingly prominent in recent decades. Using observations and climate model simulations, we show that the South Pacific Oscillation (SPO) plays a crucial, previously unrecognized role in determining whether ENSO evolves into a multi-year event. Specifically, when an El Niño (La Niña) triggers a positive (negative) SPO in the extratropical Southern Hemisphere during its decaying phase, the SPO feedbacks onto the tropical Pacific through the wind-evaporation-sea surface temperature mechanism, helping sustain ENSO into a multi-year event. This SPO–ENSO interaction is absent in single-year ENSO events. Furthermore, whether ENSO can trigger the SPO depends systematically on the central SST anomaly location for El Niños and the anomaly intensity for La Niñas, with interference from atmospheric internal variability. These findings highlight the importance of including off-equatorial processes from the Southern Hemisphere in studies of ENSO complexity dynamics. 

Abstract This study investigates boreal spring events of Pacific Meridional Mode (PMM) from 1950 to 2022, revealing that cold PMM is more effective in triggering subsequent La Niña compared to warm PMM's induction of following El Niño. This asymmetry stems from the varying origins and sub‐efficacies of PMM groups. The cold PMM is primarily initiated by pre‐existing La Niña, while the warm PMM is comparably activated by pre‐existing El Niño and internal atmospheric dynamics. PMMs initiated by pre‐existing El Niño or La Niña play a crucial role in determining the efficacies of PMMs in triggering subsequent El Niño‐Southern Oscillation (ENSO). The strong discharge of pre‐existing El Niño hampers warm PMM's induction of subsequent El Niño, whereas weak recharge from pre‐existing La Niña enhances the efficacy of cold PMM in inducing subsequent La Niña. Comprehending not only the PMM phase but also its origin is crucial for ENSO research and prediction. 

Abstract In around 1990, significant shifts occurred in the spatial pattern and temporal evolution of the El Niño‐Southern Oscillation (ENSO), with these shifts showing asymmetry between El Niño and La Niña phases. El Niño transitioned from the Eastern Pacific (EP) to the Central Pacific (CP) type, while La Niña's multi‐year (MY) events increased. These changes correlated with shifts in ENSO dynamics. Before 1990, El Niño was influenced by the Tropical Pacific (TP) ENSO dynamic, shifting to the Subtropical Pacific (SP) ENSO dynamic afterward, altering its spatial pattern. La Niña was influenced by the SP ENSO dynamic both before and after 1990 and has maintained the CP type. The strengthened SP ENSO dynamic since 1990, accompanied by enhanced precipitation efficiency during La Niña, make it easier for La Niña to transition into MY events. In contrast, there is no observed increase in precipitation efficiency during El Niño.