Mutual influences between ENSO and its two precursor modes in the extratropical South Pacific

The boreal spring South Pacific Oscillation (SPO) in the atmosphere and South Pacific Quadrupole (SPQ) in the ocean are recognized as significant precursors for the following winter’s El Niño-Southern Oscillation (ENSO). Here we delve into the generation mechanisms of the SPO and probes the potential influence and driving forces of ENSO events on this variability mode, alongside the relationships between SPO and SPQ. Employing statistical analyses with observations spanning 1979–2021 and conducting numerical simulations, this study reveals significant correlations between the SPO and the previous-ENSO-related central tropical Pacific surface temperature anomalies (SSTAs). The forced atmospheric experiments confirm the central tropical Pacific SSTAs as an essential forcing mechanism for the SPO. A quantitative analysis further demonstrates that ENSO and atmospheric internal dynamic contribute comparably to the formation of the SPO. After the SPO formation, it influences the underlying ocean, generating the SPQ mode. Investigations with coupled numerical experiments show the induced SPQ can trigger 46.6% of the original neutral events into El Niño events. This study identifies and validates the sequence of physical processes linking SPO and SPQ to ENSO. The findings highlight the existence of loop-like mutual influence processes between ENSO and the two extratropical variability modes (SPO and SPQ) in the South Pacific. This cyclic pattern bears significant implications for understanding the temporal development of ENSO and can fortify ENSO predictability.