Influence of South Pacific Subtropical Dipole on ENSO

Our previous study has identified a dipolar sea surface temperature anomaly (SSTA) in the subtropical South Pacific (SPSD), which peaks in austral summer (January–March). It’s reasonable to assume that this SPSD mode has some influence over certain ENSO events. To test such a possibility, observational analyses and numerical experiments were carried out here. The model is an atmospheric general circulation model coupled to a reduced gravity ocean model. An SPSD-like SSTA was imposed on 1 March, after which the model was free to evolve until the end of the year. The coupled model response showed that warm SSTAs extend toward the equator with northwesterly wind anomalies and then grow to El Niño–like anomalies by the end of the year. SPSD forcing weakens southeasterly trade winds and propagates warm SSTAs toward the equator through wind– evaporation–SST (WES) feedback. Meanwhile, relaxation of trade winds in the eastern equatorial Pacific depresses the thermocline and upwelling. Eastward anomalous currents near the equator cause warm horizontal advection in the central Pacific. Further experiments showed that thermodynamic coupling mainly acts on but is not essential for SSTA propagation, either from the subtropics to the equator or westward along the equator, while oceanic dynamic coupling alone also appears to be able to initiate anomalies on the equator and plays a critical role in SSTA growth in the tropical Pacific. This is consistent with observational analyses, which indicated that influence of WES feedback on SSTA propagation associated with the SPSD is limited. Meridionally, the warm pole close to the equator plays the dominant role in inducing the El Niño–like anomalies.