Multi-year La Niña frequency tied to southward tropical Pacific wind shift

Multi-year La Niña events cause prolonged climate disruptions worldwide, but a systematic understanding of the underlying mechanisms is not yet established. Using observations and CMIP6 models, the study shows that a greater frequency of consecutive La Niña events is tied to the upper equatorial Pacific Ocean when it favors more rapid heat discharge. This propensity for heat discharge is underscored by negative skewness in upper-ocean heat content, underpinned by a southward tropical Pacific wind shift during austral summer. Models with stronger westerly anomalies south of the equator simulate a steeper east-to-west upward tilt of the thermocline that favors a greater discharge rate, highlighting the crucial role of the southward wind shift in the nonlinear ENSO system. Large inter-model spread in multi-year La Niña processes underscores the need to constrain models for reliable climate prediction and projection.