The 2023 El Niño, which occurred after a triple-dip La Niña, displayed several distinct features. Although the favorable warm water volume pre-condition indicated the potential for an extreme event, its actual intensity did not meet expectations. More counterintuitively, the westerly wind anomalies were much weaker relative to the sea surface temperature (SST) warming, presenting an unusual “strong El Niño but weak Southern Oscillation” characteristic. Here we propose a tropical Pacific-rooted mechanism that the relatively westward extension of SST warming in the equatorial Pacific was the primary cause of the weak westerly anomalies by suppressing the negative convection anomaly over the equatorial western Pacific. Furthermore, the relatively westward-extended SST warming can ultimately be traced back to the preceding multi-year La Niña through the chain of causation involving weak nonlinear dynamical cooling, weak anomalous subsurface temperature gradients, and abundant warm water accumulation in the western Pacific. This mechanism similarly explains other El Niño events following multi-year La Niña, especially the 2009 event, providing insights into the rare transition from a multi-year La Niña to a strong El Niño. However, current climate models demonstrate limited skill in simulating 2023-like El Niño events and associated mechanisms, primarily due to the systematic biases in the zonal distributions of SST and convection anomalies. Our proposed mechanism, rooted in tropical Pacific dynamics, offers a new perspective to improve model simulations and predictions of such events.
 

Multi-year La Niña,El Nino diversity