![]() ![]() To successfully forecast multi-year ENSO events requires an understanding of the underlying physical mechanisms that drive their unique evolutions. For example, the 6 multi-year El Niño event surprised the ENSO scientific community by its unique evolution, and most operational forecasting models compiled at the International Research Institute for Climate and Society (IRI) failed to predict the evolution of this event 12 (Supplementary Fig. The occurrence of multi-year ENSO events results in non-cyclic evolution of ENSO and poses a challenge to the prediction of ENSO. The multi-year persistence of these El Niño and La Niña events exacerbates their induced climate impacts, causing persistent floods and droughts worldwide 9– 11. Although less frequent than multi-year La Niña events, multi-year El Niño events are also occasionally observed in the tropical Pacific (such as the 6 El Niño event) 7, 8 (Supplementary Fig. Some La Niña events persist through the following year and often re-intensify in the subsequent winter, lasting two years or longer 7. However, not all ENSO events are the same 6. Typically, El Niño and La Niña episodes develop during the boreal summer, peak during early winter, and decay rapidly during the following spring, lasting about 9–12 months. ![]() The El Niño/Southern Oscillation (ENSO) is the dominant climate phenomenon in the tropical ocean affecting the global climate and extreme weather conditions 1– 5. We conclude that properly accounting for the effects of the NPO on the evolution of El Niño events may improve multi-year El Niño prediction and projection. ![]() Future projections of Coupled Model Intercomparison Project phases 5 and 6 models demonstrate that with enhanced NPO variability under future anthropogenic forcing, more frequent multi-year El Niño events should be expected. Model experiments, with the NPO forcing assimilated to constrain atmospheric circulation, reproduce the observed connection between NPO forcing and the occurrence of multi-year El Niño events. The NPO during boreal winter can trigger a Central Pacific El Niño during the subsequent winter, which excites atmospheric teleconnections to the extratropics that re-energize the NPO variability, then re-triggers another El Niño event in the following winter, finally resulting in persistent El Niño-like states. Here we present a two-way feedback mechanism between the tropics and extratropics to argue that extratropical atmospheric variability associated with the North Pacific Oscillation (NPO) is a key source of multi-year El Niño events. Multi-year El Niño events induce severe and persistent floods and droughts worldwide, with significant socioeconomic impacts, but the causes of their long-lasting behaviors are still not fully understood. All relevant codes used in this study are available, upon request, from the corresponding author R.Q.D. The data in this study were analyzed with NCAR Command Language (NCL ). The CMIP5 simulation dataset is available at, and the CMIP6 simulation dataset is available at. The Niño3.4 index is provided by the Climate Prediction Center at. The NCEP/NCAR monthly reanalysis is available at.
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