Off-equatorial winds drive autumn growth rate shifts: contrasting the 2015 and 2023 El Niño events

Abstract El Niño represents the most prominent mode of interannual climate variability, with its growth rate critically determining its ultimate intensity. The 2015 and 2023 events were the most recent and two strongest El Niño episodes of the 21st century, yet they displayed strikingly different growth behaviors during boreal autumn: the 2015 event intensified rapidly, whereas the 2023 event stalled. This divergence led to systematic underestimation of the 2015 event and overestimation of the 2023 event in predictions issued prior to autumn. Mixed-layer heat budget analysis and ocean current diagnostics reveal that off-equatorial wind anomalies exerted a decisive control: westerly anomalies over the Northern Hemisphere (and easterly anomalies over the Southern Hemisphere) enhanced equatorward meridional transport and positively reinforced the zonal current feedback in 2015, while the opposite wind configurations in 2023 weakened equatorward transport, reversed the zonal current feedback, and suppressed further warming, which amplifies forecast uncertainties but also offers new avenues for improving ENSO predictability.