Predicting the 2023/24 El Niño from a multi-scale and global perspective

The 2023/24 El Niño ranks as the 2nd strongest El Niño in the 21st century thus far. The intensity of the event was successfully predicted as early as March 2023, based on the buildup of upper ocean heat content in the western equatorial Pacific. Nevertheless, the unusual evolution pattern of this event, including the two-step warming tendency and two warming centers during the 2023/24 El Niño, and the influence from outside of the equatorial Pacific, remain to be explained. Here we show that the 2023/24 strong El Niño was mainly contributed to by three factors. Firstly, the buildup of heat content in the western equatorial Pacific in the early months of 2023 fostered a strong canonical El Niño, characterized by a steady increase in SST anomalies across the central-eastern equatorial Pacific, peaking in late 2023. Secondly, the intense sea surface temperature warming outside of the equatorial Pacific suppressed the El Niño and confined the warming mainly to the eastern basin. Lastly, the westerly wind bursts that occurred in the western-central equatorial Pacific in autumn induced another relatively weak warm center in the central equatorial Pacific toward the end of 2023. While the latter two factors coincidentally offset each other, leaving the buildup of heat content appearing as the primary cause of the strong 2023/24 El Niño, they explained the detailed structure of this El Niño.  Our results not only confirm the essential role of equatorial ocean heat recharge for El Niño development, but also demonstrate the necessity of accounting for multi-scale interactions from a global perspective to understand and predict El Niño.