Unusual and persistent easterlies restrained the 2023/24 El Niño development after a triple-dip La Niña

The 2023/24 El Niño, emerging after a rare triple-dip La Niña from 2020 to 2023, garnered global attention due to its potential to evolve into an extreme event, given the largest accumulation of warm water in the equatorial western Pacific during its onset stage (≈ 1.92 × 1015 m3) since 1980. However, its growth rate unexpectedly decelerated after the first half of 2023, ultimately leading to El Niño’s failure to reach the anticipated extreme levels. Here, we show through observational analyses that unusual easterly anomalies over the tropical western-central Pacific, persisting after the end of the preceding La Niña, significantly contributed to this slowdown. A prominent east‒west sea surface temperature gradient in the region has been identified as the crucial factor associated with these unusual and persistent easterly anomalies. This temperature gradient is directly attributed to a negative phase of the North Pacific Meridional Mode and a deepened thermocline over the Philippine Sea—both instigated by the preceding La Niña upon its demise. These findings offer a deeper understanding of the atypical transition from a prolonged La Niña to an El Niño.