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<p>El Niño events typically develop slowly but decay rapidly, yet the processes driving this phase asymmetry remain insufficiently quantified. Here we diagnose the mixed-layer heat budget over integration depths ranging from 30 to 100 m and introduce a depth-resolved diagnostic framework. The results indicate that the asymmetry arises from thermodynamic damping, oceanic zonal advection, and thermocline feedbacks. The thermodynamic damping mainly reflects a passive linear response to warm sea surface temperature anomalies, while the seasonal modulation of the damping efficiency adds a decay-phase enhancement. Dynamical feedbacks vary strongly with depth and are amplified during a concentrated period of the decay year, driven by oceanic wave adjustment and seasonally varying background conditions. The depth-resolved diagnosis shows that attribution of El Niño phase asymmetry depends on the vertical structure of the heat budget, providing a physical benchmark for interpreting ENSO evolution and identifying potential precursors that may inform El Niño prediction systems.</p>

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Why does El Niño develop slowly but decay rapidly

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年8月5日

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  • Xiaomeng Song,
  • Renhe Zhang,
  • Xinyao Rong,
  • Jingzhi Su

摘要

El Niño events typically develop slowly but decay rapidly, yet the processes driving this phase asymmetry remain insufficiently quantified. Here we diagnose the mixed-layer heat budget over integration depths ranging from 30 to 100 m and introduce a depth-resolved diagnostic framework. The results indicate that the asymmetry arises from thermodynamic damping, oceanic zonal advection, and thermocline feedbacks. The thermodynamic damping mainly reflects a passive linear response to warm sea surface temperature anomalies, while the seasonal modulation of the damping efficiency adds a decay-phase enhancement. Dynamical feedbacks vary strongly with depth and are amplified during a concentrated period of the decay year, driven by oceanic wave adjustment and seasonally varying background conditions. The depth-resolved diagnosis shows that attribution of El Niño phase asymmetry depends on the vertical structure of the heat budget, providing a physical benchmark for interpreting ENSO evolution and identifying potential precursors that may inform El Niño prediction systems.