The amplitude asymmetry between the El Niño and La Niña is investigated by diagnosing the mixed layer heat budget during the ENSO developing phase using the three ocean assimilation products, SODA 2.0.2, SODA 1.4.2 and GODAS. It is found that the nonlinear zonal and meridional ocean temperature advections are essential to cause the asymmetry in the far eastern Pacific, while the vertical nonlinear advection has an opposite effect. The zonal current anomaly is dominated by the geostrophic current in association with the thermocline depth variation. The meridional current anomaly is primarily attributed to the Ekman current driven by the wind stress forcing. The so-induced anomalous horizontal currents lead to warm nonlinear advection during both El Niño and La Niña episodes, and thus strengthen (weaken) the El Niño (La Niña) amplitude. The convergence (divergence) of the anomalous geostrophic mixed-layer currents during El Niño (La Niña) results in anomalous downwelling (upwelling) in the far eastern equatorial Pacific, leading to a cold nonlinear vertical advection in both warm and cold episodes.
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