Mechanism for the Asymmetric Responses of Mixed‐Layer Chlorophyll‐a Concentration in the Tropical Pacific to El Niño and La Niña

Chlorophyll‐a concentration (CC) in the tropical Pacific is a key indicator of marine primary productivity and carbon cycling. This study investigates ENSO‐induced asymmetries in mixed‐layer CC across the tropical Pacific during 1994–2015 using outputs from the Darwin 3‐Day Averaged Model. Results reveal that while the spatial patterns of mixed‐layer CC anomalies are similar between El Niño and La Niña events in the model, their asymmetries lie in anomaly intensity and decay time. El Niño events induce a more pronounced decline in mixed‐layer CC than the increase shown during La Niña, with negative CC anomalies emerging earlier and decaying more rapidly. The intensity asymmetry in the CC anomalies originates from differences in how the anomalies evolve between the two ENSO phases. As El Niño develops, the earlier emergence of negative CC anomalies leads to stronger wintertime CC anomalies relative to La Niña events. Diagnostic analysis reveals that nutrient supply driven by thermocline fluctuations and horizontal advection from upper‐ocean currents contribute equally to the interannual changes in mixed‐layer CC for both event types. Importantly, during the decay time, anomalous zonal currents drive strong zonal advection that greatly accelerates the weakening of negative mixed‐layer CC anomalies during El Niño, whereas this advective effect has little impact during La Niña events. This difference in mechanism is the main cause of the asymmetric CC decay across ENSO phases. These model‐based results improve our understanding of ENSO‐related biogeochemical responses in the tropical Pacific mixed layer and offer a mechanistic basis for interpreting asymmetric CC variations.