As a vast and highly variable natural source of atmospheric carbon dioxide (CO₂), the equatorial Pacific holds a pivotal position in the global carbon cycle. Here we examine the inorganic carbon system and related parameters in the west subtropical-equatorial Pacific surface seawater during a research cruise from February to April 2023. We find that the distributions of dissolved inorganic carbon (DIC), CO₂ partial pressure (pCO₂), and their anomalies in the study area display typical “La Niña featured” patterns, despite the neutral phase of El Niño-Southern Oscillation (ENSO) then. Analyses on the interannual variations of February-to-April mean CO₂ fugacity (fCO₂) anomalies in the eastern West Pacific Warm Pool (WPWP) relative to the 3-month running mean of Niño3.4 index revealed a ~5-month lag between the ENSO events and the strongest response of fCO₂ at eastern edge of the WPWP. This lag can be attributed to the combined effect of climate system development, thermal and chemical equilibrium, and physical mixing. Our work highlights the need for sustaining observations of marine carbon system, especially in climate-sensitive areas.

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