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Export of dissolved inorganic carbon (DIC) to adjoining oceans enhances the potential of CO2 sequestration in marginal seas. By using a series of measured DIC depth profiles and reported flow transports, we estimated that the intermediate outflow (100–600 m) from the South China Sea is capable of transporting 6.5 ± 4.1 Tg (1 Tg = 1012g) of biologically mediated carbon (DICbio) annually to the East China Sea (ECS) via the northwardly flowing Kuroshio current. The mixing and transport of these DIC‐rich waters would raise 3% and 16% of DIC/TA ratio and the Revelle factor of the adjoining seawaters, respectively. Upon upwelling onto the ECS shelf, these DIC‐rich waters would counteract the potential of CO2 uptake of shelf waters that might have been enhanced by the accompanying increase in nutrient inputs, thus complicating assessment of the ECS as a net CO2 source or sink.

The eastern equatorial Pacific is the predominant source area for atmospheric CO2, and the size of this source is significantly reduced during El Niño events. Here we apply a newly constructed 1999–2008 time series carbon chemistry trend to show a similar reduction can also be identified in the northern South China Sea. The net sea‐to‐air CO2 flux during the 2002–2003 El Niño event (1.57 ± 0.13∼1.61 ± 0.28 g C m−2 a−1) at the SEATS time series site was significantly lower than that during the 2007–2008 La Niña event (10.35 ± 0.66∼10.67 ± 0.93 g C m−2 a−1). The appreciable reduction (by ∼85%) is a direct response to the diminished vertical mixing of CO2‐rich subsurface waters from below and, possibly, an increase in the lateral flow transport from the western Pacific during the weakening winter monsoon in the El Niño years. Thus, the suppression of CO2 efflux during the El Niño events is a basin‐wide phenomenon across from the eastern equatorial Pacific to the western subtropical Pacific including the northern South China Sea.

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