Trend of Air‐Sea CO ₂ Flux in the Frontal Zone of the East China Sea Investigated by a Remote‐Sensing Data Assimilation Model

Abstract Field observations are often inadequate and numerical simulations are not sufficiently accurate for determining carbon source–sink trends in estuaries, particularly those in frontal zones. To address these challenges, herein, we develop a carbon assimilation model that integrates the advantages of the total alkalinity diagnostic formula from field survey data, high‐accuracy partial pressures of CO 2 (pCO 2 ) from remote sensing data, and representations of near‐coast and frontal carbon processes from a FVCOM–NPZC model. In the regions between the 15 and 30 isohalines on the East China Sea shelf, the Changjiang River plume converges with offshore high‐salinity waters, thereby forming a typical frontal zone of a mid‐latitude estuary. This model captures pCO 2 and air–sea CO 2 flux trends in the frontal zone along with their underlying mechanisms. The frontal zone exhibits a unique pCO 2 decreasing trend, thus forming a spatial “increasing–decreasing–increasing” structure in the near‐coast, frontal, and offshore areas. The unique decreasing trend in the frontal zone shows a three‐branched structure, which is associated with the reduction in the plume’s expansion pathways. A rapid carbon source–sink trend (−0.28 mmol/m 2 /d/y) is observed in the frontal zone, which is considerably faster than the increasing sink trend (−0.10 mmol/m 2 /d/y) in the offshore area. Enhanced shoreward wind and increased chlorophyll a concentration contribute to the pCO 2 decreasing trend and the rapid carbon source–sink trend in the frontal zone. The enhanced shoreward wind in the estuary increases salinity and total alkalinity, ultimately decreasing pCO 2 levels. Enhanced phytoplankton growth in the frontal zone decreases both dissolved inorganic carbon concentration and pCO 2 levels.