Construction of a High Spatiotemporal Resolution Dataset of Satellite-DerivedpCO2and Air–Sea CO2Flux in the South China Sea (2003–2019)

The South China Sea (SCS) is one of the largest marginal seas in the world. It includes a river-dominated, highly productive ocean margin on the northern shelf and an oligotrophic ocean-dominated basin along with other sub-regions with various features. It was challenge to estimate the air-sea CO2 flux in this area. We developed a retrieval algorithm for sea surface p CO ₂ by a combination of our previously established semi-mechanistic approach (MeSAA) and machine learning (ML) method, named MeSAA-ML-SCS, built upon a large dataset of sea surface partial pressure of CO ₂ ( p CO ₂ ) collected from in situ measurements during 44 cruises/legs to the SCS in the last two decades. We set several semi-analytical parameters, includes: p CO _{2_therm} represented the combined effect of thermodynamics and the atmospheric CO ₂ forcing on seawater p CO ₂ ; upwelling index (UISST) and mixing layer depth (MLD) to characterize the mixing processes; chlorophyll-a concentration (Chl-a) with remote sensing reflectance at 443 and 555 nm (Rrs(443) and Rrs(555)), which were proxies of biological effects and other characteristics for distinguishing shelf, basin, and sub-regions. We set the difference between seawater p CO ₂ and atmospheric p CO ₂ (Δ p CO ₂ ^Sea-Air ) as the output, and the seawater p CO ₂ was finally obtained by summing atmospheric p CO ₂ and Δ p CO ₂ ^Sea-Air . We compared several ML models, and the XGBoost model was confirmed as the best. Independent cruise-based datasets that are not involved in the model training were used to validate the satellite products, with low root mean square error (RMSE = 11.69 μatm) and mean absolute percentage deviation (APD = 1.59%). The increasing trend of time-series satellite-derived p CO ₂ (2.44 ± 0.24 μatm/yr) were validated by the in situ data at the Southeastern Asia Time-series Study (SEATS) station, showing good consistency. Results indicate that the SCS as a whole is a source of atmospheric CO ₂ , releasing an average of 12.34 ± 3.11 Tg C/yr from a total area of 2.87 × 10 ⁶ km ² , while the northern shelf act as a sink (2.02 ± 0.64 Tg C/yr). With the forcing of increasing atmospheric CO ₂ , the area-integrated CO ₂ efflux over the entire SCS is decreasing with a rate of 0.41 Tg C/yr during 2003–2019. This shared long time series, high-accuracy dataset (1 km) can be helpful to further improve our understanding of the air-sea CO ₂ exchange dynamics in the SCS.