Comparison of air-sea CO2 flux and biological productivity in the South China Sea, East China Sea, and Yellow Sea: a three-dimensional physical-biogeochemical modeling study

Abstract

Marginal seas play important roles in regulating the global carbon budget, but there are great uncertainties in estimating carbon sources and sinks in the continental margins. A Pacific basin-wide physical-biogeochemical model is used to estimate primary productivity and air-sea CO2 flux in the South China Sea (SCS), the East China Sea (ECS), and the Yellow Sea (YS). The model is forced with daily air-sea fluxes which are derived from the NCEP2 reanalysis from 1982 to 2005. During the period of time, the modeled monthly-mean air-sea CO2 fluxes in these three marginal seas altered from an atmospheric carbon sink in winter to a source in summer. On annualmean basis, the SCS acts as a source of carbon to the atmosphere (16 Tg/a, calculated by carbon, released to the atmosphere), and the ECS and the YS are sinks for atmospheric carbon (–6.73 Tg/a and–5.23 Tg/a, respectively, absorbed by the ocean). The model results suggest that the sea surface temperature (SST) controls the spatial and temporal variations of the oceanic pCO2 in the SCS and ECS, and biological removal of carbon plays a compensating role in modulating the variability of the oceanic pCO2 and determining its strength in each sea, especially in the ECS and the SCS. However, the biological activity is the dominating factor for controlling the oceanic pCO2 in the YS. The modeled depth-integrated primary production (IPP) over the euphotic zone shows seasonal variation features with annual-mean values of 293, 297, and 315 mg/(m2·d) in the SCS, the ECS, and the YS, respectively. The model-integrated annual-mean new production (uptake of nitrate) values, as in carbon units, are 103, 109, and 139 mg/(m2·d), which yield the f-ratios of 0.35, 0.37, and 0.45 for the SCS, the ECS, and the YS, respectively. Compared to the productivity in the ECS and the YS, the seasonal variation of biological productivity in the SCS is rather weak. The atmospheric pCO2 increases from 1982 to 2005, which is consistent with the anthropogenic CO2 input to the atmosphere. The oceanic pCO2 increases in responses to the atmospheric pCO2 that drives air-sea CO2 flux in the model. The modeled increase rate of oceanic pCO2 is 0.91 μatm/a in the YS, 1.04 μatm/a in the ECS, and 1.66 μatm/a in the SCS, respectively.