Seasonal variability of air–sea CO2 fluxes in the Yellow and East China Seas: A case study of continental shelf sea carbon cycle model

Abstract

An inorganic carbon system module was established and coupled with a physical–ecological model based on the concept of continental shelf sea carbon cycle. Seasonal air–sea CO2 flux (FDIC) distribution in the Yellow and East China Seas (YECS) are simulated and the model results are in good agreement with observations. The simulations suggest that the YECS serve as a strong sink (−7.1±3.6mmolm−2 day−1) of atmospheric CO2 in winter and a moderate sink (−1.6±0.8mmolm−2 day−1) in spring. In summer, sink areas occupy the Yellow Sea (YS) and the adjacent sea of the Changjiang Estuary, while the middle and outer shelves of the East China Sea (ECS) act as moderate sources of atmospheric CO2. In fall, substantial carbon sources occur over the Changjiang Bank and the Subei Shoal.Dissolved inorganic carbon (DIC) variations in the euphotic layer and sea surface temperature (SST) are the key parameters to control the FDIC. DIC concentration relates to solubility, algae consumption and physical transport. According to the topographic features and the relationship between partial pressure of CO2 in surface water and SST, the YECS are divided into three typical subregions, namely the central YS, the Changjiang Bank, and the middle shelf of the ECS, to examine the key processes in regulating the total DIC variations in the euphotic layer and study the influential factors of the seasonal pattern of FDIC. The results imply that, in the central YS and the Changjiang Bank, biological effect plays a critical role in DIC removal in the euphotic layer and facilitates the sea to be a sink in spring and summer. In fall, horizontal advection transports DIC out of the central YS area leading this area to be a sink of atmospheric CO2, meanwhile vertical mixing provides DIC for the euphotic layer over the Changjiang Bank inversing this area to be a source. In winter, the low temperature exerts an essential effect on carbon sink which strength is enhanced by the intensive wind in the YECS. In the middle shelf of ECS, seasonal cycle of air–sea CO2 flux is mainly controlled by the SST seasonality. The Kuroshio Subsurface Water intrusion behaves as a net DIC source for the euphotic layer in the shelf of the ECS. The dynamic phytoplankton production and various ocean circulations cause the YECS to form distinctive subregions and thus induce the seasonal and regional changes in air–sea CO2 fluxes.