The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

The oceanic partial pressure of CO₂ (pCO₂) is one of the most critical parameters for evaluating the oceanic carbon cycle and the carbon sink capacity. Accurate calculation and proper analysis of pCO₂ on multi-tempo-spatial scales are pathways to a better understanding of oceanic carbon cycle and carbon uptake/sink structure. The study focuses on the intraseasonal-seasonal-interannual multi-time scale changes of pCO₂ in the South China Sea (SCS) and their regulating mechanisms. A high-resolution physical-biogeochemical numerical model based on ROMS-CoSiNE is constructed, and simulation for physical-biogeochemical processes in the SCS from 1992 to 2021 from the nearshore to the basin scale is accomplished. By means of EOF analysis, decomposition of influence factors, and Butterworth filter and etc., the pCO₂ is studied on multi-time dimensions, multi-spatial scales and multi-coupling processes under an unified framework.
Based on the model results for over 30 years, we have several major conclusions as follows: the SCS acts as a weak source of CO₂. The entire area is with a long-term trend rate o f increase of 1.45 μatm a^-1, which is regulated by the increasing trend atmospheric CO₂ concentration according to the EOF analysis result; besides, influence from ENSO and PDO is also observed in the interannual-decadal changes of pCO₂. The seasonal variability is dominated by non-temperature effect, which implies the input or consumption of total CO₂ (TCO₂) by water mass mixing and biological activities is predominant in the seasonal variation of the long term trend rate of pCO₂. The intraseasonal signal of pCO₂ peaks at 45 day, and the component of temperature to the pCO₂ variability is the main contribution to the intraseasonal oscillation of pCO₂ along the southeast coast of Vietnam in boreal summer.
In this study, the subseasonal-seasonal-interannual spatial-temporal changes of oceanic pCO₂ based on physical-biogeochemical model results were analyzed, and the diverse responses of pCO₂ to meso-and small-scale processes are discussed. The results illustrate the unique carbon cycle controlling mechanism in the SCS, which benefits the estimations of the carbon source/sink pattern and carbon fixation capacity in the SCS, and provides useful information for developing carbon-neutral policies.

partial pressure of CO2 (pCO2), subseasonal-seasonal-interannual variation, South China Sea, physical-biogeochemical modeling