Modeling seasonal and diurnal pCO2 variations in the northern South China Sea

This paper describes the simulated temporal variation of surface seawater CO2 partial pressure (pCO2) in the northern South China Sea. We produced the simulations with a one-dimensional (1-D) coupled physical–biogeochemical model that had high-frequency, time-dependent atmospheric forcing and that were validated with field observations. We also examined the associated processes that modulate seawater pCO2 at different time scales, from diurnal to seasonal, using a series of process-oriented experiments. At seasonal time scales, we revealed that the sea–air CO2 exchange was a primary process that modulated surface pCO2 and exceeded the role of sea surface temperature (SST) even though the phase of the pCO2 variation generally followed the strong seasonal cycle of SST. This was because sea–air CO2 exchange is a slow process and has an accumulative effect on surface water pCO2 due to the buffering effect of the carbonate system once CO2 has dissolved in the seawater, which leads to a long equilibration time of CO2 between the atmosphere and seawater. The mixing effect on pCO2 induced by total alkalinity and dissolved inorganic carbon variations was, generally, positively correlated with the seasonal evolution of wind speed. Biological processes were the smallest contributors to pCO2 variations at the seasonal scale because of the oligotrophic characteristic of the region. At diurnal time scales, the dominant pCO2 controlling factor was mainly associated with the local physical and biological conditions. Temperature and wind-induced vertical mixing played major roles in pCO2 when the winter heat flux and upward transport of low temperature and high pCO2 in deep water were intensified. Phytoplankton blooms generally occur after a period of strong wind, as a result, biological metabolism becomes the most important pCO2 regulator when the surface chlorophyll-a reached its highest level. Unlike that in the seasonal scale, the effect of sea–air CO2 exchange was minor at diurnal time scales due to the long equilibration time of CO2 between the atmosphere and seawater. We also found that the frequency of the model driving force was important in reproducing the sea surface pCO2. The high frequency forcing was important in controlling the pCO2 variation through the feedback effect to the corresponding physical and biogeochemical responses.

► The pCO2 variation in the northern South China Sea was investigated with a 1-D model.
► Air–sea CO2 exchange was a primary process that modulated surface pCO2 at seasonal time scales.
► The dominant pCO2 controlling factor was associated with the local physical and biological conditions at diurnal time scales.
► Buffering effect played important role in regulating surface water pCO2 variation.