Modeling seasonal and diurnal pCO 2 variations in the northern South China Sea

Abstract

This paper describes the simulated temporal variation of surface seawater CO 2 partial pressure ( pCO 2) 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 pCO 2 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 CO 2 exchange was a primary process that modulated surface pCO 2 and exceeded the role of sea surface temperature (SST) even though the phase of the pCO 2 variation generally followed the strong seasonal cycle of SST. This was because sea–air CO 2 exchange is a slow process and has an accumulative effect on surface water pCO 2 due to the buffering effect of the carbonate system once CO 2 has dissolved in the seawater, which leads to a long equilibration time of CO 2 between the atmosphere and seawater. The mixing effect on pCO 2 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 pCO 2 variations at the seasonal scale because of the oligotrophic characteristic of the region. At diurnal time scales, the dominant pCO 2 controlling factor was mainly associated with the local physical and biological conditions. Temperature and wind-induced vertical mixing played major roles in pCO 2 when the winter heat flux and upward transport of low temperature and high pCO 2 in deep water were intensified. Phytoplankton blooms generally occur after a period of strong wind, as a result, biological metabolism becomes the most important pCO 2 regulator when the surface chlorophyll-a reached its highest level. Unlike that in the seasonal scale, the effect of sea–air CO 2 exchange was minor at diurnal time scales due to the long equilibration time of CO 2 between the atmosphere and seawater. We also found that the frequency of the model driving force was important in reproducing the sea surface pCO 2. The high frequency forcing was important in controlling the pCO 2 variation through the feedback effect to the corresponding physical and biogeochemical responses.