Simulation of export production and biological pump structure in the South China Sea

The export flux of particulate organic carbon (POC) consumes upwelled dissolved inorganic carbon (DIC), which hinders surplus CO2 being released to the atmosphere. The export flux of POC is therefore crucial to the carbon and biogeochemical cycles. This study aims to model the long-term (1958–2009) variation of export flux and structure of the biological pump in the South China Sea (SCS) using a three-dimensional physical-biogeochemical coupled (ROMS-CoSiNE) model. The modeled POC export flux in the northeastern and north central SCS is high in winter and low in summer, whereas the flux in the central, southwestern and southern SCS varies following a “W” shape: two maxima in winter and summer, and two minima in spring and autumn. The pattern follows the variation of the East Asian monsoon and is consistent with observations. On the interannual scale, export flux is anti-phased with the El Nino-Southern Oscillation such that El Nino (La Nina) conditions correspond to low (high) export flux. Modeled annual mean POC export flux reaches up to 1.95 mmol m–2 day–1, which is underestimated comparing with field observations. The f-ratio is estimated to be ~0.4. The b value of the Martin equation for POC is 1.18±0.03. Remineralization rate of POC is greater than the classical Martin equation but is consistent with its subtropical counterparts. The modeled results indicate that the SCS is a weak source of atmospheric CO2 with a flux estimated at 1.0 mmol m–2 day–1. The modeled results provide an insight of the temporal and spatial variability of the carbon cycle in this monsoon-driven, semi-enclosed basin.