Sub-monthly Processes Contribute Significantly to CO₂ Uptake in the South China Sea

Estimating the multi-year average air–sea CO₂ flux over a large area usually involves the use of monthly mean variables from the atmosphere and ocean. Ignoring sub-monthly processes will blur the oceanic carbon cycle, especially when the synoptic and sub-seasonal scale processes are significant, like in the South China Sea (SCS). Based on an empirical relationship between the partial pressure of CO₂ in water and the sea surface temperature (SST), we recalculated the air–sea CO₂ flux of the SCS with daily products of atmospheric reanalysis and SST. Our results show that the sub-monthly process contributes 10% of the total CO₂ flux of the SCS and can even alter the sign of the CO₂ flux in the spring. In the near-surface coupling process, intramonthly variations in surface winds play the dominant role, except in regions with significant ocean eddies. The co-spectrum analysis of SST and wind speed reveals the most essential oscillation of >20 days. Therefore, a product of the sea surface environment for 10-day intervals can better estimate the air–sea CO₂ flux over the SCS than monthly data.