Air–sea CO₂ fluxes in the East China Sea based on multiple-year underway observations

Abstract. This study reports the most comprehensive data set thus far of surface seawater pCO₂ (partial pressure of CO₂) and the associated air–sea CO₂ fluxes in a major ocean margin, the East China Sea (ECS), based on 24 surveys conducted in 2006 to 2011. We showed highly dynamic spatial variability in sea surface pCO₂ in the ECS except in winter, when it ranged across a narrow band of 330 to 360 μatm. We categorized the ECS into five different domains featuring with different physics and biogeochemistry to better characterize the seasonality of the pCO₂ dynamics and to better constrain the CO₂ flux. The five domains are (I) the outer Changjiang estuary and Changjiang plume, (II) the Zhejiang–Fujian coast, (III) the northern ECS shelf, (IV) the middle ECS shelf, and (V) the southern ECS shelf. In spring and summer, pCO₂ off the Changjiang estuary was as low as < 100 μatm, while it was up to > 400 μatm in autumn. pCO₂ along the Zhejiang–Fujian coast was low in spring, summer and winter (300 to 350 μatm) but was relatively high in autumn (> 350 μatm). On the northern ECS shelf, pCO₂ in summer and autumn was > 340 μatm in most areas, higher than in winter and spring. On the middle and southern ECS shelf, pCO₂ in summer ranged from 380 to 400 μatm, which was higher than in other seasons (< 350 μatm). The area-weighted CO₂ flux on the entire ECS shelf was −10.0 ± 2.0 in winter, −11.7 ± 3.6 in spring, −3.5 ± 4.6 in summer and −2.3 ± 3.1 mmol m⁻² d⁻¹ in autumn. It is important to note that the standard deviations in these flux ranges mostly reflect the spatial variation in pCO₂ rather than the bulk uncertainty. Nevertheless, on an annual basis, the average CO₂ influx into the entire ECS shelf was 6.9 ± 4.0 mmol m⁻² d⁻¹, about twice the global average in ocean margins.