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 p CO2 (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 p CO2 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 p CO2 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, p CO2 off the Changjiang estuary was as low as < 100 μatm, while it was up to > 400 μatm in autumn. p CO2 along the Zhejiang-Fujian coast was low in spring, summer and winter (300 to 350 1/4atm) but was relatively high in autumn (> 350 μatm). On the northern ECS shelf, p CO2 in summer and autumn was > 340 μatm in most areas, higher than in winter and spring. On the middle and southern ECS shelf, p CO2 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 '1 in autumn. It is important to note that the standard deviations in these flux ranges mostly reflect the spatial variation in p CO2 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.