Air–sea CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes in the East China Sea based on multiple-year underway observations

X.-H Guo,Q Li,Y Bai,Y Xu,C Zhang,W.-D Zhai,M.-H Dai,G.-Z Wang

doi:10.5194/bg-12-5495-2015

Abstract

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.

Highlights

IntroductionWith the rapid growth of carbon flux measurements during the past decade, our estimation of the coastal ocean air– sea CO2 fluxes is that they have converged to about 0.2 to 0.5 Pg C yr−1 on a global scale (Borges et al, 2005; Cai et al, 2006; Chen and Borges, 2009; Chen et al, 2013; Dai et al, 2013; Laruelle et al, 2010; Laruelle et al, 2014), and it is safe to state that the earlier estimate of up to 0.9 to 1.0 Pg C yr−1 was an overestimate
We investigated the air–sea CO2 fluxes on the entire East China Sea (ECS) shelf based on large-scale observations of 24 mapping cruises from 2006 to 2011, resolving both spatial coverage and fully seasonal variations
The seasonal variation in the average sea surface temperature (SST) and sea surface salinity (SSS) in the five domains is further shown in Fig. 4 and Tables 3 to 7

Summary

Introduction

With the rapid growth of carbon flux measurements during the past decade, our estimation of the coastal ocean air– sea CO2 fluxes is that they have converged to about 0.2 to 0.5 Pg C yr−1 on a global scale (Borges et al, 2005; Cai et al, 2006; Chen and Borges, 2009; Chen et al, 2013; Dai et al, 2013; Laruelle et al, 2010; Laruelle et al, 2014), and it is safe to state that the earlier estimate of up to 0.9 to 1.0 Pg C yr−1 was an overestimate Having stated this, it remains, challenging to reliably assess the carbon fluxes in individual coastal systems that are often characterized by the greatest spatial and temporal variations (Cai and Dai, 2004; Dai et al, 2013; Dai et al, 2009; Zhai et al, 2013). Many coastal oceans have been impacted by anthropogenic activities, the signals of which remain, challenging to decipher (Chou et al, 2007; Omar et al, 2003).

Methods

Results

Discussion

Conclusion