Seasonal variations of sea–air CO<sub>2</sub> fluxes in the largest tropical marginal sea (South China Sea) based on multiple-year underway measurements

Abstract

Abstract. Based upon 14 field surveys conducted between 2003 and 2008, we showed that the seasonal pattern of sea surface partial pressure of CO 2 (pCO 2 ) and sea-air CO 2 fluxes differed among four different physicalbiogeochemical domains in the South China Sea (SCS) proper. The four domains were located between 7 and 23 • N and 110 and 121 • E, covering a surface area of 1344 × 10 3 km 2 and accounting for ∼ 54 % of the SCS proper. In the area off the Pearl River estuary, relatively low pCO 2 values of 320 to 390 µatm were observed in all four seasons and both the biological productivity and CO 2 uptake were enhanced in summer in the Pearl River plume waters. In the northern SCS slope/basin area, a typical seasonal cycle of relatively high pCO 2 in the warm seasons and relatively low pCO 2 in the cold seasons was revealed. In the central/southern SCS area, moderately high sea surface pCO 2 values of 360 to 425 µatm were observed throughout the year. In the area west of the Luzon Strait, a major exchange pathway between the SCS and the Pacific Ocean, pCO 2 was particularly dynamic in winter, when northeast monsoon induced upwelling events and strong outgassing of CO 2 . These episodic events might have dominated the annual sea-air CO 2 flux in this particular area. The estimate of annual sea-air CO 2 fluxes showed that most areas of the SCS proper served as weak to moderate sources of the atmospheric CO 2 , with sea-air CO 2 flux values of 0.46 ± 0.43 mol m −2 yr −1 in the northern SCS slope/basin, 1.37 ± 0.55 mol m −2 yr −1 in the central/southern SCS, and 1.21 ± 1.48 mol m −2 yr −1 in the area west of the Luzon Strait. However, the annual sea-air CO 2 exchange was nearly in equilibrium (−0.44 ± 0.65 mol m −2 yr −1 ) in the area off the Pearl River estuary. Overall the four domains contributed (18 ± 10) × 10 12 g C yr −1 to the atmospheric CO 2 .

Diapycnal Fluxes of Nutrients in an Oligotrophic Oceanic Regime: The South China Sea

“…This extra removal of DIC, if solely through air-sea gas exchange, would infer that 0.95 mmol C m À2 d À1 of CO 2 was released into the atmosphere. Such an estimate is consistent with reported air-sea CO 2 flux in the SCS basin Zhai et al, 2013).…”

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing – an overview and synthesis

“…1), the intruding Kuroshio Branch Current is the main source of the Tsushima 3 Figure 2. Observed partial pressure of CO2 in surface waters of the three marginal seas examined in this special issue, namely, (a) the Japan/East Sea , (b) the East China Sea 2014a) and the South China Sea (Tseng et al, 2007;Zhai et al, 2013) Observed partial pressure of CO 2 in surface waters of the three marginal seas examined in this special issue, namely, (a) the Japan/East Sea , (b) the East China Sea C.-M. Tseng et al, 2014) and the South China Sea (Tseng et al, 2007;Zhai et al, 2013). Also plotted are the pCO 2 in the air observed at Mauna Loa, Hawaii.…”

“…determined by more sophisticated trend fitting . b The trend is obtained by linear regression of the observed values from the SEATS station (Tseng et al, 2007) and from Domain B of the northern SCS (Zhai et al, 2013). ) and the YS (Zhai et al, 2014).…”

“…Chou et al (2013a) contrasted the different seasonal patterns of pCO 2 in the ECS in the 1990s and the 2000s. Zhai et al (2013) surveyed the entire SCS over 6 years to assess the air-sea CO 2 exchange fluxes. They concluded that most of the SCS is a source of CO 2 to the atmosphere while the Pearl River estuary zone is a slight sink, the overall SCS being a net source of CO 2 to the atmosphere.…”

“…Using a sophisticated data processing technique to delineate the seasonal variation, Kim et al (2014) obtained a moderately strong trend of 2.7 ± 1.1 µatm yr −1 . For the SCS, we have constructed a time series from observations from the South East Asia Time-Series Study (SEATS) station (Tseng et al, 2007) and observations from Domain B in the northern SCS, which covers the SEATS station (Zhai et al, 2013). The two data sets show good agreement for the observations in 2003.…”