[A-CG44] Biogeochemical linkages between the surface ocean and atmosphere

[ACG44-02] Seasonal and interannual variations of surface water carbonate chemistry in the Indian sector of the Southern Ocean

*Manami Tozawa¹, Daiki Nomura^1,2,3, Shin-ichiro Nakaoka⁴, Masaaki Kiuchi¹, Daisuke Hirano⁵, Shigeru Aoki⁵, Hiroko Sasaki⁶, Hiroto Murase⁷ (1.Faculty of Fisheries Sciences, Hokkaido University, 2.Field Science Center for Northern Biosphere, Hokkaido University, 3.Arctic Research Center, Hokkaido University, 4.National Institute for Environmental Studies, 5.Institute of Low Temperature Science, Hokkaido University, 6.Fisheries Resources Institute, Japan Fisheries Research and Education Agency, 7.Tokyo University of Marine Science and Technology )

Keywords:Southern Ocean, carbon cycling, pressure of carbon dioxide, air-sea CO2 flux

To assess carbon cycling in the Indian sector of the Southern Ocean (80°E–150°E, south of 60°S) quantitatively, we measured seawater temperature, salinity, chlorophyll-a concentration, partial pressure of carbon dioxide (pCO₂), dissolved inorganic carbon (DIC) concentration, alkalinity (TA), and nutrient concentrations.

The air-sea CO₂ flux in this region was evaluated to be –7.6±10.9 mmol C m^–2 day^–1 (–75.6 − +14.1 mmol C m^–2 day^–1) suggesting that the region was a weak sink. Then, we estimated the change in pCO₂ from winter to summer ( pCO₂) due to changes in seawater temperature, salinity, and biological activity on the basis of the assumption that the observed values of DIC concentration and TA in the temperature minimum layer remain the same as in the winter under sea-ice with a temperature of –1.8℃ and salinity of 34.25. The spatial distribution of pCO₂ in the western area (80°E–120°E) observed from December to early January was influenced by biological activity, and that in the eastern area (120°E–150°E) observed from January to February was influenced by temperature and salinity.

We also examined the annual change in oceanic and atmospheric CO₂ concentrations (xCO₂) between 1996 and 2019. The mean values of oceanic and atmospheric xCO₂ increased by 24 ppm and 45 ppm, respectively. This suggests that the main reason of the rise in oceanic xCO₂ is that the ocean have absorbed CO₂ from atmosphere as a result of the rise in atmospheric xCO₂. However, it also indicates that the oceanic xCO₂ rise is due to the rise in sea water temperature and changes in the ocean circulation.