TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 59 (No. 5), pp. 709-717, 2003

Absorption of Atmospheric CO2 and Its Transport to the Intermediate Layer in the Okhotsk Sea

Akihisa S. Otsuki*, Shuichi Watanabe and Shizuo Tsunogai

Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan

(Received 12 July 2002; in revised form 18 February 2003; accepted 24 February 2003)

Abstract: In the southwestern Okhotsk Sea off Hokkaido we observed chemical components related to the carbonate system for 1 year from August 1997 to June 1998. Using the conservative components salinity and water temperature, we confirmed the existence of two water masses flowing into the intermediate layer of the Okhotsk Sea, the East Sakhalin Current Water (ESCW) which becomes denser by mixing of brine water, and the Forerunner of Soya Warm Current Water (FSWW) which becomes denser due to cooling of the saline Kuroshio water. The DNTCx values were calculated by comparing the ESCW and the FSWW with the Pacific Deep Water (PDW). The DNTCx values obtained are 100-110 mmol/kg and 70-100 mmol/kg for the ESCW and the FSWW off Hokkaido, respectively, which are considerably larger than that of the Kuroshio water. These large DNTCx values may be due to both low DIC concentration in the surface water and intense gas exchange under the cold and stormy winter conditions for the ESCW and the cooling of the FSWW as it flows northward. Since the flow rates of dense waters concerned with the ESCW and the FSWW have previously been estimated as 0.9 Sv and 0.2 Sv, respectively, the amount of atmospheric CO2 absorbed and transported to the intermediate layer turns out to be 3.9-4.1 1013 gC/yr. This flux is small on a global scale, but the flux divided by the surface layer of the Okhotsk Sea is 30 gC/m2/yr, which is 5 times greater than the mean absorption flux of anthropogenic CO2 in the world's oceans. It is thus considered that atmospheric CO2 is efficiently absorbed in the Okhotsk Sea.

*Corresponding author E-mail: otsuki@ees.hokudai.ac.jp

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