Journal of Oceanography, Vol. 54 (No. 4), pp. 323-336, 1998
Tomonori Watai, Masaaki Kikuchi and Takakiyo Nakazawa
Center for Atmospheric and Oceanic Studies, Faculty of Science, Tohoku University, Sendai 980-77, Japan
(Received 13 September 1997; in revised form 9 May 1998; accepted 16 May 1998)
Abstract: In order to examine temporal variations of the surface oceanic and atmospheric fCO2 and the DIC concentration, we analyzed air and seawater samples collected during the period May 1992-June 1996 in the northwestern North Pacific, about 30 km off the coast of the main island of Japan. The atmospheric CO2 concentration has increased secularly at a rate of 1.9 ppmv yr-1, and it showed a clear seasonal cycle with a maximum in spring and a minimum late in summer, produced mainly by seasonally-dependent terrestrial biospheric activities. DIC also showed a prominent seasonal cycle in the surface ocean; the minimum and maximum values of the cycle appeared in early fall and in early spring, respectively, due primarily to the seasonally-dependent activities of marine biota and partly to the vertical mixing of seawater and the coastal upwelling. The oceanic fCO2 values were almost always lower than those of the atmospheric fCO2, suggesting that this area of the ocean acts as a sink for atmospheric CO2. Values varied seasonally, mainly reflecting seasonal changes of SST and DIC, with a secular increase at a rate of 3.7 matm yr-1. The average values of the annual net CO2 flux between the ocean and the atmosphere calculated by using the different bulk equations ranged between -0.8 and -1.7 mol m-2 yr-1, and its magnitude was enhanced and reduced late in spring and mid-summer, respectively, due mainly to the seasonally varying oceanic fCO2.