Seasonal Changes in Oceanic pCO2 in the Oyashio Region from Winter to Spring TAKASHI MIDORIKAWA1*, SONOKI IWANO2, KAZUHIRO SAITO3, HIROYUKI TAKANO4, HITOMI KAMIYA1, MASAO ISHII5 and HISAYUKI Y. INOUE6 1 Climate and Marine Department, Japan Meteorological Agency, Otemachi Chiyoda-ku, Tokyo 100-8122, Japan 2 Observations Department, Japan Meteorological Agency, Otemachi Chiyoda-ku, Tokyo 100-8122, Japan 3 Hakodate Marine Observatory, Mihara Hakodate 041-0806, Japan 4 Maizuru Marine Observatory, Shimohukui 901, Maizuru 624-0946, Japan 5 Geochemical Research Department, Meteorological Research Institute, Nagamine Tsukuba 305-0052, Japan 6 Graduate School of Environmental Earth Science, Hokkaido University, Kita-ku, Sapporo 060-0810, Japan Journal of Oceanography, Vol. 59, pp. 871 to 882, 2003 Received 3 February 2003; in revised form 11 August 2003; accepted 11 August 2003 *Corresponding author address: Takashi Midorikawa, Geochemical Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan. E-mail: midorika@mri-jma.go.jp Abstract We observed the partial pressure of oceanic CO2, pCO2sea, and related surface properties in the westernmost region of the subarctic North Pacific, seasonally from 1998 to 2001. The pCO2sea in the Oyashio region showed a large decrease from winter to spring. In winter, pCO2sea was higher than 400 ƒÊatm in the Oyashio region and this region was a source of atmospheric CO2. In spring, pCO2sea decreased to extremely low values, less than 200 µatm (minimum, 139 µatm in 2001), around the Oyashio region with low surface salinity and this region turned out to be a strong sink. The spatial variations of pCO2sea were especially large in spring in this region. The typical Oyashio water with minimal mixing with subtropical warm water was extracted based on the criterion of potential alkalinity. The contribution of main oceanic processes to the changes in pCO2sea from winter to spring was estimated from the changes in the concentrations of dissolved inorganic carbon and nutrients, total alkalinity, temperature and salinity observed in surface waters in respective years. These quantifications indicated that photosynthesis made the largest contribution to the observed pCO2sea decreases in all years and its magnitude was variable year by year. These year-to-year differences in spring biological contribution could be linked to those in the development of the density stratification due to the decrease in surface salinity. Thus, the changes in the surface physical structure could induce those in pCO2sea in the Oyashio region in spring. Furthermore, it is suggested that the direction and magnitude of the air-sea CO2 flux during this season could be controlled significantly by the onset time of the spring bloom.