Spatial and temporal variations of surface seawater fCO2 in the Kuroshio off Japan

To examine the annual growth rate of CO2 in the ocean, spatial and temporal variations of the fugacity of CO2 (fCO2) in surface seawater in the Kuroshio off Japan were surveyed, using the data obtained by the Japan Meteorological Agency in the period 1989–1995. Winter surface seawater fCO2 decreased distinctly in the south of the Kuroshio front, where sea surface temperature increased sharply. In other seasons, such a large spatial variability at the Kuroshio front was rarely found. The temporal variation in surface seawater fCO2 at the selected area could be expressed by a rather simple function consisting of annual and bi-annual harmonics and a linear trend. The seasonal variation has a sharp maximum peak in summer, but an indistinct minimum peak in the period from winter to spring. The annual growth rate was 1.5 μatm/yr, significant at the 95% level. The seasonal variation reproduced assuming a thermodynamic dependence of surface seawater fCO2 on water temperature and salinity explains only about 24% of the observed seasonal variation.