Distribution of the partial pressure of CO2 in surface water (pCO2w) between Japan and the Hawaiian Islands: pCO2w—SST relationship in the winter and summer

On the basis of measurements of the partial pressure of carbon dioxide in surface seawater (pCO₂^w) between Japan and the Hawaiian Islands in winter and summer, we examined the relationship between pCO₂^w and the sea surface temperature (SST) in the North Pacific Subtropical Gyre (NPSG). In winter, pCO₂^w correlated well with the SST (0.14–0.24% °C⁻¹), suggesting a monotonous change in the carbonate system. However, in summer, five different pCO₂^w–SST relationships were found in the NPSG (including the Kuroshio Extension) due to changes in the relative contribution of ocean dynamics (upwelling, vertical mixing and advection), biological activity in the absence (very low level) of macro-nutrients and thermodynamics. The increase in pCO₂^w corresponding to a unit increase in the SST from January to July was low (<2.5% °C⁻¹) west (leeward side) of the Hawaiian Islands (19–22°N, 158–168°W) and in the Kuroshio Extension (33–35°N, 140–165°E), and high (~3% °C⁻¹) south of the Kuroshio Extension (25–30°N, 180–165°W) and the Hawaiian Islands (15–19°N, 157–162°W). This suggested that the drawdown of dissolved inorganic carbon was affected by the enhanced biological activity due to upwelling events associated with eddies and/or the transport of dissolved nutrients from gyre edges to the interior.