Seasonal variation in total inorganic carbon and its controlling processes in surface waters of the western North Pacific subtropical gyre

Seasonal variation in total inorganic carbon (TCO2) in surface waters of the western North Pacific (137°–152°E) subtropical gyre was analyzed on the basis of measurements of TCO2 and partial pressure of CO2 (pCO2sw). The controlling processes including vertical mixing, horizontal advection, and net air–sea CO2 transport, as well as biological activity, were quantified. The seasonal increase in normalized TCO2 (NTCO2) from autumn to winter, ranging from 19 to 37 μmol kg−1 in the northern part of the subtropical gyre between 24°N and 30°N, was predominantly accounted for by the upward supply of TCO2 due to enhanced vertical mixing. The contribution of horizontal advection, estimated from monthly meridional NTCO2 distributions and the monthly advection field of the Meteorological Research Institute (MRI)ʹs 3D-ocean general circulation model, was insignificant. Analyses of the mixed-layer NTCO2 budget revealed that biological activity was playing an important role in the decrease in surface NTCO2 from winter to summer. Annual net community production reached 48±19 gC m−2 between 24°N and 30°N, and 19±16 gC m−2 between 15°N and 23°N.