A model study for the carbon cycle in the upper layer of the North Pacific

A three-dimensional ocean model including a bulk mixed layer model is employed to simulate the seasonal variabilities of total CO2 (TCO2) and nutrient in the mixed layer. The model is used also to determine the mean biogeochemical fields (TCO2 and nutrient distributions) in the upper ocean, which result from a balance between air–sea flux, biological flux, and horizontal and vertical advections. The model simulates the seasonal variabilities of TCO2 and phosphate in the mixed layer by specifying the difference in biological production between the eastern and the western regions during summer. The seasonal variability of air–sea flux is controlled by biological pump in spring and summer, and vertical mixing to the mixed layer in winter along with the secondary effect of the Ekman upwelling which is stronger in winter. As advection plays an important role to determine the mean biogeochemical fields, it is necessary to use a three-dimensional model. Advection effects in the subpolar region are mainly due to the Ekman upwelling, increasing TCO2 by transporting the high TCO2 from the lower layer. The high TCO2 in the subpolar region is transported to the subtropical region by the southward Ekman flow. All three components are smaller in the subtropical region. In the mean balance, the biological pump is nearly canceled by advection effects in the subpolar region, and the minor residual transports atmospheric CO2 to the ocean. In the sensitivity studies, the zonal mean distributions of the annual mean carbon flux in the upper ocean (air–sea flux, biological pump and advection) are not changed. The uncertainties in the lower layer have significant effects on carbon cycle.