Time-series observations of optical fields in the euphotic layer and particle fluxes at 150 m were made in the western North Pacific for half year of 2005. The ratio of surface photosynthetically available radiation (surface PAR) to in situ quantum irradiance at ca. 40 m (in situ QI), as an index of turbidity, began to increase in the middle of April, peaking between the end of June and the middle of July. Seasonal variability in the ratio of spectral irradiance at a wavelength of 555 nm to that at 443 nm (Ed(555)/Ed(443)) at 40 m, as an index of chlorophyll abundance, synchronized well with the surface PAR/in situ QI ratio. Organic carbon flux also increased between the end of June and the middle of July and correlated well with optical variability. This result suggests that primary produced carbon in the euphotic layer was quickly transported to the ocean interior. Using the Ed(555)/Ed(443) and an empirical equation from shipboard observations, primary productivity was estimated to be ca. 300 mgC m-2 day-1 on average. Assuming that trapping efficiency was only 20% and organic carbon flux decreased drastically between 100 m and 150m, the export ratio at 100 m was estimated to be ca. 30 ¿ 10%, which is significantly higher than that in other oceans.