The Northwest Pacific is characterized by the coexistence of the warm, nutrient-depleted Kuroshio Current and the cold, nutrient-rich Oyashio Current. In this region, surface primary production enhances nutrient consumption and air–sea CO₂ exchange. The Yellow and East China Seas (YECS) are additionally strongly influenced by freshwater input. Because each region exhibits distinct physical and biogeochemical characteristics, a high-resolution regional numerical model tailored to these specific features is required. To accurately represent the physical and biogeochemical systems, this study developed a new coupled physical–biogeochemical model for the Northwest Pacific, including the YECS, by combining the three-dimensional Regional Ocean Modeling System (ROMS) with the Generic Ocean Turbulence Model Tracers of Phytoplankton with Allometric Zooplankton (TOPAZ). The simulated physical and biogeochemical variables from the ROMS–TOPAZ model were evaluated through comparison with available observational data.
ROMS–TOPAZ successfully reproduced the seasonal variability of chlorophyll and nutrients, capturing the spring and autumn peaks that were not represented in the CMIP6 data. In particular, in the YECS, ROMS–TOPAZ effectively simulated the high phytoplankton biomass driven by riverine inputs, which is difficult to reproduce in low-resolution global biogeochemical models. However, ROMS–TOPAZ still exhibits substantial biases in the subarctic region and marginal seas. To reduce uncertainties in biogeochemical variables, improvements in initial and boundary conditions, parameter calibration, and the application of observation-based discharge forcing are required.
Despite these limitations, ROMS–TOPAZ provides an important high-resolution framework for investigating interactions between ocean physics and biogeochemistry.

Northwest pacific ocean,Regional ocean modeling system (ROMS),TOPAZ biogeochemical model,ROMS-TOPAZ coupling model,performance evaluation