Development and assessment of the physical-biogeochemical ocean regional model in the Northwest Pacific: NPRT v1.0 (ROMS v3.9–TOPAZ v2.0)

Abstract. The biogeochemical cycling system exhibits diverse characteristics in different regions owing to various factors. The Northwest Pacific is characterized by the presence of the warm and nutrient-depleted Kuroshio Current and the cold and nutrient-enriched Oyashio Current. In this region, surface primary production leads to increased nutrient consumption and CO₂ exchange. The Yellow and East China Seas (YECS) are predominantly influenced by freshwater input. A high resolution regional numerical model tailored to the specific features of each area is required to reproduce the different characteristics of each region. Therefore, to accurately analyze the physical and biogeochemical system, this study developed a new coupled physical-biogeochemical model combining the three-dimensional Regional Ocean Modeling System (ROMS) and the Generic Ocean Turbulence Model Tracers of Phytoplankton with Allometric Zooplankton (TOPAZ) for the Northwest Pacific, including the YECS. The simulated physical and biogeochemical variables in the ROMS–TOPAZ (NPRT) were evaluated by comparing them with available observational data. The spatial correlation ranges of the various variables reproduced in the NPRT were 0.5–0.7. In the upper layer (0–20 m), NPRT successfully simulated the seasonal variability of chlorophyll, capturing two peaks in spring and summer, which were not captured by the CMIP6 data. Particularly in the YECS, NPRT effectively represented the biomass driven by riverine effect, which is difficult to reproduce in global biogeochemical model with low-resolution. However, NPRT still exhibits significant biases in the subarctic region and marginal seas. To minimize the uncertainties in biogeochemical variables, it is necessary to refine the initial and boundary conditions, adjust parameters, and apply discharge forcing based on observational data. Despite these limitations, NPRT is an important tool for studying the interaction between ocean physics and biogeochemistry at a high resolution.