Development and Assessment of NEMO(v3.6)-TOPAZ(v2), a Coupled Global Ocean Biogeochemistry Model

Earth System Models (ESMs) simulating the interrelationship between atmospheric chemistry, ocean biogeochemistry, terrestrial ecology, and climate processes are used to understand current climate and predict future climate change. However, ocean biogeochemical results show wide variability between ESMs. We have implemented the Tracers of Phytoplankton with Allometric Zooplankton (TOPAZ) ocean biogeochemistry model into the National Institute of Meteorological Sciences ESM. The offline version (Nucleus for European Modelling of the Ocean – Tracers of Ocean Phytoplankton with Allometric Zooplankton v2 (NEMO-TOPAZ) of the coupled global ocean biogeochemistry model has been evaluated compared to both observational data and another biogeochemistry model (NEMO-Pelagic Interactions Scheme for Carbon and Ecosystem Studies volume 2 [PISCES]) with the same ocean physics model. Biogeochemical tracers simulated by these models showed horizontal and vertical spatial distributions similar to observations. However, limitations caused by the shared ocean physical model were found in both models. While NEMO-TOPAZ tended to overestimate surface chlorophyll and nutrients, variation of simulated equatorial surface chlorophyll has a significant relationship with the El Niño-Southern Oscillation (ENSO) consistent with the observational result. NEMO-TOPAZ achieved superior simulation of dissolved inorganic carbon and alkalinity along with vertical distributions of biogeochemical variables in the Pacific and Atlantic Oceans. For nutrients, NEMO-PISCES showed better results overall. This model will improve scientific understanding of ocean biogeochemical processes and can be used in combination with other models for other components of the Earth’s system to develop a new ESM.