An Offline Biogeochemical Model within the Regional Ocean Modelling System (ROMS): application to the Northwestern Mediterranean Sea

Modelling the distribution of biogeochemical components in the ocean is essential for further understanding climate change impacts and assessing the functioning of marine ecosystems. This requires robust and efficient physical-biological simulations of coupled ocean-ecosystem models, which are often hindered by limited data availability and computational resources. The option of running biological tracer fields offline, independently from the physical ocean simulation, is appealing due to increased computational efficiency. Here, we present an assessment and implementation of an offline biogeochemical model — the Offline Fennel model — within the Regional Ocean Modeling System (ROMS). Our methodology employs ROMS hydrodynamic outputs to run the biogeochemical model offline. This work also includes the first evaluation exercise of the referred offline biogeochemical model. We used a variety of skill metrics to compare the simulated surface chlorophyll to an ocean colour dataset (CMEMS-Mediterranean Ocean Colour) and BGC-ARGO floats for the 2015-2020 period. The model is able to reproduce the temporal and spatial structures of the main chlorophyll fluctuation patterns in the study area, the Northwestern Mediterranean Sea, as well as the vertical distribution of chlorophyll and nitrate. This area is of particular interest as it is one of the most productive regions in the entire Mediterranean Basin, with open-ocean upwellings and deep winter convection events occurring seasonally. The typical behaviour of the region is likewise effectively represented in the implementation, including offshore primary production, nutrient supplies from the Rhone and Ebro rivers, and mesoscale hydrographic structures. This study provides a baseline for ROMS users in need of executing more biogeochemical simulations independently from more computationally demanding physical simulations.