Evaluation of an online grid-coarsening algorithm in a global eddy-admitting ocean-biogeochemical model

Berthet, Sarah ; Séférian, Roland ; Bricaud, Clément ; Chevallier, Matthieu ; Voldoire, Aurore ; Ethé, Christian

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<p align="justify">In order to explore the effects of mesoscale eddies on marine biogeochemistry over climate timescales, global ocean biogeochemical general circulation models (OBGCMs) need at least to be run at a horizontal resolution of a 0.25°, the minimal resolution admitting eddies. However, their use is currently limited because of a prohibitive computational cost and storage requirements. To overcome this problem, an online coarsening algorithm is evaluated in the oceanic component (NEMO-GELATO-PISCES) of CNRM-ESM2-1. This algorithm allows to compute biogeochemical processes at a coarse resolution (0.75°) while inheriting most of the dynamical characteristics of the eddy-admitting OBGCM (0.25°). Through the coarse-graining process, the effective resolution of the ocean dynamics seen by the biogeochemical model is higher than that which would be obtained from an OBGCM run at 0.75°. In this context, we assess how much the increase from low (1°) to coarse-grained horizontal resolution impacts the ocean dynamics and the marine biogeochemistry over long-term climate simulations. The online coarsening reduces the computational cost by 60% with respect to that of the eddy-admitting OBGCM. In addition, it improves the representation of chlorophyll, nutrients, oxygen, and sea-air carbon fluxes over more than half of the open ocean area compared to the 1° OBGCM. Most importantly, the coarse-grained OBGCM captures the physical-biogeochemical coupling between sea-air carbon fluxes and sea surface height and between oxygen minimum zone boundaries and eddies, as produced by the eddy-admitting OBGCM. Such a cost-efficient coarsening algorithm offers a good trade-off to conduct process-based studies over centennial timescales at higher resolution.</p>
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