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

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.

A cost-efficient method allowing to run a global OBGCM at high resolution and over climate timescales is introduced. The "online grid-coarsening algorithm" allows to compute biogeochemical processes at a coarse resolution (0.75°) while inheriting most of the dynamical characteristics of an eddy-admitting ocean (0.25°). It is considered as cost-efficient as it reduces the computational cost by 60% with respect to that of a "classical" eddy-admitting solution (in which ocean dynamics and biogeochemistry are run at the same horizontal resolution). The key aspect of our study is to determine how close to an eddy-admitting solution is the coarse-grained solution and what it provides compared to a standard state-of-the-art 1° solution.

We demonstrate that the cost-efficient coarse-grained solution improves the representation of chlorophyll, nutrients, oxygen and sea-air carbon fluxes over more than half of the open ocean compared to the 1° solution. On top of that, we show that the coarse-grained ocean-biogeochemical solution captures the physical-biogeochemical coupling as simulated by the eddy-admitting solution (0.25°) whereas the low state-of-the-art solution (1°) does not. The existence of this physical-biogeochemical coupling is highlighted over two major biogeochemical regions: between sea-air carbon fluxes and sea surface height over the Southern Ocean, and between oxygen minimum zone boundaries and eddies over the East Pacific.