Abstract

Abstract. Global biogeochemical ocean models are often tuned to match the observed distributions and fluxes of inorganic and organic quantities. This tuning is typically carried out “by hand”. However, this rather subjective approach might not yield the best fit to observations, is closely linked to the circulation employed and is thus influenced by its specific features and even its faults. We here investigate the effect of model tuning, via objective optimisation, of one biogeochemical model of intermediate complexity when simulated in five different offline circulations. For each circulation, three of six model parameters have been adjusted to characteristic features of the respective circulation. The values of these three parameters – namely, the oxygen utilisation of remineralisation, the particle flux parameter and potential nitrogen fixation rate – correlate significantly with deep mixing and ideal age of North Atlantic Deep Water (NADW) and the outcrop area of Antarctic Intermediate Waters (AAIW) and Subantarctic Mode Water (SAMW) in the Southern Ocean. The clear relationship between these parameters and circulation characteristics, which can be easily diagnosed from global models, can provide guidance when tuning global biogeochemistry within any new circulation model. The results from 20 global cross-validation experiments show that parameter sets optimised for a specific circulation can be transferred between similar circulations without losing too much of the model's fit to observed quantities. When compared to model intercomparisons of subjectively tuned, global coupled biogeochemistry–circulation models, each with different circulation and/or biogeochemistry, our results show a much lower range of oxygen inventory, oxygen minimum zone (OMZ) volume and global biogeochemical fluxes. Export production depends to a large extent on the circulation applied, while deep particle flux is mostly determined by the particle flux parameter. Oxygen inventory, OMZ volume, primary production and fixed-nitrogen turnover depend more or less equally on both factors, with OMZ volume showing the highest sensitivity, and residual variability. These results show a beneficial effect of optimisation, even when a biogeochemical model is first optimised in a relatively coarse circulation and then transferred to a different finer-resolution circulation model.

Highlights

  • Global models of marine biogeochemistry are applied to prognostic problems, such as the future exchange of CO2 between the ocean and atmosphere, the evolution of oxygen minimum zones (OMZs) under a changing climate or future primary production, which is the ultimate food source for fish

  • When compared to model intercomparisons of subjectively tuned, global coupled biogeochemistry–circulation models, each with different circulation and/or biogeochemistry, our results show a much lower range of oxygen inventory, oxygen minimum zone (OMZ) volume and global biogeochemical fluxes

  • The misfit decreases with more realistic circulation and physics and is lowest for ECCO∗

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Summary

Introduction

Global models of marine biogeochemistry are applied to prognostic problems, such as the future exchange of CO2 between the ocean and atmosphere, the evolution of oxygen minimum zones (OMZs) under a changing climate or future primary production, which is the ultimate food source for fish. The largest uncertainty is related to OMZ volume – here Bopp et al (2013) report a range of variation that is several times the observed volume, depending on the criterion (maximum oxygen concentration) used for OMZ definition Because these coupled models differ in both their physical and biogeochemical setups, to date the contribution of the different model components to this large variation is not clear (e.g. Cabre et al, 2015). On the other hand, Kriest et al (2012) and Kriest and Oschlies (2015) showed that the impact of biogeochemical model structure and parameters on deep oxygen profiles can be large; in the latter study OMZ volume varied among different biogeochemical model setups up to 3 times the observed value (for an OMZ criterion of 8 mmol O2 m−3). Neither oxygen content nor OMZ volume seems well constrained, possibly because of both circulation and biogeochemistry

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