Exploring the role of different data types and timescales for the quality of marine biogeochemical model calibration

Abstract. Global biogeochemical ocean models help to investigate the present and potential future state of the ocean biogeochemistry, its productivity and cascading effects on higher trophic levels such as fish. They are often subjectively tuned against data sets of inorganic tracers and surface chlorophyll and only very rarely against organic components such as particulate organic carbon or zooplankton. The resulting uncertainty in biogeochemical model parameters (and parameterisations) associated with these components can explain some of the large spread of global model solutions with regard to the cycling of organic matter and its impacts on biogeochemical tracer distributions, such as oxygen minimum zones (OMZs). A second source of uncertainty arises from differences in the model spin-up length, as, so far, there seems to be no agreement on the required simulation time that should elapse before a global model is assessed against observations.

We investigated these two sources of uncertainty by optimising a global biogeochemical ocean model against the root-mean-squared error (RMSE) of six different combinations of data sets and different spin-up times. Besides nutrients and oxygen, the observational data sets also included phyto- and zooplankton, as well as dissolved and particulate organic phosphorus. We further analysed the optimised model performance with regard to global biogeochemical fluxes, oxygen inventory and OMZ volume.

The optimisations resulted in optimal model solutions that yield similar values of the RMSE of tracers mainly located in surface layers, showing a range of between 14 % of the average RMSE after 10 years and 24 % after 3000 years of simulation. Global biogeochemical fluxes, global oxygen bias and OMZ volume showed a much stronger divergence among the models and over time than RMSE, indicating that even models that are similar with regard to local surface tracer concentrations can perform very differently when assessed against the global diagnostics for oxygen. Considering organic tracers in the optimisation had a strong impact on the particle flux exponent ("Martin b") and may reduce much of the uncertainty in this parameter and the resulting deep particle flux. Independent of the optimisation setup, the OMZ volume showed a particularly sensitive response with strong trends over time even after 3000 years of simulation time (despite the constant physical forcing), a high sensitivity to simulation time, as well as the highest sensitivity to model parameters arising from the tuning strategy setup (variation of almost 80 % of the ensemble mean).

In conclusion, calibration against observations of organic tracers can help to improve global biogeochemical models even after short spin-up times; here, especially observations of deep particle flux could provide a powerful constraint. However, a large uncertainty remains with regard to global OMZ volume and its evolution over time, which can show a very dynamic behaviour during the model spin-up, that renders temporal extrapolation to a final 'equilibrium' state difficult, if non impossible. Given that the real ocean shows variations on many timescales, the assumption of observations representing a steady-state ocean may require some reconsideration.