Abstract
Abstract. Idealised and hindcast simulations performed with the stand-alone ocean carbon-cycle configuration of the Norwegian Earth System Model (NorESM-OC) are described and evaluated. We present simulation results of three different model configurations (two different model versions at different grid resolutions) using two different atmospheric forcing data sets. Model version NorESM-OC1 corresponds to the version that is included in the NorESM-ME1 fully coupled model, which participated in CMIP5. The main update between NorESM-OC1 and NorESM-OC1.2 is the addition of two new options for the treatment of sinking particles. We find that using a constant sinking speed, which has been the standard in NorESM's ocean carbon cycle module HAMOCC (HAMburg Ocean Carbon Cycle model), does not transport enough particulate organic carbon (POC) into the deep ocean below approximately 2000 m depth. The two newly implemented parameterisations, a particle aggregation scheme with prognostic sinking speed, and a simpler scheme that uses a linear increase in the sinking speed with depth, provide better agreement with observed POC fluxes. Additionally, reduced deep ocean biases of oxygen and remineralised phosphate indicate a better performance of the new parameterisations. For model version 1.2, a re-tuning of the ecosystem parameterisation has been performed, which (i) reduces previously too high primary production at high latitudes, (ii) consequently improves model results for surface nutrients, and (iii) reduces alkalinity and dissolved inorganic carbon biases at low latitudes. We use hindcast simulations with prescribed observed and constant (pre-industrial) atmospheric CO2 concentrations to derive the past and contemporary ocean carbon sink. For the period 1990–1999 we find an average ocean carbon uptake ranging from 2.01 to 2.58 Pg C yr−1 depending on model version, grid resolution, and atmospheric forcing data set.
Highlights
Earth system models (ESMs) have been developed to take into account feedbacks between the physical climate and biogeochemical processes in projections of climate change (Bretherton, 1985; Flato, 2011)
Aside from the absolute values we find similar curves of Atlantic Meridional Overturning Circulation (AMOC) strength under the forcing protocol applied here (Fig. 3, right panel) compared to the results presented in Danabasoglu et al (2014): first a 10- to 15-year decrease by 2 to 4 Sv followed by a relatively stable phase until the early 1980s, an increase by 4 to 7 Sv towards a maximum in the late 1990s, and another decrease until the end of the simulation period (2007 or 2014)
Model version 1.2 has a stronger AMOC when simulations are forced with the CORE-IAF, and we find a larger accumulation of DICant in the North Atlantic for this forcing, consistent with the larger uptake flux (Fig. 20)
Summary
Earth system models (ESMs) have been developed to take into account feedbacks between the physical climate and biogeochemical processes in projections of climate change (Bretherton, 1985; Flato, 2011). Due to the complexity of feedback processes, it can prove useful to run one or several submodels of an ESM independently by using prescribed data at the boundary between submodel domains, e.g. by using prescribed atmospheric conditions at the air–sea boundary to force the ocean and ice models of an ESM. Such “stand-alone” model configurations are useful for conducting idealised experiments, for performing hindcast simulations in which boundary conditions reflect the observed variability and trends, or for saving computer time in cases where certain feedbacks are not expected to be important. Schwinger et al.: NorESM-OC force the stand-alone configuration of the same or another model
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