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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 9 | Copyright

Special issue: The 10th International Carbon Dioxide Conference (ICDC10)...

Biogeosciences, 15, 2851-2872, 2018
https://doi.org/10.5194/bg-15-2851-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 May 2018

Research article | 15 May 2018

The seasonal cycle of pCO2 and CO2 fluxes in the Southern Ocean: diagnosing anomalies in CMIP5 Earth system models

N. Precious Mongwe1,2, Marcello Vichi2,3, and Pedro M. S. Monteiro1,2 N. Precious Mongwe et al.
  • 1Southern Ocean Carbon-Climate Observatory (SOCCO), CSIR, Cape Town, South Africa
  • 2Department of Oceanography, University of Cape Town, Cape Town, South Africa
  • 3Marine Research Institute, University of Cape Town, Cape Town, South Africa

Abstract. The Southern Ocean forms an important component of the Earth system as a major sink of CO2 and heat. Recent studies based on the Coupled Model Intercomparison Project version 5 (CMIP5) Earth system models (ESMs) show that CMIP5 models disagree on the phasing of the seasonal cycle of the CO2 flux (FCO2) and compare poorly with available observation products for the Southern Ocean. Because the seasonal cycle is the dominant mode of CO2 variability in the Southern Ocean, its simulation is a rigorous test for models and their long-term projections. Here we examine the competing roles of temperature and dissolved inorganic carbon (DIC) as drivers of the seasonal cycle of pCO2 in the Southern Ocean to explain the mechanistic basis for the seasonal biases in CMIP5 models. We find that despite significant differences in the spatial characteristics of the mean annual fluxes, the intra-model homogeneity in the seasonal cycle of FCO2 is greater than observational products. FCO2 biases in CMIP5 models can be grouped into two main categories, i.e., group-SST and group-DIC. Group-SST models show an exaggeration of the seasonal rates of change of sea surface temperature (SST) in autumn and spring during the cooling and warming peaks. These higher-than-observed rates of change of SST tip the control of the seasonal cycle of pCO2 and FCO2 towards SST and result in a divergence between the observed and modeled seasonal cycles, particularly in the Sub-Antarctic Zone. While almost all analyzed models (9 out of 10) show these SST-driven biases, 3 out of 10 (namely NorESM1-ME, HadGEM-ES and MPI-ESM, collectively the group-DIC models) compensate for the solubility bias because of their overly exaggerated primary production, such that biologically driven DIC changes mainly regulate the seasonal cycle of FCO2.

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Here we analyze seasonal cycle of CO2 biases in 10 CMIP5 models in the SO. We find two main model biases; exaggeration of primary production such that biologically driven DIC changes mainly regulates FCO2 variability, and an overestimation of the role of solubility, such that changes in temperature dominantly drive FCO2 seasonal changes to an extent of opposing biological CO2 uptake in spring. CMIP5 models show greater zonal homogeneity in the seasonal cycle of FCO2 than observational products.
Here we analyze seasonal cycle of CO2 biases in 10 CMIP5 models in the SO. We find two main...
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