The seasonal cycle of pCO(2) and CO2 fluxes in the Southern Ocean: diagnosing anomalies in CMIP5 Earth system models

Type Article
Date 2018-05
Language English
Author(s) Mongwe N. Precious1, 2, Vichi MarcelloORCID2, 3, Monteiro Pedro M. S.1, 2
Affiliation(s) 1 : CSIR, SOCCO, Cape Town, South Africa.
2 : Univ Cape Town, Dept Oceanog, Cape Town, South Africa.
3 : Univ Cape Town, Marine Res Inst, Cape Town, South Africa.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2018-05 , Vol. 15 , N. 9 , P. 2851-2872
DOI 10.5194/bg-15-2851-2018
WOS© Times Cited 29
Note Special issue | The 10th International Carbon Dioxide Conference (ICDC10) and the 19th WMO/IAEA Meeting on Carbon Dioxide, other Greenhouse Gases and Related Measurement Techniques (GGMT-2017) (AMT/ACP/BG/CP/ESD inter-journal SI) Editor(s): F. Joos, C. Heinze, C. Le Quere, J. Pongratz, I. C. Prentice, and N. Zeng Special issue jointly organized between Atmospheric Measurement Techniques, Atmospheric Chemistry and Physics, Biogeosciences, Climate of the Past, and Earth System Dynamics

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 pCO(2) 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 pCO(2) 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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