The seasonal cycle of p CO 2 and CO 2 fluxes in the Southern Ocean: diagnosing anomalies in CMIP5 Earth system models

• Published in: Biogeosciences
• Main Authors: N. P. Mongwe, M. Vichi, P. M. S. Monteiro
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2018
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Antarctic; Southern Ocean; Antarc*
• Online Access: https://doi.org/10.5194/bg-15-2851-2018; https://doaj.org/article/d3848d43dd204e038c4b496439659c54

The Southern Ocean forms an important component of the Earth system as a major sink of CO 2 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 CO 2 flux ( F CO 2 ) and compare poorly with available observation products for the Southern Ocean. Because the seasonal cycle is the dominant mode of CO 2 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 p CO 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 F CO 2 is greater than observational products. F CO 2 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 p CO 2 and F CO 2 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 F CO 2 .