Abstract
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.
Highlights
The Southern Ocean takes up about a third of the total oceanic CO2 uptake, slowing down the accumulation of CO2 in the atmosphere (Fung et al, 2005; Le Quéré et al, 2016; Takahashi et al, 2012)
The annual mean climatological distribution of F CO2 in the Southern Ocean obtained from observational products is spatially variable, but mainly characterized by two key features: (i) CO2 in-gassing north of 50–55◦ S (Polar Frontal Zone, PFZ) within and north of the Sub-Antarctic Zone, and (ii) CO2 out-gassing between the PF (∼ 58◦ S) and the marginal ice zone (MIZ, ∼ 60–68◦ S) (Fig. 1a–b)
In this study we expand on the framework proposed by Mongwe et al (2016), which examined the competing roles of temperature and dissolved inorganic carbon (DIC) as drivers of pCO2 variability and the seasonal cycle of pCO2 in the Southern Ocean, to explain the mechanistic basis for seasonal biases of pCO2 and F CO2 between observational products and Coupled Model Intercomparison Project version 5 (CMIP5) models
Summary
The Southern Ocean (south of 30◦ S) takes up about a third of the total oceanic CO2 uptake, slowing down the accumulation of CO2 in the atmosphere (Fung et al, 2005; Le Quéré et al, 2016; Takahashi et al, 2012). Recent studies suggest that the Southern Ocean CO2 sink is expected to change as a result of anthropogenic warming; the sign and magnitude of the change is still disputed (Leung et al, 2015; Roy et al, 2011; Sarmiento et al, 1998; Segschneider and Bendtsen, 2013). While some studies suggest that the Southern Ocean CO2 sink is weakening and will continue to do so (e.g., Le Quéré et al, 2007; Son et al, 2010; Thompson et al, 2011), other recent studies infer an increasing CO2 sink (Landschützer et al, 2015; Takahashi et al, 2012; Zickfeld et al, 2008).
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