The dynamics of the North Atlantic carbon cycle and its relation to the temperature of the winter mixed layer

Property—property relationships between oceanic carbon species and physical parameters have been used extensively to extrapolate sparse measurements to larger areas. However, the physical processes behind the applied relationships are not always described, implying that uncertainties are introduced to the method. In this study, it is shown that the dynamical features of the North Atlantic carbon cycle can be condensed into a simple framework by focusing on the properties of the winter mixed layer and the equilibrium concentration of the system. The change in the equilibrium concentration between the Sargasso Sea and the Nordic seas is about 250 μmol kg⁻¹, where 50 μmol kg⁻¹ is due to the corrections from the annual cycles in sea surface temperature and biological production. A theoretical framework that focuses on the relation between variables and the temperature of the winter mixed layer is presented and analyzed in some detail using results from a general circulation model. The derived model predicts that there is a tight relation between the total dissolved inorganic carbon concentration and the temperature in the winter mixed layer, and such relation can indeed be found from the TTO/NAS expedition data. Furthermore, the analysis shows that the system is relatively close to the equilibrium concentration in the Sargasso Sea but that the system is highly undersaturated in the Nordic seas. Further, the analysis shows that both the entrainment of carbon-rich deep water and the air—sea flux of CO₂ must be correctly accounted for to describe the system.