Analysis of tracer transport in the Atlantic Ocean with application in the uptake of anthropogenic carbon dioxide

This thesis addresses topics in ocean tracer transport, applications of tracer transport to ocean carbon uptake, and the role of ocean carbon uptake in the broader perturbed carbon cycle. First, an observationally-based indirect method to infer the uptake of anthropogenic carbon, Cant, by the oceans is used to reconstruct the time evolution of carbon in the Labrador Sea (LS) and subtropical mode waters (STMW), two important water masses in the North-Atlantic (NA). It is found that pre-industrial LS was neutral or a weak source of CO2 to the atmosphere. Furthermore, it is confirmed that LS is currently one of the major sinks of carbon in the ocean, but it is also found that the degree of saturation of surface LS with respect to concurrent atmospheric pCO2 is much lower than in previous studies. In contrast, STMW in the NA are found to be in approximate equilibrium with atmospheric pCO2. Second, the ocean carbon uptake analysis is expanded globally and a terrestrial biosphere component is added, resulting in a simple global carbon-cycle model. The model is then used to analyze the fraction of anthropogenic carbon emissions that remains airborne. The airborne fraction is found to depend sensitively on the variation in time of the emission rate and is therefore not a fundamental property of the carbon cycle, as has sometimes been assumed. Finally, idealized models of Deep Western Boundary Current transport are used to determine whether observed CFC fluctuations in the tropical NA can be related to either CFC fluctuations in northern source regions in the presence of steady transport or to decadal transport variability. In these models the dominant mechanism for CFC fluctuations close to the source is propagation of fluctuations at the source-region. This mechanism is negligible in the tropics compared to fluctuations induced locally by transport variability. This contradicts the common interpretation of tropical tracer fluctuations as being transported from northern source regions. However, neither mechanism in these models is able to generate CFC fluctuations as large as those observed.