Transport of inorganic carbon through, within, and below the ocean surface

Human emissions of CO2 through the burning of fossil fuel, cement production, and land use change have increased the atmospheric CO2 concentration by 39 % since 1750. Without the large uptake of anthropogenic CO2 by the ocean and land the atmospheric CO2 increase would have been much larger. The focus of this thesis is the ocean carbon sink in the North Atlantic and the Nordic Seas, and how this responds to transport of carbon. PAPER I deals with transport of carbon through the atmosphere and ocean boundary layer. Using the eddy covariance method, fluxes of CO2 were measured in the Greenland Sea, and this flux data set was used to test and verify a recently published correction method for the CO2-H2O crosstalk problems. After successful correction the average flux was in accord with calculations using parameterizations of the transfer velocity, showing that directly measuring correct CO2 fluxes over the ocean is possible. PAPER II presents a large-scale study of the distribution of the CO2 fugacity (fCO2) within the North Atlantic surface ocean, and its relationship with sea surface temperature and surface ocean circulation. The key finding is that the circulation variations driven by the North Atlantic Oscillation (NAO) impacts the large-scale distribution of fCO2, and that in most regions of the North Atlantic this can explain the recent decreasing trend in the carbon sink. The link between NAO and fCO2 also provides some evidence of reversibility of the recent changes in the North Atlantic carbon sink. PAPERS III and IV focus on the transport of carbon below the surface ocean. PAPER IV presents a carbon budget for the Nordic Seas, using the mean state of volume and carbon fluxes into and out of this region. PAPER III uses the carbon tracer C* to study the processes of vertical mixing and water mass transformations in the Nordic Seas and North Atlantic Subpolar Gyre. C* is found to be a particularly good tracer for Nordic Seas’ overflow water. PAPERS III and IV present results that have impact on how we understand the global ocean carbon uptake. Combined, the four papers in this thesis increase our understanding of the recent trends in the North Atlantic carbon sink, as well as the variability of this carbon sink. In addition, these papers are a contribution to our understanding of the possible future feedbacks on the global ocean carbon sink