PhD defense by Søren Borg Nielsen

Title: Oceanic Vertical Mixing, Marine Biogeochemistry and Atmospheric Carbon Dioxide

Abstract: The ocean holds multiple times as much carbon as the atmosphere, and is therefore of key importance to the carbon cycle. Previous studies have shown that the oceanic vertical diffusivity is important for the global carbon cycle through its impact on ocean solubility and nutrient cycling. Thus, the current use of semi- empirical parameterizations that are tuned to present day climate may damp the ocean carbon cycle response to climate change.

A recently proposed parameterization for mixing induced by breaking internal waves is implemented in a general circulation model. The first set of simulations compares the climate simulated using the new and the existing semi-empirical pa- rameterization. With the new parameterization the thermocline gets sharper and shallower, a result of too little dissipation in the thermocline compared to obser- vations, and the overturning circulation is weaker and more sensitive to changes in Southern Ocean wind stress.

In the next set of simulations, two simulations using the new, energetically consistent, stratification-dependent mixing parameterization are performed, inves- tigating the response of the carbon cycle to a collapse of the Atlantic Meridional Overturning Circulation. The global carbon cycle responds on two timescales, a centennial terrestrial release of carbon to the atmosphere, and a slow, centennial to millennial ocean outgassing. The terrestrial release is related to a southward migration of the tropical precipitation, the ocean response is due to a reduced pro- ductivity and a global ocean release of carbon to the atmosphere. The overturning collapse leads to increased oceanic stratification, particularly in the Atlantic, which reduces the diffusive transport of nutrients to the surface ocean. This adds to the surface nutrient depletion caused by the increased North Atlantic freshening, which shoals the winter mixed layer that normally brings nutrients from the abyss to the surface ocean.

As a result it is demonstrated that, in line with previous studies, oceanic verti- cal mixing is of key importance to the global carbon cycle. The current paradigm for parameterizing vertical mixing results in the ocean stratification being less sen- sitive to climate change. To fully grasp the impact it has on the carbon cycle, the response of ocean mixing to changes in ocean circulation must be better under- stood, and caution must be exercised when using constant background diffusivities in general circulation and Earth System models.

Prof. Markus Jochum, Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen
Assessment of Committee
Prof. Thomas Blunier, Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen
Prof. Fortunat Joos, Climate and Environmental Physics, Physics Institute, and Oeschger Center for Climate Change Research, University of Bern
Prof. Andreas Schmittner, College of Earth, Ocean, and Atmospheric Sciences, USA

If interested, please contact Søren Borg Nielsen (soeren.nielsen @ for a copy of the thesis.