Søren Borg Nielsen

A thesis submitted for the degree of Doctor of Philosophy defended March 2019.

The PhD School of Science, Faculty of Science, Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen

Supervisor:
Markus Jochum

Oceanic Vertical Mixing, Marine Biogeochemistry and Atmospheric Carbon Dioxide

A key feature of the ocean circulation is the meridional overturning circulation, where warm surface waters flow from the tropics to high latitudes where they are cooled and sink. One of the key mechanisms for bringing deep water back to the surface is breaking internal waves that mix watermasses of different densities.

Despite its crucial importance to the ocean circulation, this process is often parameterized through diffusion of tracers using semi-empirical parameterizations of tidal induced mixing and a constant background diffusivity. While this method generally reproduces observed ocean circulation, issues arise from such parameterization: It is not energetically consistent, violating one of the key principles of physics, and it is tuned to match the modern day internal wave field, which is believed to have been significantly different in the past due to changes in wind and tidal forcing.
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 vertical diffusivity is important for the global carbon cycle through its impact on ocean solubility and nutrient cycling. Thus, the current use of semiempirical 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 parameterization. With the new parameterization the thermocline gets sharper and shallower, a result of too little dissipation in the thermocline compared to observations, 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, investigating 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 productivity and a global ocean release of carbon to the atmosphere. The latter is in part caused by 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 vertical mixing is of key importance to the global carbon cycle. The current paradigm for parameterizing vertical mixing results in the ocean stratification being less sensitive 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 understood, and caution must be exercised when using constant background diffusivities in general circulation and Earth System models.