It has been shown that the diapycnal diffusion is one of the processes controlling the strength of the meridional overturning circulation (MOC). Most of the current climate models use parameterizations for the diapycnal diffusion that are independent of the model state. However, measurements indicate that local stratification has a strong influence on diapycnal diffusion.

In this thesis, the effect of different diffusion parameterizations on the stability of the Atlantic MOC is investigated by using both a simple three-box model of the Atlantic Ocean and a fully coupled global ocean-atmosphere model.

Two types of experiments are conducted:

First, an anomalous freshwater flux is applied to the high northern latitudes of the Atlantic Ocean. A feedback between stratification and diffusion anomalies is identified that has the potential to significantly alter the response of the Atlantic MOC to perturbations in the freshwater forcing. The sensitivity of the Atlantic MOC to the freshwater input is sharply increased, if the coupling between diffusion and stratification is larger than a certain threshold.

Secondly, the response of the Atlantic MOC to a gradual increase of the atmospheric CO2 concentration is investigated. Again, a threshold in diffusion sensitivity is identified that leads to an increased sensitivity of the Atlantic MOC to changes in the atmospheric forcing.

These results, if proven to be robust, could have several implications for the understanding of the climate system. They indicate that current state-of-the-art climate models potentially underestimate the changes in the Atlantic circulation that might be caused by the changed heat and freshwater fluxes associated with increased atmospheric CO2 concentrations. In addition, the results indicate that changes in the mechanisms responsible for ocean diapycnal diffusion, such as winds and tides, might explain changes in the sensitivity of the Atlantic MOC as observed in records of paleoclimate variability.

More information is available at http://www.marzeion.info, or by contacting me directly at ben.marzeion@nersc.no