Master thesis defence by Laurits Støvring Andreasen

Title: Time scales of the Bipolar seesaw: The role of oceanic cross-hemisphere signals, Southern Ocean eddies and wind changes

The coupling between Dansgaard-Oescher events and Antarctic Isotope Maxi-mas, together known as the bipolar seesaw, is a prominent example of interhemispheric teleconnection in the earth system. The bipolar seesaw has been reproduced in coupled earth system models, but it is still not clear which physical mechanisms facilitate this. The coupled models find changes in both ocean thermal structure and the atmospheric circulation, and their interdependence is not clear. To gain insight in this interdependence, we perturb an ocean general circulation model with a com-bination of salinity perturbations and changes in atmospheric forcing. These forcing changes are inspired by changes found in the atmosphere in coupled models, that simulates the bipolar seesaw. We find that the response time of the ocean thermocline to the salinity perturbations is insensitive to the changes in atmospheric forcing.
We also find that the response time is insensitive to whether the salinity perturbation is fresh or salt, and to the magnitude of the eddy diffusion constant. A short integration of a high resolution setup and a scale analysis indicate time scales in the same range as the coarse resolution runs. This time scale - common for all diffusivites, the
high resolution run and the scale analysis - is an order of magnitude smaller then ice core data suggest. From this mismatch it is suggested that changes in sea ice distributions and not oceanic adjustments are of major importance for the time scales revealed by ice cores.