| Summary: | Abrupt changes in Earth's climate have occurred repeatedly throughout the geological record. Evidence from paleoclimate data has revealed that climate changed most dramatically during the last glacial period, associated with the Dansgaard-Oeschger (D-O) events. These are characterized by large and rapid fluctuations in North Atlantic climate, with regional warming of up to 15 degrees C over Greenland, within a few decades. The main hypotheses for these abrupt climate changes in the past, centers around changes in the Atlantic Meridional Overturning Circulation (AMOC) and its influence on poleward ocean heat transport. Recently, the role of sea ice has also been recognized as a critical player for the D-O events; linking the abrupt changes in Greenland temperature to a retreat of Northern Hemisphere sea ice, driven by internal variability of the coupled ice-ocean and atmosphere system. However, the mechanism for triggering rapid changes in sea ice, and how it is linked to ocean circulation changes, remains elusive. This thesis focuses on the interaction between ocean circulation, sea ice and high-latitude climate in the context of abrupt climate changes in the past. The main goal is to improve our understanding of how internal dynamics of the coupled climate system can lead to rapid and unforced changes in climate. The first part of this thesis explores the mechanism behind abrupt changes in sea ice by studying the formation of open-ocean polynyas in the Southern Ocean. The second and third part, focuses on the dynamics of large-scale ocean circulation and its sensitivity to ocean bathymetry and the distribution of diapycnal mixing in the ocean interior. We find that open-ocean polynyas in the Southern Ocean provide a mechanism to trigger abrupt sea ice retreat, similar to that seen during the last glacial period. These events drive increased bottom water formation, thereby impacting the large-scale ocean circulation. The formation of the polynya is preconditioned by a gradual build-up of subsurface heat and salt ...
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