| Summary: | The first part of my dissertation focusses on the intermittent occurrence of Open Ocean Polynyas (OOPs) in a 500-year-long High-Resolution Pre-Industrial (HR-PI) Community Earth System Model 1.3 simulation (Chapter 3). During the winter season, the near-surface salinity stratification is found to be a key condition for the intermittent occurrence of the OOPs. Increased/decreased stratification, resulting from strong/weak freshwater fluxes at the surface will hamper/favor the formation of polynyas. The surface freshwater flux varies with a regional Southern Annular Mode (SAM)-like index (measured over a longitudinal section instead of circumpolar) and the associated meridional shift of the precipitation-rich westerly winds. Based on this HR-PI simulation, I detected a new possible regional ocean-atmosphere coupled mechanism that explains both the intermittent occurrence of OOPs and the simultaneous change of the regional SAM index. When large Weddell Sea Polynya (WSP) emerge, they affect the regional atmospheric sea-level pressure, thereby feeding back onto the regional SAM index. The initiation of polynya events is controlled by changes in surface properties while the location of initiation is determined by bathymetric features. The second part of my dissertation deals with the anthropogenic impact on the formation of WSPs and open ocean deep convection in an accompanying 250-year HR historical and future Transient (HR-TN) simulation (Chapter 4). In HR-PI, the (regional) SAM index does not have a clear trend, and only oscillates around its mean value. This provides a suitable environment for studying the intermittent occurrence of OOPs. In HR-TN, the anthropogenic impact forces the (regional) SAM index to become more positive. The associated poleward movement of the precipitation-rich Southern Hemisphere westerlies brings more freshwater and heat to the Weddell Sea region. At the same time, less sea ice forms due to the higher air temperature. These changes increase the stratification in the Weddell Sea, which ...
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