| Summary: | The Arctic Ocean plays a fundamental role in regulating Earth’s climate, and a changing Arctic will affect climate, weather, and life everywhere on the planet. Understanding the fundamental dynamics and mechanisms driving natural variability, and the effects of anthropogenic warming in the Arctic climate system is imperative to improve future climate predictions. Warm and saline Atlantic Water (AW) entering the region across the Greenland-Scotland Ridge is the primary heat source to the Arctic Ocean and plays an essential role in modulating the Arctic climate system. However, our knowledge is still insufficient to make skillful projections of future Arctic climate change with uncertainty levels similar to other regions. This thesis improves our understanding of the role of AW in the Arctic Ocean, focusing primarily on: its variations in the twentieth and twenty-first centuries; the underlying mechanisms governing this variability; and its proliferating regional impacts on sea ice, marine-terminating glaciers, and stratification. First, we investigate the twentieth-century variability of AW heat transport through the gates of the Arctic Ocean. The analysis is based on a simulation from the global ocean-ice Norwegian Earth System Model (NorESM) supported by an extensive set of hydrographic observations dating back to 1900. We quantify prominent variability in both AW temperature and volume transport on near-decadal time scales, as well as significant positive trends in the most recent decades. Variations in volume transport were found to be linked to the wind forcing in the Nordic Seas and Subtropical North Atlantic, as manifested through the North Atlantic Oscillation, although the correlation is not constant over time and breaks down entirely in specific periods, such as the Early Twentieth Century Warming period. Variations in temperature are a combination of advected signals originating upstream and variations in atmospheric cooling over the Nordic Seas, which effectively dampen the AW heat anomalies along ...
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