| Summary: | The Arctic Ocean remains one of the least known ocean regions due to its remote location, year-round sea ice cover, and harsh weather conditions. Today, knowledge of the Arctic atmosphere-sea ice-ocean system increases in parallel with the need to understand this changing environment under anthropogenic greenhouse warming. Indeed, the Arctic is an area where the effects of anthropogenic greenhouse warming are already visible and among the strongest on Earth; the atmosphere and the ocean are warming, the sea ice cover is diminishing, and the freshwater input to the ocean through river inflow, precipitation, and ice melt is increasing. It is this rapidly changing, but poorly understood, Arctic climate system that motivates us to study its fate in a fresh and warm future. Our objective is to assess how the ocean circulation, the ocean heat content, and the ice cover respond to increasing freshwater input and overall greenhouse warming. We also ask whether changes in the ocean affect the atmosphere, i.e., is the atmospheric surface warming modified by the changing ocean? We choose to seek answers to these questions with a hierarchy of model simulations. We focus on the North Atlantic-Arctic sector and examine changes in the ocean circulation and ocean heat content under greenhouse warming. We use idealized model simulations to assess changes in freshwater forcing, and global climate model simulations to examine the changing ocean heat budget. With this hierarchy of models we build a comprehensive understanding of the changing high latitude climate system and compile this dissertation around three main scientific findings. First, we increase the Arctic river runoff in an idealized column model which represents the large scale average conditions of the Arctic ocean-sea ice-atmosphere system. A larger Arctic river runoff leads to a new equilibrium with a fresher surface and a warmer subsurface. Interestingly, even though the fresher surface leads to larger vertical density differences and suppresses vertical mixing, ...
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