On freshwater and the density-driven circulation in the northern seas

The Arctic Ocean and the Nordic Seas are freshening, in part due to anthropogenic climate change. Within the northern seas, seawater density is in part dominated by temperature, and in part by salinity. This reflects on the density-driven circulation in the northern seas which consists of two coupled branches of circulation: an overturning branch dominated by temperature, and an estuarine branch dominated by salinity. In order to assess how observed and projected increase in freshwater input into the northern seas can change this circulation, a better understanding of its governing dynamics is required. Using a hierarchy of theory and models, this dissertation studies how freshwater impacts the strength, stability, and variability of the density-driven circulation in the northern seas. Whereas freshwater is commonly perceived as suppressing a temperaturedominated circulation, models of various complexity indicate that increased freshwater input into low-salinity surface waters can spin up the salinity-dominated circulation. This results in a freshwater-induced increase in the poleward transport of Atlantic Water and heat. The coupling of the two circulation branches stabilises the density-driven circulation with respect to perturbations in freshwater, and abrupt transitions in the temperature-dominated circulation can only be induced by sufficient, localized freshwater input into dense surface waters. Finally, the transient response of the temperatureand salinity-modification of AtlanticWater in the Arctic Ocean to perturbations in Arctic river runoff is quantified; this indicates a potential predictability of regional processes at monthly resolution. This dissertation describes the unique behaviour of coupled temperature- and salinitydominated branches of density-driven ocean circulation. It reveals a glimpse of the largely unexplored potential for interaction between subtropical and polar ocean circulation. For the northern seas, this interaction translates into freshwater-sensitivity of the density-driven circulation that appears relatively weak and less dominant than what is commonly understood. As this circulation is integrated in the large-scale ocean circulation beyond the northern seas, these findings would have implications for the sensitivity of the global ocean circulation to future perturbations in high-latitude freshwater input