Consequences of Future Increased Arctic Runoff on Arctic Ocean Stratification, Circulation, and Sea Ice Cover

The Arctic sea ice cover depends strongly on the stratification of the underlying ocean, which itself depends on processes over a range of spatial scales, from circulation to shelf processes and vertical mixing. Also important for the stratification are the freshwater sources to the Arctic Ocean, including river runoff, low evaporation, and exchange with the Pacific Ocean. In the future, we expect even larger freshwater input to the Arctic Ocean as the global hydrological cycle accelerates, increasing high latitude precipitation and river runoff. Previous modelling studies show some robust responses to high latitude freshwater perturbations, including a strengthening of Arctic stratification and a weakening of the large-scale ocean circulation; some idealized modelling studies also document a stronger cyclonic circulation within the Arctic Ocean itself. Here, we adopt a more comprehensive modelling approach to better understand both the local processes and the broader linkages between the Arctic and surrounding oceans. We increase river runoff to the Arctic Ocean in a coupled ice--ocean general circulation model, and show contrasting responses in the polar and subpolar regions. Within the Arctic, the stratification strengthens, the halocline and Atlantic Water layer warm, and the cyclonic circulation spins up, in agreement with previous work. In the subpolar gyre region of the North Atlantic, the model simulates a colder and fresher water column with weaker barotropic circulation. In contrast to the estuarine circulation theory, the volume exchange between the Arctic Ocean and the surrounding oceans does not increase with increasing runoff. While these results are robust in our model, we require experiments with other model systems and more complete observational syntheses to better constrain the sensitivity of the climate system to high latitude freshwater perturbations.