The response of the central Arctic Ocean stratification to changes in the freshwater input: a dynamically-constrained affair?

Using a state-of-the-art coupled ice-ocean circulation model, we perform a number of sensitivity experiments to examine how the central Arctic Ocean stratification responds to changes in the river runoff and precipitation. The simulations yield marked changes both in the upper-ocean salinity stratification and the temperatures in the Atlantic water layer as the freshwater supply is varied. Increased precipitation yields a warming of the Arctic Atlantic layer, which is primarily caused by changes of the sea-ice cover and air--sea heat flux over the Barents Sea that affect the temperature of the Atlantic water entering the central Arctic Ocean through St. Anna Trough. As the freshwater supply is increased, the anti-cyclonic Beaufort Gyre is weakened and a greater proportion of the Arctic Ocean freshwater input is exported via the Fram Strait, with nearly compensating decreases of the export through the Canadian Archipelago. The corresponding changes in freshwater storage tend to be controlled by advective processes, rather than by the local changes in the net surface freshwater flux. A simple conceptual model of the Arctic Ocean, based on a geostrophically-controlled discharge of the low salinity water, is introduced. A key prediction of the conceptual model is that the Arctic Ocean freshwater storage should increase with the square root of the freshwater supply. However, the rate of increase of the freshwater storage in the model simulations is weaker, in particular for high freshwater inputs. This difference stems partly from reorganizations of the Fram Strait flow structure in the simulations that are not accounted for in the simple model.