The Arctic Ocean has been covered by sea ice year-round for much of the past, inhibiting the transfer of momentum from atmosphere to ocean, with the consequence that Arctic Ocean currents are generally slow and turbulent mixing weak. However, recent decades have seen accelerated lower tropospheric warming accompanied by declines in sea ice concentration, thickness and extent, and more recently, changes in the ocean, termed ”atlantification”, are beginning to be observed. Against this background, here we explore the nature of the Arctic Ocean ”double estuary”, whereby (mainly) inflowing Atlantic-sourced waters are transformed into both lighter and denser components in a two-cell density-overturning circulation. The double estuary is quantified using measurements, and a box model is employed to determine the relative significance of surface forcing versus turbulent mixing to water mass transformation. We generate a net Arctic Ocean profile of turbulent diffusivity that is used to test the likely contribution of tides to mixing, and we find that the outcome is most sensitive to mixing efficiency. We note that Arctic Ocean dense water formation adds to the recognised sites of dense water formation in the Nordic Seas and northern North Atlantic. Finally, we discuss how mixing rates may change in future as sea ice declines and the efficiency of atmosphere-to-ocean momentum transfer increases, leading to ocean ”spin-up” and more intense turbulent mixing, and the possible consequences thereof.

For over 150 years, plans to divert Arctic Ocean-draining rivers southwards in order to relieve an ongoing water supply crisis in central Asia have been discussed. Recent insights have identified the importance of freshwater in regulating the role of heat associated with intruding intermediate depth Atlantic water in driving Arctic Ocean sea ice decline. Here we assess the potential impact of the redirection of the Ob’, Yenisey, Northern Dvina and Pechora rivers on upper ocean density structure, and by implication, the aerial sea ice extent. A simple 1D model is applied in which freshwater content of the upper ocean water column is reduced to mimic the diversion of the rivers, and the impact on water column stratification assessed. The results show that the impact is dependent on distribution of riverine freshwater in the upper water column. If the impact of reduced freshwater is spread through the entire water column, down to the Atlantic Water Layer, the level of stratification is reduced by an average of 28%, more than the seasonal variability in stratification. However, if the changes were limited to the surface layer, the resultant reduction in stratification is less, only 17%, but the direct entrainment of deeper, warmer waters is found to occur. At a time when climate change and population growth put increasing pressure on water resources, these results show the sensitivity of a region critical to global weather and climate to anthropogenic attempts to resolve water resource issues many thousands of kilometres away.