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.
