The Arctic overturning circulation: transformations, pathways and timescales

Abstract. The Arctic is the northernmost terminus of the Atlantic Meridional Overturning Circulation and is an important source of the densest waters feeding its lower limb. However, relatively little is known about the structure and timescales of the Arctic overturning circulation, and which pathways contribute most to the transformation of Atlantic Waters into dense waters and Polar Waters. In this work, we combine a Eulerian water mass transformation framework and Lagrangian tracking to decompose the time-mean Arctic overturning circulation in an eddy-rich (1/12˚) global ocean hindcast (1979–2015). We show that the Atlantic Water branch through the Barents Sea dominates dense Arctic overturning, and that a large portion of these transformed waters takes many decades to exit Fram Strait. Furthermore, we show that surface forcing in the Barents Sea and north of Svalbard dominates dense overturning, but local subsurface mixing with shelf waters and between the two Atlantic Water branches plays an important role for the Fram Strait branch. Our work identifies the dominant processes of the Arctic overturning circulation, and contributes to understanding its future changes and their impact on the stability of northern overturning.