The circulation and water mass (trans)formations in the Arctic Mediterranean Sea and their impact  on the ocean deep circulation: a review

Abstract. The Arctic Mediterranean is rapidly changing, a statement that is often made, but the follow-up statements: changing from what and towards what are often omitted. Hence its role in the global climate system, particularly regarding the Atlantic Meridional Overturning Circulations remains poorly constrained. This review of the oceanography of the Arctic Mediterranean Sea develops an unified perspective of how interacting components of the system evolve in space and time, and the processes that determine their evolution. To set the stage a succinct overview of the geographic setting and early explorations is given. We then follow the pathways of the principal water masses to describe inflows, inter-basin circulations, water mass transformations, and outflows of heat and salt to the bordering subpolar gyre and the global ocean. The fundamental connection to the global ocean, the Atlantic water, is traced along its route into and through the Arctic Mediterranean. Its transformations, driven by cooling, and by the freezing and melting of sea ice, lead to the creation of both denser and less dense waters that form and maintain the water column structures within the Arctic Mediterranean; the so-called double estuary. A second advective component to the Arctic, the low salinity water carried by the Pacific inflow, is concentrated into the Amerasian Basin and acts there to further isolate the denser waters derived from the Atlantic inflow. Hence, the waters return to the North Atlantic either as dense overflows or buoyant outflows. The water masses within diverse regions of the Arctic Mediterranean have changed over the past few decades, and this influences their exchanges with the world ocean across the Greenland-Scotland Ridge. That the water mass transformations in the Arctic Mediterranean take place beyond a ridge allows for the build-up of significant density differences that, through entrainment, can increase the impact of the Arctic Mediterranean on global overturning circulation.