A Conceptual Model of Polar Overturning Circulation

The global ocean overturning circulation is transformed in high latitudes, both in the Arctic mediterranean and the waters around Antarctica. The circulations in these high latitude termini share common features: Warm, relatively salty water flows in from lower latitudes (Atlantic Water in the Arctic and Circumpolar Deep Water in the Antarctic). Interaction with the atmosphere, and to a lesser degree with the land, transforms this inflow into three equator-ward outflow components: sea ice; buoyant fresh, freezing surface water; and dense deep (or abyssal) cold water. Understanding the controls on the properties and fluxes of these three outflows is important for oceanography and climate science. Here we present a conceptual physical model that specifies the outflow characteristics given information on the inflow and air/sea/land fluxes of heat and freshwater. The model includes budgets of mass, heat, and salt for the high latitude deep water and the continental shelves. Part of the warm salty inflow is cooled and freshened by air/sea/ice interaction to form the buoyant surface water. The remainder penetrates the shelf, where it cools, partly freezes, and thus transforms to dense saline shelf water. This dense shelf water overflows back into the deep water entraining ambient water and thus forms the dense cold part of the outflow. The model results compare well to observations in the Arctic and the Antarctic. We establish the sensitivity of the outflow characteristics to the inflow properties and air/sea/land fluxes. The principal mechanisms controlling the outflow in the conceptual model are elucidated.