Large-Scale Ocean Circulation: Deep Circulation and Meridional Overturning

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Roughly half the world ocean volume lies below 2,000m depth. This deep half of the ocean is cold (<3°C), indicating that the abyssal ocean is filled with waters that sink in high latitudes, where cold surface waters are found [1]. The deep ocean circulation transports the cold waters that sink in the polar regions throughout the deep ocean basins. The transfer of surface water to the deep ocean must be balanced by an inflow of water in the upper ocean to the deep water formation regions, to conserve mass. The result is an “overturning circulation,” in which the export of cold deep waters from the source regions is balanced by a return flow of warmer water in the upper ocean. The large temperature contrast between the upper and lower limbs of the overturning circulation makes this flow pattern an efficient means of transporting heat. The large-scale overturning circulation is the primary means by which the ocean stores and transports quantities of relevance to the Earth’s climate system and biogeochemical cycles, including heat, freshwater, carbon, and nutrients. The evolution of climate is therefore influenced strongly by the overturning circulation.