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Ocean Circulation: Thermohaline Circulation

J. R. Toggweiler, Geophysical Fluid Dynamics Laboratory, NOAA, Princeton, NJ 08542

Robert M. Key, Atmospheric and Oceanic Sciences Program, Department of Geosciences, Princeton University, Princeton, NJ 08540

The circulation of the ocean is usually divided into two parts, a wind-driven circulation that dominates in the upper few hundred meters, and a density-driven circulation that dominates below. The latter is called the ‘thermohaline’ circulation because of the role of heating, cool- ing, freshening, and salinification in producing regional density differences within the ocean. The thermohaline circulation, for the most part, is an ‘overturning’ circulation in which warm water flows poleward near the surface and is subsequently converted into cold water that sinks and flows equatorward in the in- terior. Radiocarbon measurements show that the thermohaline circulation turns over all the deep water in the ocean every 600 years or so.

The most spectacular features of the ther- mohaline circulation are seen in the sinking phase, in the formation of new deep water in the North Atlantic and the Southern Ocean. Large volumes of cold polar water can be readily observed spilling over sills, mixing violently with warmer ambient water, and otherwise descending to abyssal depths. The main features of the upwelling phase are less obvious. Most of the gaps in our knowledge about the thermohaline circulation are due to uncertainty about where the upwelling occurs and how upwelled deep water returns to the areas of deep-water formation. The main new development in studies of the thermohaline circulation is the role of Drake Passage and the Antarctic Circumpolar Current (ACC) in the upwelling phase.

The thermohaline circulation is an impor- tant factor in the earth’s climate because it transports roughly 1015 W of heat poleward

into high latitudes, about one fourth of the total heat transport of the ocean/atmosphere circu- lation system. The upwelling branch of the thermohaline circulation is important for the ocean’s biota as it brings nutrient-rich deep water up to the surface. The thermohaline circulation is thought to be vulnerable to the warming and freshening of the earth’s polar regions associated with global warming.

The Cooling Phase - Deep-Water Formation

The most vigorous thermohaline circulation in the ocean today is in the Atlantic Ocean where the overturning is often likened to a giant con- veyor belt. The upper part of the conveyor carries warm, upper ocean water through the tropics and subtropics toward the north while the deep part carries cold dense polar water southward through the Atlantic, around the tip of Africa, and into the ocean beyond. The Atlantic conveyor converts roughly 15x106 m3 s-1 of upper ocean water into deep water. (Oceanographers designate a flow rate of 1x106 m3 s-1 to be one Sverdrup, or Sv. All the world’s rivers combined deliver about one Sv of fresh water to the ocean.) Most of the 15-Sv flow of the upper part of the conveyor passes through the Florida Straits and up the east coast of North America as part of the Gulf- stream. The path of the conveyor cuts east- ward across the Atlantic and continues northward along the coast of Europe. As the conveyor moves out across the Atlantic and northward off Europe it gives up roughly 50 W of heat per square meter of ocean surface to the atmosphere. This heat flux is comparable to


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