THERMOHALINE CIRCULATION 2941 THERMOHALINE CIRCULATION J. R. Toggweiler, NOAA, Princeton, NJ, USA The Cooling Phase ^ Deep-Water R. M. Key, Princeton University, Princeton, NJ, Formation USA Copyright ^ 2001 Academic Press The most vigorous thermohaline circulation in the doi:10.1006/rwos.2001.0111 ocean today is in the Atlantic Ocean where the overturning is often likened to a giant conveyor belt. The upper part of the Atlantic thermohaline circula- Introduction tion carries warm, upper ocean water through the tropics and subtropics toward the north while the The circulation of the ocean is usually divided into deep part carries cold, dense polar water southward two parts, a wind-driven circulation that dominates through the Atlantic, around the tip of Africa, and in the upper few hundred meters, and a density- into the ocean beyond. The Atlantic thermohaline driven circulation that dominates below. The latter circulation converts roughly 15;106 m3 s\1 of is called the `thermohaline' circulation because of upper ocean water into deep water. (Oceanogra- the role of heating, cooling, freshening, and salini- phers designate a Sow rate of 1;106 m3 s\1 to be Rcation in producing regional density differences 1 Sverdrup, or Sv. All the world's rivers combined within the ocean. The thermohaline circulation, for deliver about 1 Sv of fresh water to the ocean.) Most the most part, is an `overturning' circulation in of the 15 Sv Sow of the upper part of the Atlantic which warm water Sows poleward near the surface thermohaline circulation passes through the Florida and is subsequently converted into cold, dense water Straits and up the east coast of North America as that sinks and Sows equatorward in the interior. part of the Gulf Stream. It then cuts eastward across Radiocarbon measurements show that the thermo- the Atlantic and continues northward along the haline circulation turns over all the deep water in coast of Europe. The warm water Sowing north- the ocean every 600 years or so. ward in the northern North Atlantic gives up rough- The most spectacular features of the thermohaline ly 50 W of heat per square meter to the atmosphere circulation are seen in the sinking phase, in the as it cools. This heat Sux is comparable to the solar formation of new deep water in the North Atlantic energy reaching the lowest layers of the atmosphere and the Southern Ocean. Large volumes of cold during the winter months and is a signiRcant factor polar water can be readily observed spilling over in the climate of Northern Europe. sills, mixing violently with warmer ambient water, As the upper part of the Atlantic thermohaline and otherwise descending to abyssal depths. The circulation moves northward through the tropical main features of the upwelling phase are less obvi- and subtropical North Atlantic it spans a depth ous. Most of the gaps in our knowledge about the range from the surface down to &800 m and has thermohaline circulation are due to uncertainty a mean temperature of some 15}203C. During its about where the upwelling occurs and how upwel- transit through the tropics and subtropics the Sow led deep water returns to the areas of deep-water becomes saltier due to the excess of evaporation formation. The main new development in studies of over precipitation in this region. It also becomes the thermohaline circulation is the role of Drake warmer and saltier by mixing with the salty outSow Passage and the Antarctic Circumpolar Current from the Mediterranean Sea. By the time the Sow (ACC) in the upwelling phase. has crossed the 503N parallel into the subpolar The thermohaline circulation is an important North Atlantic it has cooled to an average temper- factor in the Earth's climate because it transports ature of 11.53C. Roughly half of the water carried roughly 1015 W of heat poleward into high latitudes, northward by the thermohaline circulation Sows about one quarter of the total heat transport of the into the Norwegian Sea between Iceland and Nor- ocean/atmosphere circulation system. The upwelling way. Part of the conveyor Sow extends into the branch of the thermohaline circulation is important Arctic Ocean. for the ocean's biota as it brings nutrient-rich deep The Rnal stages of the cooling process make the water up to the surface. The thermohaline circula- salty North Atlantic water dense enough to sink. tion is thought to be vulnerable to the warming and Sinking is known to occur in three main places. The freshening of the Earth's polar regions associated densest sinking water in the North Atlantic is for- with global warming. med in the Barents Sea north of Norway where salty RWOS 0111 TJ NP INDIRA PADMA 2942 THERMOHALINE CIRCULATION water from the Norwegian Current is exposed to high oxygen, and tracer concentrations. The south- the atmosphere on the shallow ice-free Barents shelf. ward Sow of North Atlantic Deep Water (NADW) Roughly 2 Sv of water from the Norwegian Current is a prime example. NADW is identiRed as a water crosses the shelf and sinks into the Arctic basin after mass with a narrow spread of temperatures and being cooled down to 03C. This water eventually salinities between 2.03 and 3.53C and 34.9 and Sows out of the Arctic along the coast of Greenland 35.0 PSU. NADW quickly ventilates the Atlantic at a depth of 600}1000m. The volume of this Sow Ocean. Figure 1 shows an oceanographic section is increased by additional sinking and open-ocean between the southern tip of Greenland and the coast convection in the Greenland Sea north of Iceland. of Labrador showing recent ventilation by chloro- A mixture of these two water masses Sows into the Suorocarbon 11 (CFC 11, or Freon 11, a man-made North Atlantic over the sills between Greenland, trace gas used in refrigeration). High CFC water Iceland, and Scotland. with concentrations in excess of 3.0 units tags the As 03C water from the Arctic and Greenland Seas overSow water from Denmark Strait that Sows passes into the North Atlantic it mixes with 63C around the tip of Greenland along the sloping water beyond the sills and descends the Greenland bottom into the Labrador Sea. Deep water with continental slope down to a depth of 3000 m. It a slightly lower CFC concentration Sows out of the Sows southward around the southern tip of Green- Labrador Sea at the same depth. High CFC water in land into the Labrador Sea as part of a deep bound- the center of the basin (3.5}4.0 units) tags locally ary current that follows the perimeter of the formed Labrador Sea water that has been ventilated subpolar North Atlantic. A slightly warmer deep- by winter convection down to 2000 m. There is water type is formed by open-ocean convection presently no water in the North Atlantic north of within the Labrador Sea. The deep-water formed in 303N that is uncontaminated with CFCs. the Labrador Sea increases the volume Sow of the NADW Sows southward along the coasts of boundary current that exits the subpolar North North and South America until it reaches the Atlantic beyond the eastern tip of Newfoundland. circumpolar region south of the tip of Africa. Newly formed water masses are easy to track by Figure 2 shows the distribution of salinity in the their distinct temperature and salinity signatures, western Atlantic along the path of the Sow. Newly 0 5.0 4.5 4.5 4.0 4.0 1000 Labrador Greenland 3.5 3.5 2000 Depth (m) 3.0 2.5 2.0 3000 3.0 2.0 2.5 3.0 3.5 54°N 56° 58° 60°N Latitude Figure 1 Section of CFC 11 (chlorofluorocarbon 11, or Freon 11) between the southern tip of Greenland on the right and the coast of Labrador on the left (WOCE section A1W) illustrating the recent ventilation of the deep North Atlantic by the main components of North Atlantic Deep Water. (Measurements courtesy of R. Gershey of BDR Research, collected during Hudson Cruise 95011, June 1995}July 1995.) CFC concentrations are given in units of 10}15 moles kg\1 or picomoles kg\1. RWOS 0111 TJ NP INDIRA PADMA THERMOHALINE CIRCULATION 2943 0 37.0 36.0 34.2 35.0 36.0 34.6 34.9 34.4 35.0 34.6 34.9 34.7 2000 34.8 34.9 Depth (m) 4000 34.7 34.8 34.9 6000 60°S 40° 20° Eq 20° 40° 60°N Latitude Figure 2 North}south section of salinity down the western Atlantic from Iceland to Drake Passage. The salinity distribution has been contoured every 0.1 PSU between 34.0 and 35.0 PSU to highlight North Atlantic Deep Water (34.9}35.0 PSU). formed NADW (salinity (S)'34.9 PSU) is easily 34.6}34.7PSU) is observed descending the continen- distinguished from the relatively fresh intermediate tal slope to the bottom in the Weddell Sea. Bottom water above (S(34.6 PSU) and the Antarctic water water is also observed to form in the Ross Sea and below (S(34.7 PSU). NADW exits the Atlantic off the AdeH lie coast south of Australia and New south of Africa between 353 and 453S and joins the Zealand. eastward Sow of the Antarctic Circumpolar Cur- The volume of new deep water formed on the rent. Traces of NADW re-enter the Atlantic Ocean Antarctic shelves is not very large in relation to the through Drake Passage (553}653S) with a salinity in volume of deep water formed in the North Atlantic, excess of 34.7 after Sowing all the way around the perhaps 4}8 Sv in total.