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
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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.
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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. However, the deep water globe. formed around Antarctica is denser and is able to The other major site of deep-water formation is sink below NADW in the south. The deep water the coast of Antarctica. The surface waters around formed around Antarctica does not readily ventilate Antarctica, like those over most of the Arctic the Southern Ocean. It is diluted many times by Ocean, are ice covered during winter and are too mixing with the weakly ventilated water offshore. fresh to sink. Deep water below 500 m around Figure 3 shows CFC 11 concentrations along a tran- Antarctica, on the other hand, is relatively warm sect at 1403E that begins on the Antarctic continen- (1.53C) and fairly salty (34.70}34.75PSU). This tal shelf on the left. This section was occupied at water mass, known as Circumpolar Deep Water roughly the same time as the North Atlantic section (CDW), penetrates onto the relatively deep conti- in Figure 1. In contrast to Figure 1, water with nental shelves around Antarctica where it is cooled '3.0 units of CFC 11 is limited to near-surface to the freezing point. Brine rejection from the waters and the continental shelf. A small amount of formation of new sea ice maintains fairly high salin- recently ventilated water ('0.1 units) is seen de- ities on the shelf despite the freshening effects of scending to the bottom along the continental slope. precipitation and the input of glacial melt water. The mass of old CFC-free water that Rlls the Supercooled shelf water is observed to Sow out interior is Circumpolar Deep Water (CDW). The from under Soating ice shelves where contact with mixture of Antarctic shelf water and CDW descend- glacier ice cools shelf water to temperatures that ing the slope is Antarctic Bottom Water (AABW). would be below the freezing point at atmospheric AABW occupies the deepest parts of the ocean and pressure. Very cold salty shelf water (!13C, is observed penetrating northward into the Atlantic,
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Indian, and PaciRc Oceans through deep passages in oceans. Schematic diagrams often depict the closure the mid-ocean ridge system. of the thermohaline circulation as a Sow of warm Small quantities of deep water are also observed upper-ocean water that passes from the North Paci- to form in evaporative seas like Mediterranean and Rc through Indonesia, across the Indian Ocean, and Red Seas. These water masses are dense owing to then around the tip of Africa into the Atlantic. their high salinities. They tend to form intermedi- Observations made over the last 30 years have ate-depth water masses in relation to the colder failed to support the idea of broad, diffuse upwell- deep and bottom waters from the North Atlantic ing. Attempts to directly measure turbulent mixing and Antarctica. rates in the interior have shown that the actual mixing rates are generally only about 10 or 20% of The Warming Phase ^ Upwelling and the amount required. There is no indication that any the Return Flow deep water is actually upwelling to the surface in the warm parts of the Indian and PaciRc Oceans. The upwelling of deep water back to the ocean’s Mixing does seem to be effective in warming the surface was thought at one time to be widely dis- deepest part of the ocean, however. Modern circula- tributed over the whole ocean. This variety of up- tion reconstructions show Antarctic Bottom Water welling was linked to turbulent mixing processes Sowing northward into the Indian, PaciRc, and At- that were hypothesized to be active throughout the lantic basins at 03C, upwelling across 3500 m, and interior. Mixing was thought to be slowly heating exiting back to the south at (23C between 2000 the deep ocean, making the old deep water in the and 3500 m. This indicates that the circulation and interior progressively less dense so that it could be heat transport associated with mixing in the ocean displaced upward by the colder and saltier deep is probably quite weak and is limited to the volume waters forming near the poles. Since the main areas of ocean ventilated by AABW. of deep water formation are located at either end of It now appears that most, if not all, the deep the Atlantic, and since most of the ocean’s area is water sinking in the North Atlantic upwells back found in the Indian and PaciRc Oceans, it stood to to the surface around Antarctica in the Southern reason that the warming of the return Sow should Ocean in association with the Antarctic Circum- be widely distributed across the Indian and PaciRc polar Current. The upwelling in this case seems to
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60°S 64° 62° 60° 58° 56° 54°S Latitude Figure 3 Section of CFC 11 extending northward from the coast of Antarctica along 1403E (WOCE section SR3) illustrating the penetration of CFCs into the Southern Ocean. (Measurements courtesy of J. Bullister of NOAA-PMEL, collected during Aurora Australis cruise AU9404, December 1994}January 1995.)
RWOS 0111 TJ NP INDIRA PADMA THERMOHALINE CIRCULATION 2945 occur within the channel of open water that circles the southern hemisphere. The net effect is a weaken- the globe in the latitude band of Drake Passage ing of the overall heat transport in the southern (553}653S). Drake Passage lies within the belt of hemisphere and a strengthening of the heat trans- mid-latitude westerlies. The Sow in the surface port in the northern hemisphere. Ekman layer is directed northward and is divergent (upwelling favorable) within the channel. A key dy- General Theory for the Thermohaline namical factor in this problem is the lack of meridi- Circulation onal barriers in the upper 1500 m of the Drake Passage channel. Without meridional barriers, there Figure 4 is a north}south section showing the distri- can be no net geostrophically balanced Sow into the bution of density through the Atlantic Ocean. The channel above 1500 m to balance the northward main thermocline in this diagram lies close to the Sow in the surface Ekman layer. This means that surface, between the 34.0 (&203C) and 36.0 g kg\1 the winds, in driving surface waters northward out (&83C) isopycnals. The northward Sow of the of the open channel, tend to draw up old deep water Atlantic thermohaline circulation takes place largely to the surface from depths near 1500 m. within this part of the density Reld. The northward The deep water upwelled to the surface and Sow in the far south starts off a bit denser and driven northward out of the Drake Passage channel colder, as low as 36.6 g kg\1. The southward Sow is forced down into the thermocline on the north of NADW is located, for the most part, below the side of the ACC where the Sow in the surface 37.0 isopycnal. Ekman layer is convergent. Numerical experiments The isopycnals at the base of the thermocline in with ocean general circulation models (GCMs) Figure 4 are basically Sat north of 403S, but rise up show that much of the water forced down into the to the surface between 403S and 603S. The rise of thermocline all around the circumpolar belt event- these isopycnals in the south is a reSection of the ually makes its way into the Atlantic Ocean where it Antarctic Circumpolar Current which Sows to the is converted into deep water in the northern North east across the section, i.e. the north}south density Atlantic. The action of the winds in the channel, in gradient between 403 and 603S is in thermal wind drawing up deep water from the ocean’s interior, balance with an eastward Sow that increases toward has been shown to actively enhance the formation the surface. The relatively deep and Sat position of of deep water in the North Atlantic. the isopycnals at the base of the thermocline reSects, The deep water drawn up to the surface by the to a large extent, the mechanical work done by the winds in Drake Passage is quite cold. As this cold winds that drive the ACC. The convergent surface water comes into contact with the atmosphere and Sow generated by the winds pushes down relatively is carried northward, it takes up solar heat that light water north of the ACC in relation to the cold, otherwise would be available to warm the Southern dense water being lifted up by the winds south of Ocean and Antarctica. The Atlantic thermohaline the ACC. circulation carries this southern heat across the The position of the 36.0 and 36.4 g kg\1 isopyc- equator into the high latitudes of the northern nals at depths near 1000 m is important because it hemisphere where it is released to the atmosphere signals the presence of relatively warm, low-density when new deep water is formed in the north. The water at depth. Water of this density is found adjac- thermohaline circulation thereby warms the North ent to the sills in the north where new NADW spills Atlantic at the expense of a colder Southern into the deep North Atlantic. It is the contrast Ocean and colder Antarctica. Indeed, sea surface between the cold, dense water behind the sills and temperatures at 603N in the North Atlantic relatively warm water beyond the sills that provides are on average about 63C warmer than sea surface the density gradient that drives the Sow of dense temperatures at 603S. water into the deep Atlantic. Numerical experiments If the warming phase of the thermohaline circula- carried out in ocean GCMs suggest that all the tion involved a downward mixing of heat through warm isopycnals in the lower thermocline would be the thermocline in low and middle latitudes, as once squeezed up into the main thermocline if Drake thought, the thermohaline circulation would carry Passage were to be closed up and the ACC elimi- tropical heat poleward into high latitudes. The nated. warming phase now seems to take place mainly in Gnanadesikan has described a general theory for the far south where the warming is associated with the thermohaline circulation which relates the thick- an equatorward transport of heat. This means that ness of the thermocline, as illustrated in Figure 4, the heat transport by the thermohaline circulation is with the intensity of the Atlantic thermohaline cir- opposed to the heat transport of the atmosphere in culation. When deep-water formation occurs in the
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60°S 40° 20° Eq 20° 40° 60° 80°N Latitude Figure 4 Density structure of the Atlantic thermocline. Seawater density is referenced to a depth of 2000 m and has been zonally averaged across the Atlantic basin (units g kg\1). Contours have been chosen to highlight the lower part of the thermocline. The zonally averaged topography fails to capture the height of the sills in the Greenland}Scotland Ridge (653N) which are closer to 600 m.
North Atlantic it converts relatively light low- tion. This is actually not true. The cycling of fresh density water within the thermocline into new deep water between the ocean and atmosphere (the hy- water. It thereby removes mass from the thermo- drological cycle) results in a net addition of fresh cline, allowing it to be squeezed upward. When water to high latitudes which reduces the density of water is upwelled from the deep ocean and pumped polar surface waters. Thus, the haline contribution into the thermocline in the south it adds mass to the to the thermohaline circulation is nearly always in thermocline, causing it to thicken downward. In this opposition to the thermal forcing. The Earth’s way the thermocline thickness reSects a balance hydrological cycle is expected to become more between the addition of mass to the thermocline vigorous in the future with global warming. This via winds in the south, the addition of mass via is expected to weaken the thermohaline circulation upwelling from below (presumably minor), and the in a way which could be fairly abrupt and un- loss of mass by deep-water formation in the north. predictable. Gnanadesikan’s theory also includes the potentially North Atlantic Deep Water is salty because the important effect of eddies in the ACC which tend to water in the upper part of the thermohaline circula- drain mass from the thermocline in opposition to tion Sows through zones of intense evaporation in the wind effect. the tropics and subtropics. If the rate at which new deep water is forming is relatively high, as it seems Instability of the Thermohaline to be at the present time, the sinking water removes Circulation much of the fresh water added in high latitudes and carries it into the interior. The added fresh water in Cooling of the ocean in high latitudes makes polar this case dilutes the salty water being carried into surface waters denser in relation to warmer waters the deep-water formation areas, but does not erase at lower latitudes. Thus cooling contributes to a the effect of evaporation in low latitudes. Thus, stronger thermohaline circulation. The salinity sec- NADW remains fairly salty and is able to export tion through the Atlantic Ocean in Figure 1 gives a fresh water from the Atlantic basin. (The key factor superRcial impression that haline forces also make a here is that NADW at depth is actually a bit fresher positive contribution to the thermohaline circula- than the water Sowing northward.)
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If the rate of deep-water formation is relatively Ledwell JR, Watson AJ and Law CS (1993) Evidence for low, however, or the hydrological cycle is fairly slow mixing across the pycnocline from an open-ocean strong, the export of fresh water via NADW is tracer-release experiment. Nature 364: 701}703. reduced and the fresh water added in high latitudes McCartney MS and Talley LD (1984) Warm-to-cold tends to accumulate. There seems to be a critical water conversion in the northern North Atlantic } threshold of freshwater input for maintaining the Ocean. Journal of Physical Oceanography 14: 922 935. Manabe S and Stouffer R (1993) Century-scale effects of Atlantic thermohaline circulation. If the freshwater increased atmospheric CO2 on the ocean-atmosphere input is close to the threshold, the overturning system. Nature 364: 215}218. becomes unstable and may oscillate over time. If Rahmstorf S (1995) Bifurcations of the Atlantic thermo- the freshwater input exceeds the threshold, the haline circulation in response to changes in the hydro- haline effect overwhelms the thermal effect and the logical cycle. Nature 378: 145}149. thermohaline circulation stops dead. Rudels B, Jones EP, Anderson LG and Kattner G (1994) Manabe and Stouffer have projected that a four- On the intermediate depth waters of the Arctic Ocean. In: The Polar Oceans and their Role in Shaping the fold increase in atmospheric CO2 would increase the hydrological cycle sufRciently that the Atlantic Global Environment, Geophysical Monograph 85, thermohaline circulation might collapse. This would pp. 33}46. Washington, DC: American Geophysical lead to a substantially deeper thermocline and Union. Schmitz WJ (1995) On the interbasin-scale thermohaline a shift in the heat exchange between the hemi- circulation. Reviews of Geophysics 33: 151}173. spheres. It would also lead to a drastic reduction in Schmitz WJ and McCartney MS (1993) On the North the rate at which nutrients are supplied to the upper Atlantic Circulation. Reviews of Geophysics 31: 29}49. ocean biota and a reduction in the oxygen content Schmitz WJ and Richardson PL (1991) On the sources of of deep water. the Florida Current. Deep-Sea Research 38 (supple- ment 1): S379}S409. See also Toggweiler JR and Samuels B (1995) Effect of Drake Passage on the global thermohaline circulation. Deep- Antarctic Circumpolar Current. Conservative sea Research 42: 477}500. Elements. Ekman Transport and Pumping. Florida Toggweiler JR and Bjornsson H (2000) Drake Passage Current, Gulf Stream and Labrador Current. and paleoclimate. Journal of Quaternary Science 15: General Circulation Models. Heat Transport and 319}328. Climate. North Sea Circulation. Toggweiler JR and Samuels B (1998) On the ocean’s large-scale circulation near the limit of no vertical Further Reading mixing. Journal of Physical Oceanography 28: 1832}1852. Broecker WS (1991) The Great Ocean Conveyor. Tziperman E (2000) Proximity of the present-day Oceanography 4: 79}89. thermohaline circulation to an instability threshold. Cox M (1989) An idealized model of the world ocean. Journal of Physical Oceanography 30: 90}104. Part 1: The global scale water masses. Journal of Warren BA (1981) Deep circulation of the World Ocean. Physical Oceanography 19: 1730}1752. In: Warren BA and Wunsch C (eds) Evolution of Gnanadesikan A (1999) A simple predictive model for Physical Oceanography } ScientiTc Surveys in Honor the structure of the oceanic pycnocline. Science 283: of Henry Stommel, pp. 6}41. Cambridge, MA: MIT 2077}2079. Press.
TERNS
See LARIDAE, STERNIDAE AND RYNCHOPIDAE.
THREE-DIMENSIONAL (3D) TURBULENCE
W. D. Smyth and J. N. Moum, Introduction Oregon State University, Corvallis, OR, USA This article describes Suid turbulence with applica- ^ Copyright 2001 Academic Press tion to the Earth’s oceans. We begin with the doi:10.1006/rwos.2001.0134 simple, classical picture of stationary, homogeneous,
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