Thermohaline circulation of the oceans

From Coastal Wiki
Jump to: navigation, search
Fig.1. The Thermohaline Circulation. Source: IPPC 2001.

The Thermohaline Circulation (THC) is the classical name given to the large-scale circulation of the oceans driven by differences in seawater density. The term thermohaline derives from the Greek thermo- (heat) and haline (salt), reflecting the dependence of seawater density on temperature and salinity. However, the large-scale interocean circulation is not driven solely by density gradients. Winds over the Southern Ocean play a crucial role by driving northward Ekman transport and deep upwelling, while tidal mixing and wind-driven turbulence supply energy for vertical mixing throughout the ocean interior. For this reason, the circulation is more commonly referred to as the Meridional Overturning Circulation (MOC).

The overturning circulation transports heat, freshwater, dissolved gases, nutrients and other substances throughout the global ocean. It supports the largest habitat on earth and is a key regulator of Earth’s climate. Changes in the overturning circulation can strongly affect regional and global climate through their impact on ocean heat transport.[1]

See also: Ocean circulation.


Description of the Thermohaline Circulation

The global overturning circulation is often illustrated as a “conveyor belt” linking the Atlantic, Southern, Indian and Pacific Oceans. The ocean conveyor belt is a simplified conceptual representation of the global overturning circulation. The loop takes more than 1,000 years to complete. The overturning circulation is in reality more complex: it is highly three-dimensional, it involves multiple pathways and not a single continuous loop.

Dense deep water forms primarily in a few high-latitude regions where surface waters become sufficiently cold and salty to sink. In the North Atlantic, deep convection occurs mainly in the Greenland-Iceland-Norwegian Seas and the Labrador Sea, forming North Atlantic Deep Water (NADW). Around Antarctica, especially in the Weddell and Ross Seas, very dense waters form through cooling and brine rejection during sea-ice formation, producing Antarctic Bottom Water (AABW).

Fig. 2. Schematic representation of the Atlantic Meridional Overturning Current.

NADW flows southward through deep passages between Greenland, Iceland and Scotland into the Atlantic abyssal plains and eventually reaches the Southern Ocean. There it interacts with the Antarctic Circumpolar Current and contributes to the global overturning circulation. The Atlantic branch, consisting of northward surface flow and southward deep return flow, is commonly called the Atlantic Meridional Overturning Circulation (AMOC).

The Pacific Ocean does not possess an overturning circulation comparable to the AMOC. Deep-water formation in the North Pacific is inhibited because surface waters are relatively fresh and strongly stratified, preventing sufficiently high surface densities from developing. The shallow Bering Strait limits exchange with the Arctic Ocean, but the absence of a strong overturning circulation is itself also part of the reason for the persistent low-salinity stratification of the North Pacific.

The Antarctic Bottom Water (AABW) is denser than NADW and therefore spreads beneath it along the ocean floor into the Atlantic, Indian and Pacific Oceans. In these basins, deep waters gradually mix upward through turbulent mixing and wind-driven upwelling, particularly in the Southern Ocean and tropical regions, eventually returning to the upper ocean.

The upper branch of the circulation returns warm surface and thermocline waters toward the Atlantic through the Indonesian Throughflow, around South Africa via the Agulhas system, and northward through the Gulf Stream and North Atlantic Current toward the Nordic Seas. Surface waters in the North Atlantic lose heat to the atmosphere, become denser through cooling and evaporation, and sink to form deep water, thereby completing the overturning loop.


Energy for maintaining the large-scale thermohaline circulation

Although density differences are small and the flow velocity is low (of the order of a few cm/s), the water masses moving around by thermohaline circulation are huge. Water fluxes are of the order of 20 Sv or more (Sv = Sverdrup = 1 million m3/s). Density gradients alone are not sufficient for sustaining the deep ocean circulation. Upwelling and mixing processes, to bring deep ocean water back to the surface, are required too [2].

To maintain the large-scale thermohaline circulation of the ocean, it has been estimated that about 2.1 TW ([math]10^{12}[/math] Watts) of mixing energy is required (Munk and Wunsch, 1998 [3]). It has long been recognized that winds and tides are two important sources of mechanical energy to drive the ocean interior mixing. Although most of the tidal energy from Moon and Sun on the global ocean is dissipated in the shallow seas, perhaps 1.0 TW or more of the tidal energy dissipation occurs in the deep ocean through the scattering by ocean-bottom topography of surface tides into internal tidal waves (Egbert and Ray, 2000[4]). The breaking of internal waves is believed to be a principal contributor to pelagic turbulence.

The winds can also generate internal gravity waves in the surface layer of Earth’s oceans, which are called near-inertial oscillations due to the peak wave energy near the inertial frequency. They are thought to play an important role in diapycnal mixing to sustain the global system of thermohaline circulation. But the exact contribution of wind power to these near-inertial motions and wind’s relative importance compared to tidal forces remain topics of vigorous debate. Estimations on near‐inertial wind power input varied widely from 0.3 to 1.5 TW using numerical models. However, recent calculations on the basis of observations suggest that the wind power is only 0.3–0.6 TW, and the strongest flux of energy occurs between 30° and 60° latitudes during the winter season, when storms are the most prevalent (Liu et al., 2019[5]).

In recent decades, biogenic mixing is thought to be another significant contributor to ocean mixing (Katija and Dabiri, 2009[6]). From small zooplankton to large mammals, swimming animals are capable of carrying bottom water with them as they migrate upward, and that movement indeed creates an inversion that results in ocean mixing. The global power input from this process is estimated in the order of a TW of energy, comparable with levels caused by winds and tides. After all, each day, billions of tiny krill and some jellyfish migrate hundreds of meters from the deep ocean toward the surface where they feed. However, the possible contribution of biogenic mixing to large-scale ocean mixing remains debated.


Bipolar Character of the Thermohaline Circulation

The global overturning circulation has a strongly bipolar character, with deep and bottom waters formed in both the North Atlantic and around Antarctica. Interactions between these northern and southern deep-water sources are thought to play an important role in glacial-interglacial climate variability.

Evidence from ice core analysis and from several complementary sources indicate that during the last ice age many rather abrupt climate fluctuations have occurred. These fluctuations are called Dansgaard-Oeschger (D-O) events after the main discoverers.[7]. The D-O events are abrupt climate changes on centennial to millennial time scales during the mid-glacial period. They are often associated with changes in the Atlantic overturning circulation and with asynchronous temperature variations between Greenland and Antarctica.[8] This bipolar ‘seesaw’ behavior is generally interpreted as a redistribution of heat between the hemispheres associated with variations in the strength of the AMOC, although atmospheric and sea-ice feedbacks likely also contributed.[9]

The Dansgaard-Oeschger events suggest that the Atlantic Meridional Overturning Current can become unstable under certain conditions and sensitive to positive feedback from relatively minor perturbations. The dynamical origin of D-O events is not well known. Several mechanisms have been proposed, in particular positive feedbacks involving rapid reorganizations of North Atlantic overturning circulation coupled to sea-ice advance-retreat, together with atmospheric and ocean feedbacks.[10][11]

Particular attention has been given to the role of Heinrich events, episodic massive iceberg discharges from Northern Hemisphere ice sheets, occurring during some of the D-O cold periods (so-called stadials).[12] It has been suggested that the freshwater from iceberg discharges into the North Atlantic reduced surface salinity and density sufficiently to substantially weaken the AMOC and North Atlantic Deep Water (NADW) formation, possibly leading to near-collapse states during some D-O stadials. Proxy records and model studies suggest that such reductions in overturning circulation contributed to cooling in the North Atlantic region while promoting gradual warming in the Southern Hemisphere through the bipolar seesaw mechanism.[13]

A similar meltwater-induced weakening of the AMOC is considered a possible explanation for the Younger Dryas cold event, although the precise freshwater sources and triggering mechanisms remain debated. Atmospheric circulation and sea-ice feedbacks may also have contributed.

Some studies have suggested that changes in Southern Ocean ventilation, Antarctic Bottom Water formation and Southern Ocean sea-ice extent may have contributed to past climate variability. Variations in Southern Ocean salinity, sea-ice formation and deep-water ventilation may influence the global overturning circulation through feedbacks between the northern and southern deep-water formation regions.[14][15][16] Suggestions for more recent climate fluctuations, such as the Little Ice Age (roughly the period 1400-1900 AD) include volcanic activity and reduced solar activity, among others.[17]

The relative importance of ocean circulation, sea ice, atmospheric feedbacks and Southern Ocean processes in driving abrupt glacial climate variability remains an active area of research. The lack of clear understanding of past climate fluctuations illustrates that future climate predictions, apart from a general trend, are still fraught with uncertainty.


The thermohaline circulation and global climate change

The global ocean is accumulating heat, with strongest warming in the upper 2000 m; abyssal warming is detectable but regionally variable (Levitus, S., 2000[18]). Although historical observations and paleoclimatic data reveal significant climate variability on decadal to millennial time scales, this ocean warming during the last several decades is linked to global climate change. Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation and land surface properties have altered the energy balance of the climate system.

Fig. 3. Melting sea ice

The thermohaline circulation is primarily driven by sinking of cold and dense water in the North Atlantic and the Antarctic region. Therefore, any factor that changes the conditions for deep water formation can result in a slow-down of the thermohaline circulation. This raises concerns about potential abrupt climate changes in the future, related to strong weakening or even complete shutdown of the Atlantic Meridional Overturning Current. Direct observations since 2004 and proxy/fingerprint studies suggest weakening, but the trend length is short and attribution remains uncertain. Current models do not provide a robust assessment of the role of anthropogenic forcing in the observed AMOC weakening.[19] There is also some evidence that current changes in the AMOC are related to the North Atlantic Oscillation (NAO)[20].

The Arctic has warmed during recent decades at roughly twice the global average rate or more. Increased Greenland ice melt freshens North Atlantic surface waters, reducing their density and thereby suppressing deep convection and the formation of North Atlantic Deep Water (NADW). A weaker formation of NADW is expected to reduce the strength of the Atlantic Meridional Overturning Circulation (AMOC). Model studies further suggest that AMOC weakening may lead to a shoaling of the NADW layer and a greater northward influence of Antarctic Bottom Water, although the magnitude of these changes remains uncertain.

Weakening of the Atlantic Meridional Overturning Current can raise dynamic sea levels along parts of the North American Atlantic coast by altering density structure, western boundary current transport, and geostrophic sea-surface slopes (see Geostrophic flow). This may lead to an increase of the dynamic sea level along the east US Atlantic coast of a few tens of centimeters.[21]

A major effect of AMOC weakening is a lesser heat transport between the tropics and the Arctic, leading to an increase of meridional temperature gradients. This will alter global atmospheric circulation patterns, including, for example, a southward shift of the Intertropical Convergence Zone.[22] Increased meridional temperature gradients can have an impact on local wind climate, storm tracks and precipitation, leading to increased vulnerability of some coastal zones to erosion and flooding. An important issue is the possibility that changes in the atmospheric circulation pattern could strengthen changes in global ocean circulation patterns through freshwater input, reduced northward salt transport, sea-ice changes, and altered air–sea fluxes, but feedback strengths differ among models.

Reduced northward ocean heat transport during AMOC weakening tends to cool the North Atlantic and redistribute heat toward the Southern Hemisphere. Paleoclimate records and model simulations show a bipolar-seesaw response in which Antarctic temperatures can rise gradually after northern cooling, although the magnitude, timing and regional pattern are uncertain.[23]. In some scenarios, this can contribute to ice-shelf basal melt, accelerate melting of the Antarctic ice sheet, leading eventually to acceleration of the sea level rise.

The high sensitivity of global climate to the AMOC exacerbates the consequences of global warming due to greenhouse gas emissions. Analyses of sedimentary records and ice cores provide evidence for the coupling between the global climate system and oceanic heat transport. This coupling also follows from simulations with integrated earth system models. However, the details of several underlying processes are still insufficiently understood for accurate projections of all the consequences[19].


Related articles

Ocean circulation
Sea level rise
Ekman transport
Geostrophic flow
Seawater density
Shelf sea exchange with the ocean


References

  1. ↑ Broecker, W., 1991. The great ocean conveyor. Oceanography 4, 79–89.
  2. ↑ Rahmstorf, S. 2006. Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences, Edited by S. A. Elias. Elsevier.
  3. ↑ Munk, W. H., and Wunsch C. 1998. Abyssal recipes II: Energetics of tidal and wind mixing. Deep-Sea Research 45: 1977-2010
  4. ↑ Egbert, G. D., and Ray R. D. 2000. Significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data. Nature 405: 775-778
  5. ↑ Liu, Y., Z. Jing, and Wu L. 2019. Wind power on oceanic near-inertial oscillations in the global ocean estimated from surface drifters. Geophysical Research Letters 46: 2647-2653
  6. ↑ Katija, K., and Dabiri J. O. 2009. A viscosity-enhanced mechanism for biogenic ocean mixing'. Nature. 460: 624-626
  7. ↑ Dansgaard, W., Clausen, H.B., Gundestrup, N., et al. 1984. North Atlantic climatic oscillations revealed by deep Greenland ice cores. In: Climate Processes and Climate Sensitivity, AGU Geophysical Monograph 29: 288–298
  8. ↑ Blunier, T. et al., 1998. Asynchrony of Antarctic and Greenland climate change during the last glacial period. Nature 394, 739–743.
  9. ↑ Stocker, T.F. and Johnsen, S.J. 2003. A Minimum Thermodynamic Model for the Bipolar Seesaw. Paleoceanography 18(4), 1087
  10. ↑ Lynch-Stieglitz, J. 2017. The Atlantic Meridional Overturning Circulation and Abrupt Climate Change. Annual Review of Marine Science 9: 83–104
  11. ↑ Li, C. and Born, A. 2019. Coupled atmosphere-ice-ocean dynamics in Dansgaard-Oeschger events. Quaternary Sci. Rev. 203: 1–20
  12. ↑ Broecker, W.S., 1994. Massive iceberg discharges as triggers for global climate change. Nature 372, 421–424.
  13. ↑ Vidal, L. et al., 1999. Link between the North and South Atlantic during the Heinrich events of the last glacial period. Clim. Dyn. 15: 909–919
  14. ↑ Birchfield, G.E. and Broecker, W.S. 1990. A salt oscillator in the glacial Atlantic? 2. A scale analysis model. Paleoceanography 5: 835–843
  15. ↑ Broecker, W.S. 2000. Was a change in thermohaline circulation responsible for the Little Ice Age? Proc. Natl. Acad. Sci. 97: 1339–1342
  16. ↑ Willeit, M., Ganopolski, A., Kaufhold, C., Dalmonech, D., Liu, B. and Ilyina, T. 2025. Earth System Response to Heinrich Events Explained by a Cascade of Glacial Feedbacks. Nature Geoscience 18: 1159–1166
  17. ↑ Owens, M.J., Lockwood, M., Hawkins, E., Usoskin, I., Jones, G.S., Barnard, L., Schurer, A. and Fasullo, J. 2017. The Maunder minimum and the Little Ice Age: an update from recent reconstructions and climate simulations. Journal of Space Weather and Space Climate 7, A33
  18. ↑ Levitus, S., Antonov, J.I., Boyer, T.P., Stephens, C., 2000. Warming of the world ocean. Science 287, 2225–2229.
  19. ↑ 19.0 19.1 Dong, B., Aksenov, Y., Colfescu, I., Harvey, B., Hirschi, J., Josey, S., Lu, H., Mecking, J., Oltmanns, M., Osprey, S., Robson, J., Rynders, S., Shaffrey, L., Sinha, B., Sutton, R. and Weisheimer, A. 2025. Key drivers of large scale changes in North Atlantic atmospheric and oceanic circulations and their predictability. Climate Dynamics 63: 113
  20. ↑ Robson, J., Sutton, R.T., Lohmann, K., Smith, D. and Palmer, M.D. 2012. Causes of the rapid warming of the North Atlantic ocean in the mid-1990s. J. Clim. 25: 4116–4134
  21. ↑ Little, C.M., Hu, A., Hughes, C.W., McCarthy, G.D., Piecuch, C.G., Ponte, R.M. and Thomas, M.D. 2019. The Relationship between U.S. East Coast sea level and the Atlantic Meridional Overturning Circulation: A review. Journal of Geophysical Research: Oceans 124: 6435–6458
  22. ↑ Zhang, R., Kang, S.M. and Held, I.M. 2010. Sensitivity of climate change induced by the weakening of the Atlantic meridional overturning circulation to cloud feedback. Journal of Climate 23: 378-389
  23. ↑ Pedro, J.B., Jochum, M., Buizert, C., He, F., Barker, S. and Rasmussen, S.O. 2018. Beyond the bipolar seesaw: Toward a process understanding of interhemispheric coupling. Quaternary Science Reviews 192: 27-46


The main authors of this article are Tange, Hannli, Dehai Song and Job Dronkers
Please note that others may also have edited the contents of this article.
Citation: Tange, Hannli; Dehai Song; Job Dronkers ; (2026): Thermohaline circulation of the oceans. Available from http://www.coastalwiki.org/wiki/Thermohaline_circulation_of_the_oceans [accessed on 4-06-2026]
Retrieved from "https://www.coastalwiki.org/w/index.php?title=Thermohaline_circulation_of_the_oceans&oldid=82055"