What Is Thermohaline Circulation?

Straddling the world's oceans is a serpentine network of important ocean currents. Their purpose is to distribute cold water from the polar regions towards the equator, and warm water from equatorial regions to the poles. As well as wind-driven surface currents, this network includes a number of massive deep-water currents known as the thermohaline circulation. We explain how these currents work, how they exert a critical influence on global climate and the marine biome, and why climate change is threatening to destabilize them.
Thermohaline Circulation: NoMorePlanet.com
Thermohaline circulation refers to ocean currents driven by water temperature and salinity. Image Credit: NASA.

The Global Conveyor Belt

For thousands of years, the deep water thermohaline circulation has been helping Earth’s climate system to distribute heat and cold around the world. At the same time, it absorbs carbon dioxide from the polar regions and keeps it deep underwater for centuries.

Today, with a climate crisis looming, this continuous oceanic circulation is more important than ever, as it helps to dial down the effects of global warming on the oceans and safeguard precious ecosystems on land and sea.

The term “thermohaline circulation” refers to a serpentine pattern of deep ocean currents that are driven by density differences – mainly caused by variations in salinity and temperature – as opposed to near-surface currents that are driven mainly by wind. 1 The density-related thermohaline circulation drives a series of ocean currents, often referred to as the “global conveyor belt”, or the “meridional overturning circulation.”

These currents are an important feature of the hydrosphere and play a key role in the global water cycle, moderating temperature and revitalizing water content around the globe. Flowing like huge underwater rivers, they transport heat, salts, dissolved gases from the atmosphere like carbon dioxide, as well as nutrients vital to phytoplankton and other marine life.

Thermohaline circulation currents are yet another example of how the oceans influence climate and safeguard the marine environment.

How Does Thermohaline Circulation Work?

Thermohaline currents work on the relative principle that denser water is heavier than less dense water, and will therefore sink. Sea water density is affected by salinity and temperature. (1) Salinity is usually controlled by the presence or absence of freshwater fluxes. For example, sea ice or evaporation raises salinity, while precipitation, land ice-melt and rivers reduce it. 2 Water temperature is influenced by heating/cooling that takes place at the ocean surface: typically, from sunlight, or from contact with warm or cold (icy) water, or cold winds. Generally speaking, cold water with higher salinity is the most dense, while warm water with a lower salinity is least dense.

Ma: Pattern of Thermohaline Circulation
Map showing the pattern of thermohaline circulation with cold dense deep water currents marked in blue and warmer near-surface currents marked in red. Photo: © NASA

Thermohaline circulation starts in the Earth’s polar regions. When ocean water in these areas gets very cold, sea ice forms. The ice causes the temperature of the surrounding seawater to fall, but it also causes the water’s salinity to rise, because when sea ice forms, salt is left behind. As the water becomes colder and saltier, its density increases, and it starts to sink.

Why does its density increase? Because cooling a substance makes its molecules slow down and move closer together, thus occupying a smaller volume which results in higher density. And adding salt increases its mass, which raises its density further.

As the dense water sinks, new surface water is drawn in to replace the sinking water. This new water in turn becomes affected by the cold and extra salt, and it too sinks. It is this downwelling that initiates the thermohaline circulation and that drives the deep-ocean currents around the world.

The Global Conveyor Belt Of Ocean Currents

The so-called global conveyor belt (a concept conceived by the geoscientist Wally Broecker) is a snake-like pattern of currents that rise and fall around the oceans. It includes both slow-moving deep-water cold currents (like the Antarctic Circumpolar Current) and faster moving near-surface warm currents (like the Gulf Stream), although only the cold-water currents are density driven. The Earth’s spin and the topography of the ocean floor are other influences that affect the direction in which these currents flow.

The global conveyor belt moves much more slowly (a few centimeters per second) than wind-driven or tidal currents (tens to hundreds of centimeters per second). Even so, it transports a huge volume of water – more than one hundred times larger than the flow of the Amazon River. For example, the volume of water moved by the Antarctic Circumpolar Current alone, is estimated to be at least 110–150 million cubic metres of water per second. 2 3

Did You Know That Oceans Slope?
Water expands as it heats up. Water at the equator is the warmest, and rises enough (8 cms/3 inches) to create a slight incline in the direction of the poles. A second slope forms between the Atlantic and the Pacific. The undersea of cold, salty water leaving the Atlantic causes the sea level of the Atlantic to dip slightly lower than the Pacific. This (along with a difference in salinity) leads to a large (albeit slow) flow of warmer water from the Pacific to the Atlantic via the Indonesian Archipelago. A third type results from a combination of wind and the Coriolis effect, which creates a raised area in the middle of a system of circulating ocean currents, known as an ocean gyre.

What Route Does The Global Conveyor Belt Take?

Here is an extremely simplified outline of the route taken by the global conveyor belt around the world.

We’ll start at the equator, where the warm surface current known as the Gulf Stream and its northerly extension, the North Atlantic Drift, carry tropical heat from the Gulf of Mexico to the Arctic, warming the UK and the west coast of Europe en route. Cooling and becoming saltier due to evaporation, this dense current finally sinks into the depths at two sites: the Greenland-Norwegian Sea, and the Labrador Sea (southwest of Greenland). These downwellings form a body of dense water, known as North Atlantic Deep Water (NADW), a key feature of the thermohaline circulation in the Northern Hemisphere.

The shallow ocean floor of the Bering Straight (between Russia and the USA) prevents deep water currents from flowing out of the Arctic Ocean into the Pacific. So dense water on the floor of the North Atlantic moves southwards and flows along the bottom of the Atlantic, at a depth of between 1500 and 4000 metres, until it finally reaches the Southern Ocean around Antarctica. Here it rises to the surface, pulled by the upwelling effect of strong coastal winds, aided by the Ekman divergence. 4 However, due to increased density created by surface water cooling in polynyas and below the ice shelf, the water downwells once again. 5 6

This downwelling occurs at two different places: in the Weddell Sea (off northwest Antarctica), and the Ross Sea (off south Antarctica). The resulting mass of deep water is known as Antarctic Bottom Water (AABW), and this forms part of the cold Antarctic Circumpolar Current (ACC), the strongest current system in the world and the only one to link all of the world’s major oceans (Atlantic, Indian, and Pacific).

The ACC flows clockwise from west to east around Antarctica. The Drake Passage, between the Antarctic Peninsula and the southern tip of South America, prevents the current from flowing west, so it turns eastwards. Soon after, two sections split off and turn northward. One section (which includes AABW formed in the Weddell Sea) moves along the east coast of Africa into the Indian Ocean. The other (which includes AABW formed in the Ross Sea) rounds New Zealand before snaking up into the north Pacific. As they move towards the tropics, becoming warmer and less dense as they go, these two currents blend with other water masses to form what is known as Circumpolar Deep Water (CDW). 

The Indian Ocean branch finally upwells south of the Indian sub-continent; the Pacific branch rises up from the deep waters north of the Hawaiian Ridge. It then returns from the Pacific via the Indonesian throughflow, and rejoins the first branch in the southern Indian Ocean.

Together they round the tip of Africa before returning to the tropical waters of the Caribbean, where the circuit recommences. Oceanographers calculate that a cubic meter of water takes between 1,000 and 1,600 years to complete the journey along the global conveyor belt.

Note: There are no downwellings of deep water in the Indian Ocean because its surface waters are too warm to sink. Northern Pacific waters are cold enough, but not saline enough to sink into the depths, because prevailing winds run into the mountains of the western United States and Canada and dump their rainfall. The resulting freshwater runs into the Pacific, diluting the Pacific’s salinity and depriving its water of the density to initiate thermohaline circulation.

What Are The Benefits Of Thermohaline Circulation Currents?

  • The currents of the global conveyor belt absorb solar heat from tropical ocean waters and redistribute it to mid-latitudes and polar regions. This action, which has helped to even out sea temperatures for thousands of years, is particularly useful in regulating some of the effects of global warming across the different latitudes.

For example, the Gulf Stream warms the climate of the American east coast all the way from Florida to Newfoundland, and also the west coast of Europe. This current, part of the Atlantic meridional overturning circulation, carries up to 25 percent of the northward atmosphere-ocean heat transfer in the northern hemisphere. 7

  • Conversely, the deep-water thermohaline circulation currents redistribute cold water from the poles, thus helping to cool ocean temperatures around the globe.
  • These currents perform a critical function in the carbon cycle by absorbing carbon dioxide from the air and redistributing it around the world, thus spreading the chemical and ecological risk. For example, the Atlantic Meridional Overturning Circulation (AMOC) – the most important section of the thermohaline circulation – is now the largest carbon sink in the Northern Hemisphere, sequestering an estimated 700 million tonnes of CO2 from the atmosphere every year. 8
  • Deep water currents revitalize the ocean surface with nutrients. Warm surface waters are replenished by the upwelling of cold nutrient-dense waters from the ocean floor. This helps the growth of seaweed, algae and other phytoplankton – the base of the marine food web – which in turn attracts swarms of krill, a keystone species in marine ecosystems, which provides food for fish, marine mammals, and birds. See also: Marine Microbes Drive the Aquatic Food Web.
  • Coastal upwelling induced by the “Ekman transport” effect, helps to create some of the world’s most fertile ecosystems. A 25,900-square-kilometer (10,000-square-mile) area off the west coast of Peru, for instance, experiences regular upwelling making it one of the richest fishing grounds in the world. 9 In total, coastal upwelling regions only account for about 1 percent of the total area of the world’s oceans, but they account for roughly half of all fish caught by the world’s fishing fleets. 10

Does Global Warming Affect The Global Conveyor Belt?

Yes. There are two ways that global warming can affect the downwelling of deep water in the Arctic that drives the thermohaline circulation of the global conveyor belt. First, ocean warming can spread to the surface of the subpolar regions of the North Atlantic. Second, it can melt more Arctic sea ice or glacial ice from the Greenland ice sheet or Antarctic ice sheet, thus lowering the salinity of the ocean. Both these actions would reduce the density of high-latitude surface waters and thus inhibit deep water formation.

Is The Atlantic Meridional Overturning Circulation Weakening?

Yes, but no one knows by how much, exactly.

A 2015 study suggested that the AMOC has weakened by 15-20 percent in 200 years and that Greenland ice melt is a possible contributor. 11 12 The weakening of the AMOC was confirmed in two further studies which appeared in 2018. 13 14

The IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate (2019) stated that the Atlantic meridional overturning circulation – described as a key tipping point of the Earth’s climate system – was very likely to weaken over the 21st century, but was very unlikely to collapse – although it stated it was “physically-plausible.” 15

The report said that a weakening of the AMOC would result in “a decrease in marine productivity in the North Atlantic, widespread, increased winter storms in Europe, further reduced rainfall in the already semi-arid Sahel region, and an increase in sea-levels around the Atlantic especially along the northeast coast of North America.”

Other experts claim that a “breakdown” of the AMOC, which happens only in highly pessimistic projections, is a far less likely prospect than a weakening of 20-50 percent, which has been predicted by many coupled climate models on ocean currents. 16

One UK report cites the fact that 11,000 years ago the North Atlantic Deep Water downwelling shut down in response to small changes in global climate. This slowed the course of the Gulf Stream to such a degree that the climate of the northeast Atlantic became significantly cooler. As a result, within tens of years, northwestern Europe reverted to ice age conditions. It went on to warn that global warming may trigger a similar effect, leading to colder climates throughout the UK and Northwest Europe. 17

A new study (2020) of the Atlantic Meridional Overturning Circulation (AMOC) – part of the ‘global conveyor belt’ – examined foraminifera fossils obtained from a clay core extracted from the sea-floor off southern Greenland, and analyzed their chemical footprints to see what they said about the history of the AMOC current.

The study found that today’s climate change may replicate prehistoric events, when multiple episodes of warming caused rapid swings in the current’s strength, unbalancing the climate system and alternately chilling and warming Europe. 18

Does A Slowdown In The Atlantic Meridional Overturning Circulation Make Global Warming Worse?

Yes. The deep sea is kept at a cold temperature by downwelling of cold, dense water in the polar regions of the North Atlantic and Southern oceans, from where it spreads around the globe via the global conveyor belt. All things being equal, the temperatures of the deep ocean are maintained at a constant level by the balance of two adversarial trends: a warming tendency from above – caused by climate change – and this downwelling water from the poles. If the rate of thermohaline circulation slows down – that is to say, if less downwelling occurs – heat diffusion will gain the upper hand and the deep ocean will warm. 19

References

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  4. Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences, Edited by S. A. Elias. Elsevier, Amsterdam. Rahmstorf, S., 2006 []
  5. Some aspects of ocean heat transport by the shallow, intermediate and deep overturning circulations”. Geophysical Monographs. 112: 1–22. Talley, Lynne (1999). []
  6. Antarctic offshore polynyas linked to Southern Hemisphere climate anomalies.” Ethan C. Campbell, Earle A. Wilson, G. W. Kent Moore, Stephen C. Riser, Casey E. Brayton, Matthew R. Mazloff & Lynne D. Talley. Nature volume 570, pages 319–325 (2019). []
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  9. Ocean biogeochemical dynamics. Princeton University Press. Sarmiento, Jorge L.; Gruber, Nicolas (2006). []
  10. “Ocean Motion: Definition: Wind Driven Surface Currents – Upwelling and Downwelling”. Lindstrom, Eric J. oceanmotion.org. []
  11. Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation.” (PDF) Rahmstorf, Stefan; Box, Jason E.; Feulner, Georg; Mann, Michael E.; Robinson, Alexander; Rutherford, Scott; Schaffernicht, Erik J. (2015). Nature Climate Change. 5 (5): 475–480. []
  12. The Atlantic Meridional Overturning Circulation and Abrupt Climate Change.” Lynch-Stieglitz, Jean. Annual Review of Marine Science, vol. 9, p.83-104. January 2017. []
  13. Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years”. Nature. 556 (7700): 227–230. Thornalley, David J. R.; Oppo, Delia W.; Ortega, Pablo; Robson, Jon I.; Brierley, Chris M.; Davis, Renee; Hall, Ian R.; Moffa-Sanchez, Paola; Rose, Neil L.; Spooner, Peter T.; Yashayaev, Igor; Keigwin, Lloyd D. (11 April 2018). []
  14. Observed fingerprint of a weakening Atlantic Ocean overturning circulation”. Nature. 556 (7700): 191–196. Caesar, L.; Rahmstorf, S.; Robinson, A.; Feulner, G.; Saba, V. (11 April 2018). []
  15. Chapter 6: Extremes, Abrupt Changes and Managing Risks (PDF). IPCC (Report). Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). September 25, 2019. []
  16. Shifting seas in the greenhouse?” Nature, 1999. 399: p.523-524. Rahmstorf, S. []
  17. “North Atlantic Drift (Gulf Stream).” Weather Online. weatheronline.co.uk/ []
  18. Interglacial instability of North Atlantic Deep Water ventilation.” Eirik Vinje Galaasen, et al. Science 27 Mar 2020: Vol. 367, Issue 6485, pp. 1485-1489. []
  19. Does a slow AMOC increase the rate of global warming?” Stefan Rahmstorf, Michael Mann. RealClimate.org. July 18, 2018. []
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