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— CH. 1 · GLOBAL CIRCULATION FRAMEWORK —

Southern Ocean overturning circulation

~4 min read · Ch. 1 of 7
7 sections

  • The Southern Ocean overturning circulation operates as the southern half of Earth's global thermohaline system. This massive current connects water basins across the entire planet, linking with its better-known northern counterpart known as the Atlantic meridional overturning circulation. Warm, oxygenated, and nutrient-poor water travels into the deep ocean through downwelling processes. Cold, oxygen-limited, and nutrient-rich water moves upward at specific points in a process called upwelling. Thermohaline circulation transports not only vast volumes of warm and cold water but also dissolved oxygen and organic carbon. These movements play an essential role in Earth's climate system by affecting the energy budget and oceanic carbon cycle.

  • Southern Ocean overturning circulation consists of two distinct parts: the upper cell and the lower cell. The smaller upper cell is most strongly affected by winds due to its proximity to the surface. Around 27 ± 7 Sverdrup of deep water wells up to the surface in the Southern Ocean. This upwelled water transforms into lighter water measuring 22 ± 4 Sv and denser water measuring 5 ± 5 Sv. The larger lower cell behavior is defined by the temperature and salinity of Antarctic bottom water. Observations suggest that approximately 80 percent of global deep water is upwelled in the Southern Ocean. Circulation remains a slow process where upwelling from depths to the surface mixed layer takes 60, 90 years for just half of the water mass.

  • Scientists face a challenge regarding vertical mixing in the thermocline when dense water returns to the surface. The missing-mixing paradox assumes that dense water must be upwelled through the thermocline to close the circulation loop. To achieve this, vertical mixing is needed in the thermocline, which is not observed in measurements. Instead, dense water from sinking regions returned to the surface in nearly adiabatic pathways along density isopycnals. Harald Sverdrup already wrote about this phenomenon in his 1933 research on vertical circulation in the ocean due to wind action within the Antarctic Circumpolar Current. This pathway allows water to move without the expected turbulent mixing that models previously predicted would occur.

  • The Southern Ocean plays the greatest role in carbon uptake among all oceans, responsible for around 40% of human-caused emissions absorption. In the 2000s, some research suggested climate-driven changes were reducing carbon absorption, but subsequent findings showed the sink was even stronger by 14% to 18%. Deep water brings dissolved carbon concentrations much lower than modern surface waters before being transported back to depths. Regions where deep warm circumpolar carbon-rich waters reach the surface outgas through exposure to the atmosphere. Ocean upwelling also brings mineral nutrients like iron to the surface for consumption by phytoplankton. The biological pump moves dead phytoplankton and organic matter to depths before decomposition releases carbon back into the atmosphere.

  • Human-caused greenhouse gas emissions have caused increased warming with over 90% of total global heating since 1971 occurring in extratropical Southern Hemisphere oceans south of 30°S. West Antarctica upper layer ocean temperature has warmed significantly since 1955 while the Antarctic Circumpolar Current warms faster than the global average. Greater warming increases ice loss from Antarctica generating more fresh meltwater at a rate of 1100, 1500 billion tons per year. This freshening causes increased stratification and stabilization of layers with the single largest impact on long-term circulation properties. Since the 1970s, the upper cell has strengthened by 3, 4 sverdrup representing 50, 60% of its flow while the lower cell weakened by a similar amount but represents only 10, 20% weakening due to larger volume.

  • Paleoclimate evidence shows that the entire circulation had strongly weakened or outright collapsed before during past periods both warmer and colder than now. During the Last Glacial Maximum, it was too weak to flow out of the Weddell Sea making the overturning circulation much weaker than today. It was also weaker during periods warmer than current conditions. Some preliminary research suggests such a collapse may become likely once global warming reaches levels between unknown thresholds. There is far less certainty than with estimates for most other tipping points in the climate system. Southern Hemisphere contains only 10% of the world's population so this ocean has historically received much less attention than the Atlantic meridional overturning circulation.

  • A study suggests the circulation would lose half its strength by 2050 under the worst climate change scenario with greater losses occurring afterwards. Even if the circulation's collapse starts in the near future, it is unlikely to be complete until close to 2300. Impacts such as reduction in precipitation in the Southern Hemisphere with corresponding increase in the North are expected to unfold over multiple centuries. Scientists expect loss of nutrients from Antarctic bottom water diminishing ocean productivity potentially leading to extinction of some fish species. Reduced marine productivity means the ocean absorbs less carbon which could increase ultimate long-term warming response to anthropogenic emissions. Climate models currently disagree on whether the circulation would continue responding to changes in the Southern Annular Mode or adjust differently.

Common questions

What is the Southern Ocean overturning circulation?

The Southern Ocean overturning circulation operates as the southern half of Earth's global thermohaline system. This massive current connects water basins across the entire planet, linking with its better-known northern counterpart known as the Atlantic meridional overturning circulation.

How much deep water wells up to the surface in the Southern Ocean?

Around 27 ± 7 Sverdrup of deep water wells up to the surface in the Southern Ocean. Observations suggest that approximately 80 percent of global deep water is upwelled in the Southern Ocean.

When did Harald Sverdrup write about vertical circulation in the ocean due to wind action within the Antarctic Circumpolar Current?

Harald Sverdrup already wrote about this phenomenon in his 1933 research on vertical circulation in the ocean due to wind action within the Antarctic Circumpolar Current. This pathway allows water to move without the expected turbulent mixing that models previously predicted would occur.

Why does the Southern Ocean play the greatest role in carbon uptake among all oceans?

The Southern Ocean plays the greatest role in carbon uptake among all oceans, responsible for around 40% of human-caused emissions absorption. Deep water brings dissolved carbon concentrations much lower than modern surface waters before being transported back to depths.

What happens to the upper cell and lower cell of the Southern Ocean overturning circulation since the 1970s?

Since the 1970s, the upper cell has strengthened by 3, 4 sverdrup representing 50, 60% of its flow while the lower cell weakened by a similar amount but represents only 10, 20% weakening due to larger volume. Greater warming increases ice loss from Antarctica generating more fresh meltwater at a rate of 1100, 1500 billion tons per year.

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All sources

47 references cited across the entry

  1. 1journalMassive Southern Ocean phytoplankton bloom fed by iron of possible hydrothermal originCasey M. S. Schine et al. — 22 February 2021
  2. 2journalClosure of the meridional overturning circulation through Southern Ocean upwellingJohn Marshall et al. — 26 February 2012
  3. 3journalThe southern ocean meridional overturning in the sea-ice sector is driven by freshwater fluxesViolaine Pellichero et al. — 3 May 2018
  4. 4journalSlowdown of Antarctic Bottom Water export driven by climatic wind and sea-ice changesShenjie Zhou et al. — 12 June 2023
  5. 5webSlowing deep Southern Ocean current may be linked to natural climate cycle—but melting Antarctic ice is still a concernAlessandro Silvano et al. — The Conversation — 17 June 2023
  6. 6journalResponse of the Southern Ocean Overturning Circulation to Extreme Southern Annular Mode ConditionsK. D. Stewart et al. — 2 November 2020
  7. 7journalStratification constrains future heat and carbon uptake in the Southern Ocean between 30°S and 55°STimothée Bourgeois et al. — 17 January 2022
  8. 8journalFreshening by glacial meltwater enhances the melting of ice shelves and reduces the formation of Antarctic Bottom WaterAlessandro Silvano et al. — 18 April 2018
  9. 9journalWarm Modified Circumpolar Deep Water Intrusions Drive Ice Shelf Melt and Inhibit Dense Shelf Water Formation in Vincennes Bay, East AntarcticaN. Ribeiro et al. — 15 July 2021
  10. 10journalReduced Deep Convection and Bottom Water Formation Due To Antarctic Meltwater in a Multi-Model EnsembleJia-Jia Chen et al. — 28 December 2023
  11. 11journalHuman-induced changes in the global meridional overturning circulation are emerging from the Southern OceanSang-Ki Lee et al. — 13 March 2023
  12. 12webNOAA Scientists Detect a Reshaping of the Meridional Overturning Circulation in the Southern OceanNOAA — 29 March 2023
  13. 13journalAbyssal ocean overturning slowdown and warming driven by Antarctic meltwaterQian Li et al. — 29 March 2023
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  24. 24journalSaturation of the Southern Ocean CO 2 Sink Due to Recent Climate ChangeCorinne Le QuéRé et al. — 22 June 2007
  25. 25journalStrong Southern Ocean carbon uptake evident in airborne observationsMatthew C. Long et al. — 2 December 2021
  26. 26journalSouthern Ocean anthropogenic carbon sink constrained by sea surface salinityJens Terhaar et al. — 28 April 2021
  27. 27journalDynamically and Observationally Constrained Estimates of Water-Mass Distributions and Ages in the Global OceanTim Devries et al. — 2011
  28. 28journalA deep-learning estimate of the decadal trends in the Southern Ocean carbon storageVarvara E. Zemskova et al. — 13 July 2022
  29. 29journalThe impact of Southern Ocean residual upwelling on atmospheric CO2 on centennial and millennial timescalesJonathan M. Lauderdale et al. — 2017
  30. 30journalBiogenic carbon pool production maintains the Southern Ocean carbon sinkYibin Huang et al. — 26 April 2023
  31. 31journalEmissions – the 'business as usual' story is misleadingZeke Hausfather et al. — 29 January 2020
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  33. 33inlineIPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Chan
  34. 34journalHeat stored in the Earth system: where does the energy go?K. von Schuckmann et al. — 7 September 2020
  35. 35journalImproved estimates of ocean heat content from 1960 to 2015Lijing Cheng et al. — 2017
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  39. 39journalIntense ocean freshening from melting glacier around the Antarctica during early twenty-first centuryXianliang L. Pan et al. — 10 January 2022
  40. 40journalSea-ice transport driving Southern Ocean salinity and its recent trendsF. Alexander Haumann et al. — September 2016
  41. 41newsOcean current scientists are very upset and you should be tooFirst Dog on the Moon — 7 July 2025
  42. 42journalOcean warming and accelerating Southern Ocean zonal flowJia-Rui Shi et al. — Springer Science and Business Media LLC — 2021-11-29
  43. 43journalReversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010sS. Aoki et al. — 15 September 2020
  44. 44journalRecent reduced abyssal overturning and ventilation in the Australian Antarctic BasinKathryn L. Gunn et al. — 25 May 2023
  45. 45journalMarine ice sheet instability amplifies and skews uncertainty in projections of future sea-level riseAlexander A. Robel et al. — 23 July 2019
  46. 46journalFate of the Atlantic Meridional Overturning Circulation: Strong decline under continued warming and Greenland meltingP Bakker et al. — 11 November 2016
  47. 47journalNo detectable Weddell Sea Antarctic Bottom Water export during the Last and Penultimate Glacial MaximumHuang Huang et al. — 22 January 2020