Southern Ocean overturning circulation

Last updated
A schematic overview of the Southern Ocean overturning circulation. The arrows point in the direction of the water movement. The lower cell of the circulation is depicted by the upwelling arrows south of the Antarctic Circumpolar Current (ACC) and the formation of Antarctic Bottom Water beneath the sea ice of Antarctica due to buoyancy loss. The upper cell is depicted by the upwelling arrows north of the ACC and the formation of lighter Antarctic Intermediate water due to buoyancy gain north of the ACC. Southern Ocean overturning circulation.jpg
A schematic overview of the Southern Ocean overturning circulation. The arrows point in the direction of the water movement. The lower cell of the circulation is depicted by the upwelling arrows south of the Antarctic Circumpolar Current (ACC) and the formation of Antarctic Bottom Water beneath the sea ice of Antarctica due to buoyancy loss. The upper cell is depicted by the upwelling arrows north of the ACC and the formation of lighter Antarctic Intermediate water due to buoyancy gain north of the ACC.

Southern Ocean overturning circulation (sometimes referred to as the Southern Meridional overturning circulation (SMOC) [1] or Antarctic overturning circulation) is the southern half of a global thermohaline circulation, which connects different water basins across the global ocean. Its better-known northern counterpart is the Atlantic meridional overturning circulation (AMOC). This circulation operates when certain currents send warm, oxygenated, nutrient-poor water into the deep ocean (downwelling), while the cold, oxygen-limited, nutrient-rich water travels upwards (or upwells) at specific points. Thermohaline circulation transports not only massive volumes of warm and cold water across the planet, but also dissolved oxygen, dissolved organic carbon and other nutrients such as iron. [2] Thus, both halves of the circulation have a great effect on Earth's energy budget and oceanic carbon cycle, and so play an essential role in the Earth's climate system. [3] [4]

Contents

Southern ocean overturning circulation itself consists of two parts, the upper and the lower cell. The smaller upper cell is most strongly affected by winds due to its proximity to the surface, while the behaviour of the larger lower cell is defined by the temperature and salinity of Antarctic bottom water. [5] The strength of both halves had undergone substantial changes in the recent decades: the flow of the upper cell has increased by 50-60% since 1970s, while the lower cell has weakened by 10-20%. [6] [3] Some of this has been due to the natural cycle of Interdecadal Pacific Oscillation, [7] [8] but climate change has also played a substantial role in both trends, as it had altered the Southern Annular Mode weather pattern, [9] [7] while the massive growth of ocean heat content in the Southern Ocean [10] has increased the melting of the Antarctic ice sheets, and this fresh meltwater dilutes salty Antarctic bottom water. [11] [12]

As the formation of dense and cold waters weakens near the coast while the flow of warm waters towards the coast strengthens, the surface waters become less likely to sink downwards and mix with the lower layers. [13] Consequently, ocean stratification increases. [6] [3] One study suggests that the circulation would lose half its strength by 2050 under the worst climate change scenario, [14] with greater losses occurring afterwards. [15] This slowdown would have important effects on the global climate due to the strength of the Southern Ocean as a global carbon sink and heat sink. For instance, global warming will reach 2 °C (3.6 °F) in all scenarios where greenhouse gas emissions have not been strongly lowered, but the exact year depends on the status of the circulation more than any factor other than the overall emissions. [16]

Paleoclimate evidence shows that the entire circulation had strongly weakened or outright collapsed before: some preliminary research suggests that such a collapse may become likely once global warming reaches levels between 1.7 °C (3.1 °F) and 3 °C (5.4 °F). However, there is far less certainty than with the estimates for most other tipping points in the climate system. [16] Even if initiated in the near future, the circulation's collapse is unlikely to be complete until close to 2300, [1] Similarly, impacts such as the reduction in precipitation in the Southern Hemisphere, with a corresponding increase in the North, or a decline of fisheries in the Southern Ocean with a potential collapse of certain marine ecosystems, are also expected to unfold over multiple centuries. [15]

Dynamics

3D representation of North Atlantic Deep Water upwelling in the Southern Ocean basin, which closes the connection between the Atlantic and Southern circulation, and takes place along the defined pathways with limited mixing. Tamsitt 2017 NADW upwelling.jpg
3D representation of North Atlantic Deep Water upwelling in the Southern Ocean basin, which closes the connection between the Atlantic and Southern circulation, and takes place along the defined pathways with limited mixing.

Southern Ocean overturning circulation consists of two cells in the Southern Ocean, which are driven by upwelling and downwelling. The upwelling in the upper cell is associated with mid-deep water that is brought to the surface, whereas the upwelling in the lower cell is linked to the fresh and abyssal waters around Antarctica. Around 27 ± 7 Sverdrup (Sv) of deep water wells up to the surface in the Southern Ocean. This upwelled water is partly transformed to lighter water and denser water, respectively 22 ± 4 Sv and 5 ± 5 Sv. The densities of these waters change due to heat and buoyancy fluxes which result in upwelling in the upper cell and downwelling in the lower cell. [5]

The Southern Ocean plays a key role in the closure of the Atlantic meridional overturning circulation by compensating for the North Atlantic downwelling by upwelling of North Atlantic Deep Water and connects the interior ocean to the surface. This upwelling is induced by the strong westerly winds that blow over the ACC. [4] [17] Observations suggest that approximately 80 percent of global deep water is upwelled in the Southern Ocean. [18] Circulation is a slow process - for instance, the upwelling of North Atlantic Deep Water from the depths of 1,000–3,500 m (3,281–11,483 ft) to the surface mixed layer takes 60–90 years for just half of the water mass, and some water travels to the surface for more than a century. [17]

Upper cell

The upper cell is driven by wind generated flow, a result of the Westerlies, that brings water from the Circumpolar Deep Water (CDW) to the surface. [19] Zonal wind stress induces upwelling near the pole and downwelling at the equator due to the zonal surface-wind maximum. This wind-driven circulation is also called the Deacon cell and acts to overturn water supporting the thermal wind current of the Antarctic Circumpolar Current (ACC) and creating a storage of potential energy. This upper cell process is also known as Ekman transport. [4]

The meridional overturning flow is from the north to the south in deep waters and from the south to the north at the ocean surface. At the surface deep waters are exposed to the atmosphere and surface buoyancy forces. There is a net gain of buoyancy in the upper cell as a result of the freshening of the water caused by precipitation and the melting of sea ice during summer (on the Southern Hemipshere). This buoyancy gain transforms the waters into lighter, less dense waters, such as Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). Around 22 ± 4 Sv of the total upwelled water in the overturning circulation is transformed into lighter waters in the upper cell. The overturning process of density surfaces is balanced through the baroclinic instability of the thermal wind currents. This instability flattens the density surfaces and the transport towards the poles, resulting in energetic, time-dependent eddying motions. The potential energy from the wind-driven circulation is then flattened out by eddies. [5]

Missing-mixing paradox

The missing-mixing paradox assumes that dense water is upwelled through the thermocline to close the circulation. To achieve this, vertical mixing is needed in the thermocline, which is not observed. [20] Instead, dense water from sinking regions returned to the surface in nearly adiabatic pathways along density isopycnals, which was already written by Harald Sverdrup (oceanographer). [21]

Lower cell

The role of seasonal meltwater from the Antarctic ice sheet in driving the lower-cell circulation. Pellichero 2018 Southern Ocean mixed layer.jpg
The role of seasonal meltwater from the Antarctic ice sheet in driving the lower-cell circulation.

The lower cell is driven by freshwater fluxes where sea-ice formation and melting play an important role. [5] The formation of sea-ice is accompanied by brine rejection, resulting in water with a higher salinity and density and therefore buoyancy loss. When ice melts there is a freshwater flow and exposure to the atmosphere. If water turns into ice, there is more salt in the water and less exposure to the atmosphere. Due to seasonal variations, there is a gain of buoyancy during summer and a loss of buoyancy in winter. This cold and dense water filled with salt is called Dense Shelf Water (DSW). DSW is then transformed into Antarctic Bottom Water (AABW), originating from the Ross Sea, Weddell Sea and along the eastern coast of Antarctica. Around 5 ± 5 Sv of AABW is formed in the lower cell of the Southern Ocean circulation, which is around a third of the total AABW formation. [22] [23] [24]

Global carbon cycle

In the 1990s and 2000s, the concentration of dissolved organic carbon at the surface had been decreasing, as more was pushed to the depths through the circulation. In the 2010s, however, the weakening circulation moved less carbon downwards, and its concentration started to increase across the surface. Zemskova 2022 Southern Ocean DIC.png
In the 1990s and 2000s, the concentration of dissolved organic carbon at the surface had been decreasing, as more was pushed to the depths through the circulation. In the 2010s, however, the weakening circulation moved less carbon downwards, and its concentration started to increase across the surface.

The ocean is in normally in equilibrium with the atmospheric carbon dioxide concentration. The increase in atmospheric CO2 since the Industrial Revolution had turned the oceans into a net carbon sink, and they absorb around 25% of human-caused emissions. [26] Out of all oceans, the Southern Ocean plays the greatest role in carbon uptake, and on its own, it is responsible for around 40%. [27] [28] [29] In 2000s, some research suggested that climate-driven changes to Southern Hemisphere winds were reducing the amount of carbon it absorbed, [30] but subsequent research found that this carbon sink had been even stronger than estimated earlier, by some 14% to 18%. [27] [28] Ocean circulation is very important for this process, as it brings deep water to the surface, which has not been there for centuries and so wasn't in contact with anthropogenic emissions before. Thus, deep water's dissolved carbon concentrations are much lower than of the modern surface waters, and it absorbs a lot more carbon before it's transported back to the depths through downwelling. [31] [25]

On the other hand, regions where deep warm circumpolar carbon rich waters are brought to the surface through upwelling, outgas CO2 through exposure to the atmosphere, partly compensating the carbon sink effect of the overturning circulation. [32] Additionally, ocean upwelling brings mineral nutrients such as iron from the depths to the surface, which are then consumed by phytoplankton and allow them to increase their numbers, enhancing ocean primary production and boosting the carbon sink due to greater photosynthesis. [2] At the same time, downwelling circulation moves much of dead phytoplankton and other organic matter to the depths before it could decompose at the surface and release CO2 back to the atmosphere. This so-called biological pump is so important that a completely abiotic Southern Ocean, where this pump would be absent, would also be a net source of CO2. [29]

Climate change impacts

Even under the most intense climate change scenario, which is currently considered unlikely, the Southern Ocean would continue to function as a strong sink in the 21st century, and take up an increasing amount of carbon dioxide (left) and heat (middle). However, it would take up a smaller fraction of heat per every additional degree of warming than it does now (right), as well as a smaller fraction of emissions. Bourgeois 2022 Southern Ocean uptake.png
Even under the most intense climate change scenario, which is currently considered unlikely, the Southern Ocean would continue to function as a strong sink in the 21st century, and take up an increasing amount of carbon dioxide (left) and heat (middle). However, it would take up a smaller fraction of heat per every additional degree of warming than it does now (right), as well as a smaller fraction of emissions.

As human-caused greenhouse gas emissions cause increased warming, one of the most notable effects of climate change on oceans is the increase in ocean heat content, which accounted for over 90% of the total global heating since 1971. [36] Since 2005, from 67% to 98% of this increase has occurred in the Southern Ocean. [9] In West Antarctica, the temperature in the upper layer of the ocean has warmed 1 °C (1.8 °F) since 1955, and the Antarctic Circumpolar Current (ACC) is also warming faster than the global average. [37] This warming directly affects the flow of warm and cold water masses which make up the overturning circulation, and it also has negative impacts on sea ice cover in Southern Hemisphere, (which is highly reflective and so elevates the albedo of Earth's surface), as well as mass balance of Antarctica's ice shelves and peripheral glaciers. [38] For these reasons, climate models consistently show that the year when global warming will reach 2 °C (3.6 °F) (inevitable in all climate change scenarios where greenhouse gas emissions have not been strongly lowered) depends on the status of the circulation more than any other factor besides the emissions themselves. [16]

Greater warming of this ocean water increases ice loss from Antarctica, and also generates more fresh meltwater, at a rate of 1100-1500 billion tons (GT) per year. [38] : 1240  This meltwater from the Antarctic ice sheet then mixes back into the Southern Ocean, making its water fresher. [39] This freshening of the Southern Ocean results in increased stratification and stabilization of its layers, [40] [38] : 1240  and this has the single largest impact on the long-term properties of Southern Ocean circulation. [14] These changes in the Southern Ocean cause the upper cell circulation to speed up, accelerating the flow of major currents, [41] while the lower cell circulation slows down, as it is dependent on the highly saline Antarctic bottom water, which already appears to have been observably weakened by the freshening, in spite of the limited recovery during 2010s. [11] [42] [43] [38] : 1240  Since the 1970s, the upper cell has strengthened by 3-4 sverdrup (Sv; represents a flow of 1 million cubic meters per second), or 50-60% of its flow, while the lower cell has weakened by a similar amount, but because of its larger volume, these changes represent a 10-20% weakening. [6] [3] However, they weren't fully caused by climate change, as the natural cycle of Interdecadal Pacific Oscillation had also played an important role. [7] [8]

Since the 1970s, the upper cell of the circulation has strengthened, while the lower cell weakened. NOAA SMOC changes.png
Since the 1970s, the upper cell of the circulation has strengthened, while the lower cell weakened.

Additionally, the main controlling pattern of the extratropical Southern Hemisphere's climate is the Southern Annular Mode (SAM), which has been spending more and more years in its positive phase due to climate change (as well as the aftermath of ozone depletion), which means more warming and more precipitation over the ocean due to stronger westerlies, freshening the Southern Ocean further. [9] [38] : 1240  Climate models currently disagree on whether the Southern Ocean circulation would continue to respond to changes in SAM the way it does now, or if it will eventually adjust to them. As of early 2020s, their best, limited-confidence estimate is that the lower cell would continue to weaken, while the upper cell may strengthen by around 20% over the 21st century. [38] A key reason for the uncertainty is the poor and inconsistent representation of ocean stratification in even the CMIP6 models - the most advanced generation available as of early 2020s. [10] Further, the largest long-term role in the state of the circulation is played by Antarctic meltwater, [14] and Antarctic ice loss had been the least-certain aspect of future sea level rise projections for a long time. [44]

Evidence suggests that the Antarctic bottom water requires a temperature range close to current conditions to be at full strength. During the Last Glacial Maximum (a cold period), it was too weak to flow out of the Weddell Sea and the overturning circulation was much weaker than now. It was also weaker during the periods warmer than now. Huang 2020 AABW Eemian.png
Evidence suggests that the Antarctic bottom water requires a temperature range close to current conditions to be at full strength. During the Last Glacial Maximum (a cold period), it was too weak to flow out of the Weddell Sea and the overturning circulation was much weaker than now. It was also weaker during the periods warmer than now.

Similar processes are taking place with Atlantic meridional overturning circulation (AMOC), which is also affected by the ocean warming and by meltwater flows from the declining Greenland ice sheet. [46] It is possible that both circulations may not simply continue to weaken in response to increased warming and freshening, but eventually collapse to a much weaker state outright, in a way which would be difficult to reverse and constitute an example of tipping points in the climate system. [16] There is paleoclimate evidence for the overturning circulation being substantially weaker than now during past periods that were both warmer and colder than now. [45] However, Southern Hemisphere is only inhabited by 10% of the world's population, and the Southern Ocean overturning circulation has historically received much less attention than the AMOC. Consequently, while multiple studies have set out to estimate the exact level of global warming which could result in AMOC collapsing, the timeframe over which such collapse may occur, and the regional impacts it would cause, much less equivalent research exists for the Southern Ocean overturning circulation as of the early 2020s. There has been a suggestion that its collapse may occur between 1.7 °C (3.1 °F) and 3 °C (5.4 °F), but this estimate is much less certain than for many other tipping points. [16]

The impacts of Southern Ocean overturning circulation collapse have also been less closely studied, though scientists expect them to unfold over multiple centuries. A notable example is the loss of nutrients from Antarctic bottom water diminishing ocean productivity and ultimately the state of Southern Ocean fisheries, potentially leading to the extinction of some species of fish, and the collapse of some marine ecosystems. [15] Reduced marine productivity would also mean that the ocean absorbs less carbon (though not within the 21st century [10] ), which could increase the ultimate long-term warming in response to anthropogenic emissions (thus raising the overall climate sensitivity) and/or prolong the time warming persists before it starts declining on the geological timescales. [1] There is also expected to be a decline in precipitation in the Southern Hemisphere countries like Australia, with a corresponding increase in the Northern Hemisphere. However, the decline or an outright collapse of the AMOC would have similar but opposite impacts, and the two would counteract each other up to a point. Both impacts would also occur alongside the other effects of climate change on the water cycle and effects of climate change on fisheries. [15]

Related Research Articles

<span class="mw-page-title-main">Antarctic Circumpolar Current</span> Ocean current that flows clockwise from west to east around Antarctica

Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100–150 Sverdrups, or possibly even higher, making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.

<span class="mw-page-title-main">Antarctic</span> Polar region around Earths South Pole

The Antarctic is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole.

<span class="mw-page-title-main">Downwelling</span> Process of accumulation and sinking of higher density material beneath lower density material

Downwelling is the downward movement of a fluid parcel and its properties within a larger fluid. It is closely related to upwelling, the upward movement of fluid.

<span class="mw-page-title-main">Ocean current</span> Directional mass flow of oceanic water generated by external or internal forces

An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents are primarily horizontal water movements.

<span class="mw-page-title-main">Thermohaline circulation</span> Part of large-scale ocean circulation

Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes. This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. The water in these circuits transport both energy and mass around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.

<span class="mw-page-title-main">Ocean gyre</span> Any large system of circulating ocean surface currents

In oceanography, a gyre is any large system of circulating ocean surface currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction determine the circulatory patterns from the wind stress curl (torque).

<span class="mw-page-title-main">West Antarctic Ice Sheet</span> Segment of the continental ice sheet that covers West (or Lesser) Antarctica

The West Antarctic Ice Sheet (WAIS) is the segment of the continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains that lies in the Western Hemisphere. It is classified as a marine-based ice sheet, meaning that its bed lies well below sea level and its edges flow into floating ice shelves. The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.

<span class="mw-page-title-main">Antarctic ice sheet</span> Earths southern polar ice cap

The Antarctic ice sheet is a continental glacier covering 98% of the Antarctic continent, with an area of 14 million square kilometres and an average thickness of over 2 kilometres (1.2 mi). It is the largest of Earth's two current ice sheets, containing 26.5 million cubic kilometres of ice, which is equivalent to 61% of all fresh water on Earth. Its surface is nearly continuous, and the only ice-free areas on the continent are the dry valleys, nunataks of the Antarctic mountain ranges, and sparse coastal bedrock. However, it is often subdivided into East Antarctic ice sheet (EAIS), West Antarctic ice sheet (WAIS), and Antarctic Peninsula (AP), due to the large differences in topography, ice flow, and glacier mass balance between the three regions.

Paleoceanography is the study of the history of the oceans in the geologic past with regard to circulation, chemistry, biology, geology and patterns of sedimentation and biological productivity. Paleoceanographic studies using environment models and different proxies enable the scientific community to assess the role of the oceanic processes in the global climate by the re-construction of past climate at various intervals. Paleoceanographic research is also intimately tied to paleoclimatology.

<span class="mw-page-title-main">Antarctic bottom water</span> Cold, dense, water mass originating in the Southern Ocean surrounding Antarctica

The Antarctic bottom water (AABW) is a type of water mass in the Southern Ocean surrounding Antarctica with temperatures ranging from −0.8 to 2 °C (35 °F) and absolute salinities from 34.6 to 35.0 g/kg. As the densest water mass of the oceans, AABW is found to occupy the depth range below 4000 m of all ocean basins that have a connection to the Southern Ocean at that level. AABW forms the lower branch of the large-scale movement in the world's oceans through thermohaline circulation.

<span class="mw-page-title-main">Atlantic meridional overturning circulation</span> System of surface and deep currents in the Atlantic Ocean

The Atlantic meridional overturning circulation (AMOC) is the "main current system in the South and North Atlantic Oceans". As such, it is a component of Earth's oceanic circulation system and plays an important role in the climate system. If the strength of the AMOC changes this could have impacts on some elements of the climate system. The AMOC includes currents at the surface as well as at great depths in the Atlantic Ocean. These currents are driven by changes in the atmospheric weather as well as by changes in temperature and salinity. They collectively make up one half of the global thermohaline circulation that encompasses the flow of major ocean currents. The other half is the Southern Ocean overturning circulation.

<span class="mw-page-title-main">Ocean heat content</span> Thermal energy stored in ocean water

Ocean heat content (OHC) is the energy absorbed and stored by oceans. To calculate the ocean heat content, it is necessary to measure ocean temperature at many different locations and depths. Integrating the areal density of ocean heat over an ocean basin or entire ocean gives the total ocean heat content. Between 1971 and 2018, the rise in ocean heat content accounted for over 90% of Earth's excess thermal energy from global heating. The main driver of this increase was anthropogenic forcing via rising greenhouse gas emissions. By 2020, about one third of the added energy had propagated to depths below 700 meters. In 2023, the world's oceans were again the hottest in the historical record and exceeded the previous 2022 record maximum. The five highest ocean heat observations to a depth of 2000 meters occurred in the period 2019–2023. The North Pacific, North Atlantic, the Mediterranean, and the Southern Ocean all recorded their highest heat observations for more than sixty years of global measurements. Ocean heat content and sea level rise are important indicators of climate change.

<span class="mw-page-title-main">Tipping points in the climate system</span> Large and possibly irreversible changes in the climate system

In climate science, a tipping point is a critical threshold that, when crossed, leads to large, accelerating and often irreversible changes in the climate system. If tipping points are crossed, they are likely to have severe impacts on human society and may accelerate global warming. Tipping behavior is found across the climate system, for example in ice sheets, mountain glaciers, circulation patterns in the ocean, in ecosystems, and the atmosphere. Examples of tipping points include thawing permafrost, which will release methane, a powerful greenhouse gas, or melting ice sheets and glaciers reducing Earth's albedo, which would warm the planet faster.

<span class="mw-page-title-main">Ross Gyre</span> Circulating system of ocean currents in the Ross Sea

The Ross Gyre is one of three gyres that exists within the Southern Ocean around Antarctica, the others being the Weddell Gyre and Balleny Gyre. The Ross Gyre is located north of the Ross Sea, and rotates clockwise. The gyre is formed by interactions between the Antarctic Circumpolar Current and the Antarctic Continental Shelf. The Ross Gyre is bounded by the Polar Front of the Antarctic Circumpolar Current to the north, the Antarctic Slope Current to the south, the Balleny Gyre to the west, and a variable boundary to the east from semiannual changes in sea surface height (SSH) in the Amundsen Sea. Circulation in the Ross Gyre has been estimated to be 20 ± 5 Sverdrup (Sv) and plays a large role in heat exchange in this region.

<span class="mw-page-title-main">Southern Ocean</span> Ocean around Antarctica

The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of 20,327,000 km2 (7,848,000 sq mi), it is regarded as the second-smallest of the five principal oceanic divisions: smaller than the Pacific, Atlantic, and Indian oceans but larger than the Arctic Ocean.

Deglaciation is the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice sheet or frozen surface layer, and the resulting exposure of the Earth's surface. The decline of the cryosphere due to ablation can occur on any scale from global to localized to a particular glacier. After the Last Glacial Maximum, the last deglaciation begun, which lasted until the early Holocene. Around much of Earth, deglaciation during the last 100 years has been accelerating as a result of climate change, partly brought on by anthropogenic changes to greenhouse gases.

The Great Salinity Anomaly (GSA) originally referred to an event in the late 1960s to early 1970s where a large influx of freshwater from the Arctic Ocean led to a salinity anomaly in the northern North Atlantic Ocean, which affected the Atlantic meridional overturning circulation. Since then, the term "Great Salinity Anomaly" has been applied to successive occurrences of the same phenomenon, including the Great Salinity Anomaly of the 1980s and the Great Salinity Anomaly of the 1990s. The Great Salinity Anomalies were advective events, propagating to different sea basins and areas of the North Atlantic, and is on the decadal-scale for the anomalies in the 1970s, 1980s, and 1990s.

<span class="mw-page-title-main">Climate change in Antarctica</span> Impacts of climate change on Antarctica

Climate change caused by greenhouse gas emissions from human activities occurs everywhere on Earth, and while Antarctica is less vulnerable to it than any other continent, climate change in Antarctica has already been observed. There has been an average temperature increase of >0.05 °C/decade since 1957 across the continent, although it had been uneven. While West Antarctica warmed by over 0.1 °C/decade from the 1950s to the 2000s and the exposed Antarctic Peninsula has warmed by 3 °C (5.4 °F) since the mid-20th century, the colder and more stable East Antarctica had been experiencing cooling until the 2000s. Around Antarctica, the Southern Ocean has absorbed more heat than any other ocean, with particularly strong warming at depths below 2,000 m (6,600 ft) and around the West Antarctic, which has warmed by 1 °C (1.8 °F) since 1955.

<span class="mw-page-title-main">Cold blob</span> Cold temperature anomaly North Atlantic surface waters

The cold blob in the North Atlantic describes a cold temperature anomaly of ocean surface waters, affecting the Atlantic Meridional Overturning Circulation (AMOC) which is part of the thermohaline circulation, possibly related to global warming-induced melting of the Greenland ice sheet.

Jean Lynch-Stieglitz is a paleoceanographer known for her research on reconstructing changes in ocean circulation over the last 100,000 years.

References

  1. 1 2 3 Liu, Y.; Moore, J. K.; Primeau, F.; Wang, W. L. (22 December 2022). "Reduced CO2 uptake and growing nutrient sequestration from slowing overturning circulation". Nature Climate Change. 13: 83–90. doi:10.1038/s41558-022-01555-7. OSTI   2242376. S2CID   255028552.
  2. 1 2 Schine, Casey M. S.; Alderkamp, Anne-Carlijn; van Dijken, Gert; Gerringa, Loes J. A.; Sergi, Sara; Laan, Patrick; van Haren, Hans; van de Poll, Willem H.; Arrigo, Kevin R. (22 February 2021). "Massive Southern Ocean phytoplankton bloom fed by iron of possible hydrothermal origin". Nature Communications. 12 (1): 1211. Bibcode:2021NatCo..12.1211S. doi:10.1038/s41467-021-21339-5. PMC   7900241 . PMID   33619262.
  3. 1 2 3 4 5 "NOAA Scientists Detect a Reshaping of the Meridional Overturning Circulation in the Southern Ocean". NOAA. 29 March 2023.
  4. 1 2 3 Marshall, John; Speer, Kevin (26 February 2012). "Closure of the meridional overturning circulation through Southern Ocean upwelling". Nature Geoscience. 5 (3): 171–180. Bibcode:2012NatGe...5..171M. doi:10.1038/ngeo1391.
  5. 1 2 3 4 5 Pellichero, Violaine; Sallée, Jean-Baptiste; Chapman, Christopher C.; Downes, Stephanie M. (3 May 2018). "The southern ocean meridional overturning in the sea-ice sector is driven by freshwater fluxes". Nature Communications. 9 (1): 1789. Bibcode:2018NatCo...9.1789P. doi:10.1038/s41467-018-04101-2. PMC   5934442 . PMID   29724994.
  6. 1 2 3 Lee, Sang-Ki; Lumpkin, Rick; Gomez, Fabian; Yeager, Stephen; Lopez, Hosmay; Takglis, Filippos; Dong, Shenfu; Aguiar, Wilton; Kim, Dongmin; Baringer, Molly (13 March 2023). "Human-induced changes in the global meridional overturning circulation are emerging from the Southern Ocean". Communications Earth & Environment. 4 (1): 69. Bibcode:2023ComEE...4...69L. doi:10.1038/s43247-023-00727-3.
  7. 1 2 3 Zhou, Shenjie; Meijers, Andrew J. S.; Meredith, Michael P.; Abrahamsen, E. Povl; Holland, Paul R.; Silvano, Alessandro; Sallée, Jean-Baptiste; Østerhus, Svein (12 June 2023). "Slowdown of Antarctic Bottom Water export driven by climatic wind and sea-ice changes". Nature Climate Change. 13: 701–709. doi:10.1038/s41558-023-01667-8.
  8. 1 2 Silvano, Alessandro; Meijers, Andrew J. S.; Zhou, Shenjie (17 June 2023). "Slowing deep Southern Ocean current may be linked to natural climate cycle—but melting Antarctic ice is still a concern". The Conversation.
  9. 1 2 3 Stewart, K. D.; Hogg, A. McC.; England, M. H.; Waugh, D. W. (2 November 2020). "Response of the Southern Ocean Overturning Circulation to Extreme Southern Annular Mode Conditions". Geophysical Research Letters. 47 (22): e2020GL091103. Bibcode:2020GeoRL..4791103S. doi:10.1029/2020GL091103. hdl: 1885/274441 . S2CID   229063736.
  10. 1 2 3 4 Bourgeois, Timothée; Goris, Nadine; Schwinger, Jörg; Tjiputra, Jerry F. (17 January 2022). "Stratification constrains future heat and carbon uptake in the Southern Ocean between 30°S and 55°S". Nature Communications. 13 (1): 340. Bibcode:2022NatCo..13..340B. doi:10.1038/s41467-022-27979-5. PMC   8764023 . PMID   35039511.
  11. 1 2 Silvano, Alessandro; Rintoul, Stephen Rich; Peña-Molino, Beatriz; Hobbs, William Richard; van Wijk, Esmee; Aoki, Shigeru; Tamura, Takeshi; Williams, Guy Darvall (18 April 2018). "Freshening by glacial meltwater enhances the melting of ice shelves and reduces the formation of Antarctic Bottom Water". Science Advances. 4 (4): eaap9467. doi:10.1126/sciadv.aap9467. PMC   5906079 . PMID   29675467.
  12. ↑ Ribeiro, N.; Herraiz‐Borreguero, L.; Rintoul, S. R.; McMahon, C. R.; Hindell, M.; Harcourt, R.; Williams, G. (15 July 2021). "Warm Modified Circumpolar Deep Water Intrusions Drive Ice Shelf Melt and Inhibit Dense Shelf Water Formation in Vincennes Bay, East Antarctica". Journal of Geophysical Research: Oceans. 126 (8). doi:10.1029/2020JC016998. ISSN   2169-9275.
  13. ↑ Chen, Jia‐Jia; Swart, Neil C.; Beadling, Rebecca; Cheng, Xuhua; Hattermann, Tore; Jüling, André; Li, Qian; Marshall, John; Martin, Torge; Muilwijk, Morven; Pauling, Andrew G.; Purich, Ariaan; Smith, Inga J.; Thomas, Max (28 December 2023). "Reduced Deep Convection and Bottom Water Formation Due To Antarctic Meltwater in a Multi‐Model Ensemble". Geophysical Research Letters. 50 (24). doi: 10.1029/2023GL106492 . ISSN   0094-8276.
  14. 1 2 3 Li, Qian; England, Matthew H.; Hogg, Andrew McC.; Rintoul, Stephen R.; Morrison, Adele K. (29 March 2023). "Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater". Nature. 615 (7954): 841–847. Bibcode:2023Natur.615..841L. doi:10.1038/s41586-023-05762-w. PMID   36991191. S2CID   257807573.
  15. 1 2 3 4 Logan, Tyne (29 March 2023). "Landmark study projects 'dramatic' changes to Southern Ocean by 2050". ABC News.
  16. 1 2 3 4 5 Lenton, T. M.; Armstrong McKay, D.I.; Loriani, S.; Abrams, J.F.; Lade, S.J.; Donges, J.F.; Milkoreit, M.; Powell, T.; Smith, S.R.; Zimm, C.; Buxton, J.E.; Daube, Bruce C.; Krummel, Paul B.; Loh, Zoë; Luijkx, Ingrid T. (2023). The Global Tipping Points Report 2023 (Report). University of Exeter.
  17. 1 2 3 Tamsitt, Veronica; Drake, Henri F.; Morrison, Adele K.; Talley, Lynne D.; Dufour, Carolina O.; Gray, Alison R.; Griffies, Stephen M.; Mazloff, Matthew R.; Sarmiento, Jorge L.; Wang, Jinbo; Weijer, Wilbert (2 August 2017). "Spiraling pathways of global deep waters to the surface of the Southern Ocean". Nature Communications. 8 (1): 172. Bibcode:2017NatCo...8..172T. doi:10.1038/s41467-017-00197-0. PMC   5541074 . PMID   28769035.
  18. ↑ Talley, Lynne (2013). "Closure of the Global Overturning Circulation Through the Indian, Pacific, and Southern Oceans: Schematics and Transports". Oceanography. 26 (1): 80–97. doi: 10.5670/oceanog.2013.07 . JSTOR   24862019.
  19. ↑ Gill, A.E.; Green, J.S.A.; Simmons, A.J. (1974). "Energy partition in the large-scale ocean circulation and the production of mid-ocean eddies". Deep Sea Research and Oceanographic Abstracts. 21 (7): 499–528. Bibcode:1974DSRA...21..499G. doi:10.1016/0011-7471(74)90010-2.
  20. ↑ St. Laurent, L. C.; Ledwell, J. R.; Girton, J. B.; Toole, J. M. (2011). "Diapycnal Mixing in the Antarctic Circumpolar Current". Journal of Physical Oceanography. 41 (1): 241–246. Bibcode:2011JPO....41..241L. doi:10.1175/2010JPO4557.1. hdl: 1912/4409 . S2CID   55251243.
  21. ↑ Sverdrup, H. U. On vertical circulation in the ocean due to the action of the wind with application to conditions within the Antarctic Circumpolar Current. Discov. Rep. VII, 139–170 (1933).
  22. ↑ Tamura, Takeshi; Ohshima, Kay I.; Nihashi, Sohey (2008). "Mapping of sea ice production for Antarctic coastal polynyas". Geophysical Research Letters. 35 (7). Bibcode:2008GeoRL..35.7606T. doi: 10.1029/2007GL032903 . S2CID   128716199.
  23. ↑ Williams, G. et al. Antarctic bottom water from the adélie and george v land coast, east antarctica (140–149°e). J. Geophys. Res. Oceans 115 (2010)
  24. ↑ Ohshima, Kay I.; Fukamachi, Yasushi; Williams, Guy D.; Nihashi, Sohey; Roquet, Fabien; Kitade, Yujiro; Tamura, Takeshi; Hirano, Daisuke; Herraiz-Borreguero, Laura; Field, Iain; Hindell, Mark; Aoki, Shigeru; Wakatsuchi, Masaaki (2013). "Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya". Nature Geoscience. 6 (3): 235. Bibcode:2013NatGe...6..235O. doi:10.1038/ngeo1738.
  25. 1 2 Zemskova, Varvara E.; He, Tai-Long; Wan, Zirui; Grisouard, Nicolas (13 July 2022). "A deep-learning estimate of the decadal trends in the Southern Ocean carbon storage". Nature Communications. 13 (1): 4056. Bibcode:2022NatCo..13.4056Z. doi:10.1038/s41467-022-31560-5. PMC   9279406 . PMID   35831323.
  26. ↑ Friedlingstein, Pierre; O'Sullivan, Michael; Jones, Matthew W.; et al. (5 December 2023). "Global Carbon Budget 2023". Earth System Science Data. 15 (12): 5301–5369. doi: 10.5194/essd-15-5301-2023 . hdl: 10871/134742 .
  27. 1 2 Long, Matthew C.; Stephens, Britton B.; McKain, Kathryn; Sweeney, Colm; Keeling, Ralph F.; Kort, Eric A.; Morgan, Eric J.; Bent, Jonathan D.; Chandra, Naveen; Chevallier, Frederic; Commane, Róisín; Daube, Bruce C.; Krummel, Paul B.; Loh, Zoë; Luijkx, Ingrid T.; Munro, David; Patra, Prabir; Peters, Wouter; Ramonet, Michel; Rödenbeck, Christian; Stavert, Ann; Tans, Pieter; Wofsy, Steven C. (2 December 2021). "Strong Southern Ocean carbon uptake evident in airborne observations". Science. 374 (6572): 1275–1280. Bibcode:2021Sci...374.1275L. doi:10.1126/science.abi4355. PMID   34855495. S2CID   244841359.
  28. 1 2 Terhaar, Jens; Frölicher, Thomas L.; Joos, Fortunat (28 April 2021). "Southern Ocean anthropogenic carbon sink constrained by sea surface salinity" (PDF). Science Advances. 7 (18): 1275–1280. Bibcode:2021Sci...374.1275L. doi:10.1126/science.abi4355. PMID   34855495. S2CID   244841359.
  29. 1 2 Huang, Yibin; Fassbender, Andrea J.; Bushinsky, Seth M. (26 April 2023). "Biogenic carbon pool production maintains the Southern Ocean carbon sink". Proceedings of the National Academy of Sciences. 120 (18): e2217909120. Bibcode:2023PNAS..12017909H. doi:10.1073/pnas.2217909120. PMC   10160987 .
  30. ↑ Le QuéRé, Corinne; RöDenbeck, Christian; Buitenhuis, Erik T.; Conway, Thomas J.; Langenfelds, Ray; Gomez, Antony; Labuschagne, Casper; Ramonet, Michel; Nakazawa, Takakiyo; Metzl, Nicolas; Gillett, Nathan; Heimann, Martin (22 June 2007). "Saturation of the Southern Ocean CO 2 Sink Due to Recent Climate Change". Science. 316 (5832): 1735–1738. doi: 10.1126/science.1136188 . PMID   17510327. S2CID   34642281.
  31. ↑ Devries, Tim; Primeau, François (2011). "Dynamically and Observationally Constrained Estimates of Water-Mass Distributions and Ages in the Global Ocean". Journal of Physical Oceanography. 41 (12): 2381–2401. Bibcode:2011JPO....41.2381D. doi: 10.1175/JPO-D-10-05011.1 . S2CID   42020235.
  32. ↑ Lauderdale, Jonathan M.; Williams, Richard G.; Munday, David R.; Marshall, David P. (2017). "The impact of Southern Ocean residual upwelling on atmospheric CO2 on centennial and millennial timescales". Climate Dynamics. 48 (5–6): 1611–1631. doi: 10.1007/s00382-016-3163-y . hdl: 1721.1/107158 . S2CID   56324078.
  33. ↑ Hausfather, Zeke; Peters, Glen (29 January 2020). "Emissions – the 'business as usual' story is misleading". Nature. 577 (7792): 618–20. Bibcode:2020Natur.577..618H. doi: 10.1038/d41586-020-00177-3 . PMID   31996825.
  34. ↑ Phiddian, Ellen (5 April 2022). "Explainer: IPCC Scenarios". Cosmos . Retrieved 30 September 2023. "The IPCC doesn't make projections about which of these scenarios is more likely, but other researchers and modellers can. The Australian Academy of Science, for instance, released a report last year stating that our current emissions trajectory had us headed for a 3°C warmer world, roughly in line with the middle scenario. Climate Action Tracker predicts 2.5 to 2.9°C of warming based on current policies and action, with pledges and government agreements taking this to 2.1°C.
  35. ↑ IPCC, 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 Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, New York, US, pp. 3−32, doi:10.1017/9781009157896.001.
  36. ↑ von Schuckmann, K.; Cheng, L.; Palmer, M. D.; Hansen, J.; et al. (7 September 2020). "Heat stored in the Earth system: where does the energy go?". Earth System Science Data. 12 (3): 2013–2041. Bibcode:2020ESSD...12.2013V. doi: 10.5194/essd-12-2013-2020 . hdl: 20.500.11850/443809 . CC-BY icon.svg Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License
  37. ↑ "Impacts of climate change". Discovering Antarctica. Retrieved 15 May 2022.
  38. 1 2 3 4 5 6 Fox-Kemper, B.; Hewitt, H.T.; Xiao, C.; Aðalgeirsdóttir, G.; Drijfhout, S.S.; Edwards, T.L.; Golledge, N.R.; Hemer, M.; Kopp, R.E.; Krinner, G.; Mix, A. (2021). "Ocean, Cryosphere and Sea Level Change". In Masson-Delmotte, V.; Zhai, P.; Pirani, A.; Connors, S.L.; Péan, C.; Berger, S.; Caud, N.; Chen, Y.; Goldfarb, L. (eds.). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I. Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Vol. 2021. Cambridge University Press. pp. 1239–1241. doi:10.1017/9781009157896.011. ISBN   9781009157896.
  39. ↑ Pan, Xianliang L.; Li, Bofeng F.; Watanabe, Yutaka W. (10 January 2022). "Intense ocean freshening from melting glacier around the Antarctica during early twenty-first century". Scientific Reports. 12 (1): 383. Bibcode:2022NatSR..12..383P. doi:10.1038/s41598-021-04231-6. ISSN   2045-2322. PMC   8748732 . PMID   35013425.
  40. ↑ Haumann, F. Alexander; Gruber, Nicolas; Münnich, Matthias; Frenger, Ivy; Kern, Stefan (September 2016). "Sea-ice transport driving Southern Ocean salinity and its recent trends". Nature. 537 (7618): 89–92. Bibcode:2016Natur.537...89H. doi:10.1038/nature19101. hdl: 20.500.11850/120143 . ISSN   1476-4687. PMID   27582222. S2CID   205250191.
  41. ↑ Shi, Jia-Rui; Talley, Lynne D.; Xie, Shang-Ping; Peng, Qihua; Liu, Wei (2021-11-29). "Ocean warming and accelerating Southern Ocean zonal flow". Nature Climate Change. 11 (12). Springer Science and Business Media LLC: 1090–1097. Bibcode:2021NatCC..11.1090S. doi:10.1038/s41558-021-01212-5. ISSN   1758-678X. S2CID   244726388.
  42. ↑ Aoki, S.; Yamazaki, K.; Hirano, D.; Katsumata, K.; Shimada, K.; Kitade, Y.; Sasaki, H.; Murase, H. (15 September 2020). "Reversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010s". Scientific Reports. 10 (1): 14415. doi:10.1038/s41598-020-71290-6. PMC   7492216 . PMID   32934273.
  43. ↑ Gunn, Kathryn L.; Rintoul, Stephen R.; England, Matthew H.; Bowen, Melissa M. (25 May 2023). "Recent reduced abyssal overturning and ventilation in the Australian Antarctic Basin". Nature Climate Change. 13 (6): 537–544. Bibcode:2023NatCC..13..537G. doi: 10.1038/s41558-023-01667-8 . ISSN   1758-6798.
  44. ↑ Robel, Alexander A.; Seroussi, Hélène; Roe, Gerard H. (23 July 2019). "Marine ice sheet instability amplifies and skews uncertainty in projections of future sea-level rise". Proceedings of the National Academy of Sciences. 116 (30): 14887–14892. Bibcode:2019PNAS..11614887R. doi: 10.1073/pnas.1904822116 . PMC   6660720 . PMID   31285345.
  45. 1 2 Huang, Huang; Gutjahr, Marcus; Eisenhauer, Anton; Kuhn, Gerhard (22 January 2020). "No detectable Weddell Sea Antarctic Bottom Water export during the Last and Penultimate Glacial Maximum". Nature Communications. 11. doi:10.1038/s41467-020-14302-3. PMC   6976697 .
  46. ↑ Bakker, P; Schmittner, A; Lenaerts, JT; Abe-Ouchi, A; Bi, D; van den Broeke, MR; Chan, WL; Hu, A; Beadling, RL; Marsland, SJ; Mernild, SH; Saenko, OA; Swingedouw, D; Sullivan, A; Yin, J (11 November 2016). "Fate of the Atlantic Meridional Overturning Circulation: Strong decline under continued warming and Greenland melting". Geophysical Research Letters. 43 (23): 12, 252–12, 260. Bibcode:2016GeoRL..4312252B. doi:10.1002/2016GL070457. hdl: 10150/622754 . S2CID   133069692.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.