Southern Ocean overturning circulation

[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.

[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.

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).

[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.

[5] The formation of sea-ice is accompanied by brine rejection, resulting in water with a higher salinity and density and therefore buoyancy loss.

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.

[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 was not in contact with anthropogenic emissions before.

[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.

[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.

[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.

[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.

[6][3] However, they were not fully caused by climate change, as the natural cycle of Interdecadal Pacific Oscillation had also played an important role.

[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.

[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.

[44] 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] The impacts of Southern Ocean overturning circulation collapse have also been less closely studied, though scientists expect them to unfold over multiple centuries.

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.
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. [ 17 ]
The role of seasonal meltwater from the Antarctic ice sheet in driving the lower-cell circulation. [ 5 ]
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. [ 25 ]
Even under the most intense climate change scenario, which is currently considered unlikely, [ 33 ] [ 34 ] 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), [ 10 ] as well as a smaller fraction of emissions. [ 35 ]
Since the 1970s, the upper cell of the circulation has strengthened, while the lower cell weakened. [ 3 ]
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. [ 45 ]