Southern Ocean overturning circulation

takes 60–90 years for just half of the water mass, and some water travels to the surface for more than a century. 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. 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. Instead, dense water from sinking regions returned to the surface in nearly adiabatic pathways along density isopycnals, which was already written by Harald Sverdrup. Lower cell from the Antarctic ice sheet in driving the lower-cell circulation. == Global carbon cycle ==

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. Out of all oceans, the Southern Ocean plays the greatest role in carbon uptake, and on its own, it is responsible for around 40%. but subsequent research found that this carbon sink had been even stronger than estimated earlier, by some 14% to 18%. 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. 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. On the other hand, regions where deep warm circumpolar carbon rich waters are brought to the surface through upwelling, outgas through exposure to the atmosphere, partly compensating the carbon sink effect of the overturning circulation. 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. == Climate change impacts ==

(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 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. Much of this increase has occurred in the extratropical Southern Hemisphere ocean south of 30°S. In West Antarctica, the temperature in the upper layer of the ocean has warmed since 1955, and the Antarctic Circumpolar Current (ACC) is also warming faster than the global average. This warming directly affects the flow of warm and cold water masses which make up the overturning circulation, and it also reduces the cover of sea ice (which is highly reflective and so elevates the albedo of Earth's surface) in the Southern Hemisphere, as well as mass balance of Antarctica's ice shelves and peripheral glaciers. For these reasons, climate models consistently show that the year when global warming will reach (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. This freshening of the Southern Ocean causes increased stratification and stabilization of its layers, and this has the single largest impact on the long-term properties of Southern Ocean circulation. 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. (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. 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. 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 and , but this estimate is much less certain than for many other tipping points. 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. Reduced marine productivity would also mean that the ocean absorbs less carbon (though not within the 21st century), 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. 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. == References ==