Southern Ocean overturning circulation

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]

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]