Southern Ocean overturning circulation

Southern Ocean overturning circulation (sometimes referred to as the Southern Meridional overturning circulation (SMOC) 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. 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. 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. 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%. This has been partly due to the natural cycle of Interdecadal Pacific Oscillation, and climate change has played a substantial role in both trends, as it had altered the Southern Annular Mode weather pattern, while the massive growth of ocean heat content in the Southern Ocean has increased the melting of the Antarctic ice sheets, and this fresh meltwater dilutes salty Antarctic bottom water. 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. Consequently, ocean stratification increases. One study suggests that the circulation would lose half its strength by 2050 under the worst climate change scenario, with greater losses occurring afterwards. 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 when this will happen depends on the status of the circulation more than any factor other than overall emissions. 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. Even if the circulation's collapse starts in the near future, it is unlikely to be complete until close to 2300, 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.

Dynamics

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. 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. Observations suggest that approximately 80 percent of global deep water is upwelled in the Southern Ocean. 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.

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. 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 Hemisphere). 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.

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