Thermohaline circulation
Thermohaline circulation (THC) is a part of the large-scale
The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt, coined by climate scientist
This global circulation has two major limbs - Atlantic Meridional Overturning circulation (AMOC), centered in the north Atlantic Ocean, and Southern Ocean overturning circulation or Southern Ocean meridional circulation (SMOC), around Antarctica. Because 90% of the human population lives in the Northern Hemisphere,[8] the AMOC has been far better studied, but both are very important for the global climate. Both of them also appear to be slowing down due to climate change, as the melting of the ice sheets dilutes salty flows such as the Antarctic bottom water.[9][10] Either one could outright collapse to a much weaker state, which would be an example of tipping points in the climate system. The hemisphere which experiences the collapse of its circulation would experience less precipitation and become drier, while the other hemisphere would become wetter. Marine ecosystems are also likely to receive fewer nutrients and experience greater ocean deoxygenation. In the Northern Hemisphere, AMOC's collapse would also substantially lower the temperatures in many European countries, while the east coast of North America would experience accelerated sea level rise. The collapse of either circulation is generally believed to be more than a century away and may only occur under high warming, but there is a lot of uncertainty about these projections.[10][11]
History of research
It has long been known that
In the 1920s, Sandström's framework was expanded by accounting for the role of salinity in ocean layer formation.[1] Salinity is important because like temperature, it affects water density. Water becomes less dense as its temperature increases and the distance between its molecules expands, but more dense as the salinity increases, since there is a larger mass of salts dissolved within that water.[15] Further, while fresh water is at its most dense at 4 °C, seawater only gets denser as it cools, up until it reaches the freezing point. That freezing point is also lower than for fresh water due to salinity, and can be below -2 °C, depending on salinity and pressure.[16]
Structure
These density differences caused by temperature and salinity ultimately separate ocean water into distinct
In the
The out-flowing undersea of cold and salty water makes the sea level of the Atlantic slightly lower than the Pacific and salinity or halinity of water at the Atlantic higher than the Pacific. This generates a large but slow flow of warmer and fresher upper ocean water from the tropical Pacific to the
Upwelling
As the deep waters sink into the ocean basins, they displace the older deep-water masses, which gradually become less dense due to continued ocean mixing. Thus, some water is rising, in what is known as
Computer models of ocean circulation increasingly place most of the deep upwelling in the Southern Ocean, associated with the strong winds in the open latitudes between South America and Antarctica.[28] Direct estimates of the strength of the thermohaline circulation have also been madeat 26.5°N in the North Atlantic, by the UK-US RAPID programme. It combines direct estimates of ocean transport using current meters and subsea cable measurements with estimates of the geostrophic current from temperature and salinity measurements to provide continuous, full-depth, basin-wide estimates of the meridional overturning circulation. However, it has only been operating since 2004, which is too short when the timescale of the circulation is measured in centuries.[29]
Effects on global climate
The thermohaline circulation plays an important role in supplying heat to the polar regions, and thus in regulating the amount of sea ice in these regions, although poleward heat transport outside the tropics is considerably larger in the atmosphere than in the ocean.
Large influxes of low-density meltwater from Lake Agassiz and deglaciation in North America are thought to have led to a shifting of deep water formation and subsidence in the extreme North Atlantic and caused the climate period in Europe known as the Younger Dryas.[31]
Slowdown or collapse of AMOC
Slowdown or collapse of SMOC
Additionally, the main controlling pattern of the extratropical Southern Hemisphere's climate is the
See also
- Atlantic multidecadal oscillation – Climate cycle that affects the surface temperature of the North Atlantic
- Brunt-Väisälä frequency– Measure of fluid stability against vertical displacement
- Contourite – Type of sedimentary deposit
- Downwelling – Process of accumulation and sinking of higher density material beneath lower density material
- Halothermal circulation – Part of world ocean circulation system
- Hydrothermal circulation – Circulation of water driven by heat exchange
- Temperature-salinity diagram– Diagrams used to identify water masses
References
- ^ S2CID 4414604.
- ^ Lappo, SS (1984). "On reason of the northward heat advection across the Equator in the South Pacific and Atlantic ocean". Study of Ocean and Atmosphere Interaction Processes. Moscow Department of Gidrometeoizdat (in Mandarin): 125–9.
- ^ NOAA. Archived from the originalon 31 December 2017.
- S2CID 130736022.
- ISSN 0362-4331. Retrieved 5 June 2022.
- S2CID 186242350.
- S2CID 129518576.
- ^ Collins, Kevin (3 November 2023). "El Niño may be drying out the southern hemisphere – here's how that affects the whole planet". The Conversation.
- NOAA. 29 March 2023.
- ^ a b 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.
- ^ Logan, Tyne (29 March 2023). "Landmark study projects 'dramatic' changes to Southern Ocean by 2050". ABC News.
- ^ Schmidt, Gavin (26 May 2005). "Gulf Stream slowdown?". RealClimate. Archived from the original on 20 February 2006.
- ISBN 0-444-52747-8.
- ISBN 978-3-642-23449-1.
- .
- Nature Magazine. Retrieved 11 March 2024.
- ^ Stommel, H., & Arons, A. B. (1960). On the abyssal circulation of the world ocean. – I. Stationary planetary flow patterns on a sphere. Deep Sea Research (1953), 6, 140-154.
- PMC 5995860.
- .
- ISSN 0094-8276.
- PMID 32494658.
- PMID 27552365.
- ISSN 2169-9275.
- ^ The Thermohaline Circulation - The Great Ocean Conveyor Belt Archived 19 December 2022 at the Wayback Machine NASA Scientific Visualization Studio, visualizations by Greg Shirah, 8 October 2009. This article incorporates text from this source, which is in the public domain.
- ^ United Nations Environment Programme / GRID-Arendal, 2006, [1] Archived 28 January 2017 at the Wayback Machine. Potential Impact of Climate Change
- ISBN 0-87590-095-X.
- )
- doi:10.1038/ngeo1391.
- ^ "RAPID: monitoring the Atlantic Meridional Overturning Circulation at 26.5N since 2004". www.rapid.ac.uk.
- .
- S2CID 39544213.
- ^ "Explainer: Nine 'tipping points' that could be triggered by climate change". Carbon Brief. 10 February 2020. Retrieved 4 September 2021.
- S2CID 229063736.
- ^ ISBN 9781009157896.
- PMID 35039511.
- S2CID 257807573.
- PMID 31285345.
- S2CID 133069692.
- ^ a b 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.
- PMC 6976697.
Other sources
- Apel, JR (1987). Principles of Ocean Physics. Academic Press. ISBN 0-12-058866-8.
- Gnanadesikan, A.; R. D. Slater; P. S. Swathi; G. K. Vallis (2005). "The energetics of ocean heat transport". Journal of Climate. 18 (14): 2604–16. .
- Knauss, JA (1996). Introduction to Physical Oceanography. Prentice Hall. ISBN 0-13-238155-9.
External links
- Ocean Conveyor Belt
- THOR FP7 projects http://arquivo.pt/wayback/20141126093524/http%3A//www.eu%2Dthor.eu/ investigates on the topic "Thermohaline overturning- at risk?" and the predictability of changes of the THC. THOR is financed by the 7th Framework Programme of the European Commission.