Thermohaline circulation (THC) is the density‑driven component of the global ocean circulation, arising from horizontal and vertical contrasts in seawater temperature (thermo) and salinity (haline). It underpins the widely used “global ocean conveyor belt” framework and interacts with wind‑driven currents to redistribute heat, freshwater, nutrients, and carbon. In the Atlantic basin, the Atlantic Meridional Overturning Circulation constitutes a major regional expression and is dynamically linked to the surface flow of the Gulf Stream. According to the Encyclopaedia Britannica and the U.S. National Oceanic and Atmospheric Administration (NOAA), THC is slower than wind‑driven gyres yet moves immense volumes of water, influencing climate and biogeochemistry on basin to global scales. Encyclopaedia Britannica; NOAA Ocean Service. (britannica.com)

Mechanisms and water masses. High‑latitude surface waters become dense enough to sink when cooled and/or made saltier (for example, by sea‑ice formation and brine rejection). In the Northern Hemisphere this yields North Atlantic Deep Water, while around Antarctica very cold, saline waters form Antarctic Bottom Water that spread into the abyss of most ocean basins. Typical THC speeds are on the order of centimeters per second, far slower than surface currents, but the flow extends to the seafloor. Authoritative summaries identify principal formation regions in the Nordic, Labrador, and Weddell–Ross Seas. Encyclopaedia Britannica; Antarctic Bottom Water | Britannica. (britannica.com)

Pathways and timescales. A commonly cited schematic frames THC as a global conveyor: dense waters formed in the high‑latitude North Atlantic and around Antarctica flow equatorward at depth, while upwelling—especially across the Southern Ocean—and mixing return waters to the surface. NOAA educational materials estimate that a parcel of water takes roughly 1,000 years to complete the global loop, while modern syntheses emphasize that the return pathway is strongly mediated by Southern Ocean wind‑driven upwelling and eddies. NOAA Currents Tutorial; [Marshall & Speer, “Closure of the meridional overturning circulation through Southern Ocean upwelling,” Nature Geoscience](journal://Nature Geoscience|Closure of the meridional overturning circulation through Southern Ocean upwelling|2012). (oceanservice.noaa.gov)

Observation and quantification. Since 2004, the RAPID‑MOCHA‑WBTS array at ~26.5°N has provided continuous estimates of the Atlantic overturning strength. Analyses of the record through 2012 yielded a mean overturning transport of about 17.2 Sv (1 Sv = 10^6 m³ s⁻¹) and a meridional heat transport near 1.2–1.3 PW, with substantial variability on seasonal to interannual timescales; an extended assessment through 2018 reports a mean of ~17.1 Sv. In the subpolar North Atlantic, the OSNAP array (deployed in 2014) measures overturning across two trans‑basin sections, with a reported 2014–2020 mean MOC of ~16.5 Sv and most variability concentrated in the eastern (Irminger–Iceland) sector. NERC/RAPID methods paper (McCarthy et al., 2015 summary); Moat et al., 2020, Ocean Science; GMD (2025) citing RAPID mean 2004–2018; OSNAP program site. (nora.nerc.ac.uk)

Climate influence. THC and, in particular, the Atlantic overturning transport heat northward, tempering North Atlantic climates and affecting regional sea level, storm tracks, and rainfall patterns. NOAA’s state‑of‑science fact sheet notes that the AMOC carries heat, salt, carbon, and nutrients; variations in its strength can modulate North Atlantic sea level and influence ecosystems. Peer‑reviewed assessments also connect AMOC variability to coastal sea‑level changes along the U.S. East Coast via dynamic adjustments. NOAA “State of the Science: AMOC”; Review on AMOC and U.S. East Coast sea level, open‑access. (repository.library.noaa.gov)

Biogeochemistry and the carbon cycle. By ventilating the ocean interior and returning deep waters to the surface, THC helps regulate the ocean’s uptake and storage of carbon. The IPCC describes how the solubility and biological pumps, coupled to overturning, maintain vertical CO₂ gradients and sequester anthropogenic carbon on decadal‑to‑millennial timescales; emerging work highlights potential feedbacks in which weakened overturning could diminish biological drawdown and alter outgassing. IPCC AR4 WGI, Ch. 7.3 (carbon cycle); MIT News (2024) on overturning–iron–ligand feedbacks. (archive.ipcc.ch)

Variability and change. An extensive IPCC AR6 assessment concludes that the AMOC is very likely to weaken during the 21st century for all emissions scenarios, with medium confidence that an abrupt collapse will not occur before 2100; evidence for historical weakening since the late 19th century is mixed, with medium to low confidence depending on the line of evidence. The directly observed 2005–2012 decline at 26.5°N and subsequent partial recovery illustrate pronounced decadal variability; Copernicus reanalyses and NOAA analyses emphasize that these changes may reflect internal variability superimposed on longer‑term forcings. IPCC AR6 WG1 Ch. 4 and Ch. 12, (https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-12/); Copernicus Marine AMOC indicators; NOAA/AOML (2025) communication on recent trends. (ipcc.ch)

Research foundations and terminology. Classical theory by Henry Stommel and Arnold Arons (1960) framed abyssal circulation as deep western boundary currents fed by localized high‑latitude sources and uniform upwelling—an idealization that successfully predicted boundary currents but does not capture the full modern picture. The “global conveyor belt” remains a useful heuristic, yet modern observations and theory stress the roles of wind‑driven Southern Ocean upwelling, mesoscale eddies, and interbasin pathways; accordingly, many authors use “meridional overturning circulation (MOC)” to describe zonally integrated overturning, with “thermohaline circulation” reserved for its buoyancy‑driven aspects. [Stommel & Arons, 1960, Deep‑Sea Research](document://WBC Library|On the abyssal circulation of the world ocean|1960); NOAA Ocean Service; Lozier, “Deconstructing the Conveyor Belt,” Science. (po.gso.uri.edu)

Observing systems and methods. Contemporary monitoring combines moored arrays (e.g., RAPID at 26.5°N; OSNAP across the subpolar North Atlantic), satellite altimetry and gravimetry, autonomous floats and gliders, hydrographic sections, and tracer budgets. The RAPID data set (v2024.1) documents continuous 10‑day estimates of transports and heat fluxes since 2004; OSNAP reports monthly overturning and flux time series from 2014 onward. RAPID data portal; OSNAP program site. (rapid.ac.uk)

Further reading. Standard oceanography texts synthesize THC structure, water‑mass formation, and basin‑scale budgets, including Talley et al.’s Descriptive Physical Oceanography (6th ed.), and reviews on Southern Ocean upwelling and eddy compensation that situate THC within the coupled climate system. [Talley et al., Descriptive Physical Oceanography](book://Lynne D. Talley|Descriptive Physical Oceanography: An Introduction (6th ed.)|Elsevier|2011); Annual Review of Marine Science (Gent, 2016). (talleylab.ucsd.edu)